ScienceWeek

SCIENCE-WEEK

A Weekly Email Digest of the News of Science

A journal devoted to the improvement of communication
between the scientific disciplines, and between scientists,
science educators, and science policy-makers.

June 1, 2001 -- Vol. 5 Number 22

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It took a million years to move from counting
pebbles to the elaborations of quantum mechanics.
Certainly this was an arduous migration of the
multitude -- not a private party of physicists,
but the Long March of the entire human race.
-- Anonymous

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=-=-=-=-=-=-=-=-=
Section 1
=-=-=-=-=-=-=-=-=

Contents of this Issue (Full reports in Section 2):

1. HUMAN EVOLUTION:
A NEW HOMININ FOSSIL DATED AT 3.5 MILLION YEARS AGO
A new hominin fossil discovered near the shore of Lake Turkana in
Kenya is almost certainly a new species with a startling mosaic
of features. None of these features is in itself new, but the
combination of features is not found in any other known species.
The discoverers of the fossil propose the fossil is a new genus
of hominin and have named it Kenyanthropus platyops.

2. MOLECULAR BIOLOGY:
ON NUTRITIONAL STATUS AND GENE ACTIVATION IN BACTERIA
All living organisms require nutrients, and the behavior of
organisms is directly dependent on the availability of nutrients
in their local environments. This dependence has been intensively
studied in bacteria, organisms that can be maintained in large
populations in the laboratory, in experiments involving careful
manipulation of environmental nutrient entities.

3. MEDICAL BIOLOGY:
FIRST CLINICAL TOTAL TRANSPLANT OF THE HUMAN LARYNX
Clinicians report the first successful total transplantation of
the human larynx. The patient was without a voice after
destruction of his larynx in a motorcycle accident 20 years ago.
40 months after transplantation of the entire pharyngolaryngeal
complex, including six tracheal rings and the thyroid and
parathyroid glands, the patient now speaks with a normal voice.

4. GEOPHYSICS: ON THE CLIMATE OF EARLY EARTH
During the cooling of early Earth, how long did conditions
favorable for thermophilic life forms exist? A new analysis
indicates that the time of 100 degC temperature was geologically
brief, with approximately 100 degC surface conditions maintained
for at most a few million years. This is an important constraint
for origin-of-life scenarios.

5. ASTROPHYSICS: A BROWN DWARF RADIO STAR
Brown dwarf stars older than approximately 10 million years are
expected to have short-lived magnetic fields and to emit only
weak radio waves and x-rays from their coronas. Astronomers now
report the discovery of both quiescent and flaring radio emission
from a 500-million-year old brown dwarf star, the radio emissions
several orders of magnitude larger than expected.

6. CHEMICAL PHYSICS:
MEASUREMENT OF SHORT-RANGE CHEMICAL BONDING FORCES
Although the atomic force microscope was originally intended to
be used as a tool capable of measuring the forces acting between
a single pair of atoms, it has evolved into an instrument capable
of producing atomically resolved images of surfaces. A new
technique demonstrates the possibility of directly measuring and
quantifying local surface reactivity with this instrument.

7. IN FOCUS: ON QUANTUM WEIRDNESS
A misinterpretation of the finding in quantum theory that
subatomic particles do not acquire some of their characteristics
until they are observed is that 'ultimate reality is in the mind
of the observer'. But quantum theory says _nothing_ about the
role of human consciousness or mental processes in the physical
world.

8. FROM THE SCIENCEWEEK ARCHIVE:
PUBLIC HEALTH: EXPECTED CONSEQUENCES OF GLOBAL WARMING
Climate change produced by global warming is expected to result
in melting ice caps, rising sea levels, torrential floods,
devastating droughts, and severe harvest failures. What are often
not considered in discussions of global warming are the effects
of global warming on public health.

=-=-=-=-=-=-=-=-=
Section 2
=-=-=-=-=-=-=-=-=

1. HUMAN EVOLUTION:
A NEW HOMININ FOSSIL DATED AT 3.5 MILLION YEARS AGO
     The terms hominine, hominin, hominoid, hominid, are not
interchangeable, but their classification criteria are variously
in a state of flux. In general, the hominoids are a primate
superfamily, the hominid family comprises the great apes within
the hominoid superfamily, the hominini are a "tribe" within the
hominids characterized by a number of features including
bipedalism, and the hominini are further partitioned into three
genera, Homo, Paranthropus, and Australopithecus (until recently
there were only two genera, with Paranthropus included in
Australopithecus). In general, the term "hominin" refers to any
human-related fossil group.
     Concerning research in human evolution, most
paleoanthropologists agree that what is important is to achieve
an understanding of the evolutionary transitions and
transformations of the human lineage, and any classification
scheme must be secondary to this objective. In other words, in
this context, classification must ultimately reflect phylogeny
(the actual evolutionary relationships), and as knowledge of
phylogeny changes, so must the extant classification schemes.
     In general, the term "Australopithecus" refers to a now
extinct genus believed to have existed between 4.4 and 1 million
years ago, with this genus believed to have been the precursor of
the genus Homo. All australopithecines are apparently
characterized by an ape-like form, rather than the human-like
form of the genus Homo. (Australopithecine fossils have been
found only in Africa: the prefix "australo-" in this case means
"south", the word "australopithecus" meaning "southern ape".)
     The fossil Australopithecus afarensis, which provided the
first record of human footprints, of hominids walking upright,
was discovered at Laetoli in East Africa, and has been dated at
3.6 million years ago. A. afarensis, probably weighed 25 to 50
kilograms (60 to 120 lbs.) as an adult.
     Concerning the early hominids, Ardipithecus ramidus is the
oldest, and is currently considered the "oldest human". This
species was found in Ethiopia in 1994 and dated as 4.4 million
years old. Australopithecus anamensis, found at Lake Turkana in
Kenya, is dated as 4.1 to 3.9 million years old, and is
considered to be an intermediate between A. ramidus and later
hominids.
     Lake Turkana (formerly called Lake Rudolf) is the fourth
largest of the east African lakes, and is named after the Turkana
people, a group living in northwestern Kenya. The lake lies
mainly in northern Kenya, with its northern end extending into
Ethiopia, and is situated in the eastern arm of the east Africa's
Rift Valley. The lake covers an area of 6405 square kilometers at
375 meters above sea level. Lake Turkana is 248 kilometers long
and only 16 to 32 kilometers wide. It is relatively shallow, with
its greatest recorded depth at 73 meters. Since 1970, a number of
valuable hominid fossils have been unearthed at localities on the
eastern shore of Lake Turkana, these localities now called the
"Koobi Fora sites". The present report concerns an important find
on the opposite (western) shore of Lake Turkana.
     The Pliocene time-frame extends from 5.2 to 1.64 million
years ago, and the Pleistocene time-frame extends from 1.64
million years ago to 10,000 years ago.
     The term "adaptive radiation" refers to the rapid evolution
of one or a few forms into many different species that occupy
different habitats within a new geographical area. In general,
the term "genus" is a taxonomic category that stands between
"family" and "species", with each genus containing a number of
species that are presumably related to each other by descent from
a common ancestor. There are various definitions in use for the
term "species": a) the biological species concept: a species is
an interbreeding or potentially interbreeding group of
populations reproductively isolated from other groups; b) the
evolutionary species concept: a species is a lineage evolving
separately from others with its own unitary evolutionary role and
tendencies; c) the population genetics concept: a species is a
population of individuals bearing distinctive genes and gene
frequencies and separated from other species by biological
barriers preventing gene exchange. In the context of
paleoanthropology, taxonomic categorizations of Pliocene hominids
based on fossil remains are far from straightforward, and the
criteria used for genus and species demarcations have been and
continue to be controversial.
     In this context, a "derived" characteristic is a
characteristic not present in the closest known ancestor, i.e., a
characteristic that has evidently newly evolved. In contrast, a
"primitive" characteristic is a characteristic that is present in
the closest known ancestor.
... ... M.G. Leakey et al (7 authors at 4 installations, KE UK US
AU) present a report on a new hominin genus from eastern Kenya,
the authors making the following points:
     1) The authors point out that the eastern African hominin
record between 4 and 3 million years ago has been represented
exclusively by a single species, A. afarensis and its possible
ancestor A. anamensis, which are commonly thought to belong to
the lineage ancestral to all later hominins. This apparent lack
of diversity in the middle Pliocene contrasts markedly with the
increasingly varied phylogeny in the later hominin fossil record.
To further study the time interval 4 to 3 million years ago,
fieldwork by the authors in 1998 and 1999 focused on sites of
this age at Lomekwi in the Nachukui Formation west of Lake
Turkana.
     2) The authors report the discovery of new fossils west of
Lake Turkana, the new fossils differing markedly from those of
contemporary A. afarensis, indicating that hominin taxonomic
diversity extended back into the middle Pliocene. A 3.5-million-
year-old cranium, showing a unique combination of derived facial
and primitive neurocranial features, is assigned by the authors
to a new genus of hominin. The authors suggest their findings
point to an early diet-driven adaptive radiation, provide new
insight into the association of hominin craniodental features,
and have implications for our understanding of Pliocene-
Pleistocene hominin phylogeny. The new fossil (KNM-WT 40000) is
named by the authors as Kenyanthropus platyops (platyops from the
Greek "platus", meaning flat, and "opsis", meaning face; thus
referring to the characteristically flat face of this species.)
The genus name Kenyanthropus is chosen "in recognition of Kenya's
contribution to the understanding of human evolution through the
many specimens recovered from its fossil sites.")
... ... In a commentary on the Leakey et al report, Daniel E.
Lieberman (George Washington University, US) makes the following
points:
     1) The author (Lieberman) suggests that the new fossil is
almost certainly a new species and with a "dizzying mosaic" of
features. None of these characteristics is in itself new, but the
combination of features is not found in any other known species.
The fossil resembles chimpanzees and one of the australopithecine
species, A. anamensis, in having a small earhole. The fossil also
shares many other features of primitive hominins with A.
afarensis and A. anamensis, such as cheek teeth with thick
enamel, a small brain the size of that of a chimpanzee, and flat
nasal margins.
     2) The author (Lieberman) suggests that we can now say with
confidence that hominin evolution, like that of many other
mammalian groups, occurred through a series of complex
radiations, in which many new species evolve and rapidly
diversify. Apparently, between 3.5 and 2 million years ago, there
were several human-like species that were well adapted to life in
different environments, although in ways that we have yet to
appreciate fully.
     3) The author (Lieberman) concludes: "A challenge for the
next decade will be for skeletal biologists, paleontologists, and
molecular biologists to work together, to devise new analytical
methods with which to tease trustworthy signals from [existing]
data. My guess is that it will be quite a while before we can
confidently determine the position of Kenyanthropus platyops in
the human evolutionary tree."
-----------
M.G. Leakey et al: New hominin genus from eastern Africa shows
diverse middle Pliocene lineages.
(Nature 22 Mar 01 410:433)
QY: Meave G. Leakey: meave@swiftkenya.com
-----------
Daniel E. Lieberman: Another face in our family tree.
(Nature 22 Mar 01 410:419)
QY: Daniel E. Lieberman: danlieb@gwu.edu
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 1Jun01
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ANTHROPOLOGY:
DIET AND THE EVOLUTION OF THE EARLIEST HUMAN ANCESTORS
     In general, the term "hominids" refers to any primate in the
human family, and the "Pliocene epoch", the geological epoch with
the time-frame 5.2 to 1.64 million years ago, is the epoch during
which the first hominids appeared. There are presently 12 species
of hominids recognized, based on fossil evidence, but some of the
classifications are controversial. Of the 12 hominid species, 6
belong to the early hominids generally known as
"australopithecines" and 6 species to the later hominid genus
"Homo".
     The geological epoch preceding the Pliocene, was the
Miocene, with the time-frame 23.3 to 5.2 million years ago, and
it is during the Miocene that the precursors to the hominids, the
"hominoids", first appeared. In general, the hominoids are a
primate superfamily that includes the gibbons, the great apes
(Pongidae) and the hominids.
     Perhaps the most important characteristic of the hominids,
at least as revealed by extant fossils, is the apparent existence
of "bipedalism", walking upright on the hind limbs. The
appearance of bipedalism would have led to anatomical changes in
all parts of the body system, and many of these changes are
distinguishing characteristics in hominid fossils. The evidence
for the evolution of bipedalism includes the oldest hominid
skeletons, dated as 3.6 to 4.4 million years old, and a trackway
of footprints in volcanic ash dated as 3.75 million years old.
     Another important characteristic in the anthropology of the
hominids concerns dentition and its relation to diet. In general,
hominid dental features show a reduction of the anterior teeth
(canines and incisors) that is interpreted as indicating a
dietary shift of significance. Recent research with the scanning
electron microscope has revealed tiny pits and scratches on the
surfaces of tooth enamel ("microwear patterns") that provide
evidence of dietary conditions. Grasses, for example, leave
linear scratches on teeth; leaves produce a polished effect; and
the bone crunching involved in eating meat gouges out tiny pits
in the enamel.
     Concerning the early hominids, Ardipithecus ramidus is the
oldest, and is currently considered the "oldest human". This
species was found in Ethiopia in 1994 and dated as 4.4 million
years old.
     Australopithecus anamensis, found in Lake Turkana in Kenya,
is dated as 4.1 to 3.9 million years old, and is considered to be
an intermediate between A. ramidus and later hominids.
     The hominid Australopithecus afarensis has provided many
fossils, all in Africa, and has been dated as 3.2 to 2.9 million
years ago.
     The fossil Australopithecus africanus has been dated as
approximately 3 to 1.5 million years ago.
     During the last 50 years, ever-increasing numbers of hominid
fossils have been found, and their taxonomy and apparent place in
the pattern of human evolution continues to involve detailed
controversy.
... ... M.F. Teaford and P.S. Ungar (2 installations, US) present
a report on diet and the evolution of the early hominids, the
authors making the following points:
     1) The authors point out that since the discovery of A.
afarensis, many researchers have emphasized the importance of
bipedality in scenarios of human origins, but less attention has
been focused on the role played by diet in the ecology and
evolution of the early hominids. Recent work in a broad range of
disciplines, such as paleo-environmental studies, behavioral
ecology, primatology, and isotope analyses has renewed interest
in early hominid diets. In addition, important new fossils from
the early Pliocene raise major questions about the role of
dietary changes in the origins and early evolution of the
hominids. The authors state: "We need to focus not just on how
the early hominids moved between food patches, but also on what
they ate when they got there."
     2) The authors present a review of the fossil evidence for
the diets of the Pliocene hominids A. ramidus, A. anamensis, A.
afarensis, and A. africanus. These hominids offer evidence for
the first half of human evolution, from our split with the
prehistoric apes to the earliest members of our own genus. The
authors view the taxa under consideration as a roughly linear
evolutionary sequence from A. ramidus to A. africanus, spanning
the time-frame 4.4 to 2.5 million years ago. As such, these
species provide a unique opportunity to examine changes in
dietary adaptations of human ancestors over nearly 2 million
years. The authors also trace what has been inferred concerning
the diets of the Miocene hominoids in order to place changes in
Pliocene hominid diets into a broader temporal perspective.
     3) The authors report that from their analysis it becomes
clear that the dietary capabilities of the early hominids changed
dramatically in the time period between 4.4 to 2.3 millions years
ago. Most of the evidence has come from five sources: analyses of
tooth size, tooth shape, enamel structure, dental microwear, and
jaw biomechanics. Taken together, the evidence suggests a dietary
shift in the early australopithecines, a shift to increased
dietary flexibility in conjunction with apparent climate
variability. Moreover, changes in diet-related adaptations from
A. anamensis to A. afarensis to A. africanus suggest that hard
abrasive foods became increasingly important through the
Pliocene, perhaps as critical items in the diet.
-----------
M.F. Teaford and P.S. Ungar: Diet and the evolution of the
earliest human ancestors.
(Proc. Natl. Acad. Sci. US 5 Dec 00 97:13506)
QY: Mark F. Teaford: mteaford@jhmi.edu
-------------------
Related Background:
HUMAN EVOLUTION:
FOSSIL EVIDENCE OF EARLY HOMINID KNUCKLE-WALKING
     There are 185 species of living primates, of which modern
humans, Homo sapiens, are one species. All primates, from bush
babies and tarsiers to gorillas and humans are characterized by
approximately 30 characters that relate to 3 major sets of
adaptations: a) agility in the trees; b) acute brain and visual
functions; c) parental care.
     The consensus view in paleoanthropology is that humans are a
very young group that has had a separate evolutionary history for
only 5 to 7 million years, with two sets of characters that
apparently set humans apart from other apes: locomotion on two
legs (bipedalism) and large brain size. Fossil evidence indicates
that by 4.1 million years ago, and perhaps earlier, *hominids
exhibited adaptations to bipedal walking. At present, however,
the fossil record offers little information about the origin of
bipedalism, and despite nearly a century of research on existing
fossils and on comparative anatomy, there is apparently still no
consensus concerning the mode of locomotion that preceded
bipedalism (*Note #1).
... ... B.G. Richmond and D.S. Strait (George Washington
University, US) now present evidence that fossils attributed to
*Australopithecus anamensis and *Australopithecus afarensis
retain specialized wrist morphology associated with knuckle-
walking. This morphology differs from that of later hominids and
non-knuckle-walking anthropoid primates, suggesting that knuckle-
walking is a derived feature of the African ape and human *clade.
The authors suggest this removes key morphological evidence for a
chimpanzee(Pan)-gorilla(Gorilla) clade, and indicates that
bipedal hominids evolved from a knuckle-walking ancestor that was
already partly terrestrial.
... ... In a commentary in the same journal on the work of
Richmond and Strait, M.Collard and L.G. Aiello (University
College London, UK) state: "Richmond and Strait's study... bears
on the long-standing debate over the evolutionary history -- the
'*phylogenetic' relationships -- of modern humans (Homo),
chimpanzees (Pan), and gorillas (Gorilla). [Molecular] genetic
analyses overwhelmingly indicate that chimpanzees and humans are
more closely related to one another than either is to gorillas.
Until the study by Richmond and Strait, however, the anatomical
evidence largely ran counter to this conclusion... Richmond and
Strait's results offer strong support for the idea that knuckle-
walking characters were present in the common ancestor of modern
humans and the African apes, and provide grounds for
reconciliation between the molecular and anatomical evidence."
-----------
B.G. Richmond and D.S. Strait: Evidence that humans evolved from
a knuckle-walking ancestor.
(Nature 23 Mar 00 404:382)
QY: B.G. Richmond [brich@gwu.edu]
-----------
M. Collard and L.C. Aiello: From Forelimbs to Two Legs.
(Nature 23 Mar 00 404:339)
QY: M. Collard [m.collard@ucl.ac.uk]
-----------
Text Notes:
... ... *hominids: In general, any primate in the human family.
... ... *Note #1: The hominid fossil record is far from
extensive, and a common quip in paleoanthropology is that there
are more working paleoanthropologists than discovered hominid
fossils to be worked on.
... ... *Australopithecus anamensis: Found by Meave Leakey in
4.1- to 3.9-million-year-old sediments near Lake Turkana in
Kenya. The remains include two jaws, a humerus, a tibia, and
isolated teeth. It has a primitive jaw with a shallow palate and
large canine teeth. The tibia indicates that A. anamensis was a
biped.
... ... *Australopithecus afarensis: The famous skeleton of
"Lucy", discovered in the 1970s, is an example of an
Australopithecus afarensis fossil. A. afarensis specimens range
from 3.2 to 2.9 million years in age. The individuals are 1 to
1.2 meters tall, with a brain size of only 415 cubic centimeters
and a generally ape-like face. Primitive characters include long
arms, short legs, and curved finger and toe bones that imply the
use of hands and feet in grasping branches. However, A. afarensis
is fully human in some significant ways, including hind limbs and
pelvis fully adapted for a type of bipedal locomotion.
... ... *clade: In general, a "clade" is a cluster of taxa
apparently derived from a single common ancestor.
... ... *phylogenetic: The term "phylogeny" refers to the
evolutionary history of a species or group of species in terms of
their derivation and relationships.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 23Jun00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ISOTOPIC ANALYSIS OF THE DIET OF AN EARLY HOMINID
The term "hominid" refers to any primate in the human family
(Hominidae) of which Homo sapiens (modern man) is the only living
specimen. The current consensus, based on molecular evidence, is
that our closest living relatives, gorillas and chimpanzees,
probably separated from our line (or us from them) approximately
6 to 8 million years ago. Humans, gorillas, common chimpanzees
and pygmy chimpanzees, the 4 African species of "great apes",
apparently had common ancestors perhaps 10 to 12 million years
ago. The first record of human footprints, of hominids walking
upright, was discovered at *Laetoli in East Africa, and has been
dated at 3.6 million years ago. This ancestor, Australopithecus
afarensis, probably weighed 25 to 50 kilograms (60 to 120 lbs.)
as an adult (*Note #1). Apparently derived from Australopithecus
afarensis were several species, including Australopithecus
africanus, a species which is believed to have appeared
approximately 3 million years ago. Little is known about the
diets of hominids that predate the Homo genus, because these
hominids did not leave archeological traces such as "*kitchen
middens" and stone tools. Consequently, researchers have made
inferences concerning hominid diet on the basis of craniodental
morphology, gross dental wear, and dental microwear. The current
consensus is that the 3-million-year-old Australopithecus
africanus hominid subsisted on fruits and leaves, similar to the
modern chimpanzee. Early hominid diets are of some theoretical
significance, since one current view is that the emergence of the
more intelligent Homo genus depended on the consumption of high-
quality animal foods that made possible biological changes
resulting in the evolution of a larger brain.
... ... M. Sponheimer and  J.A. Lee-Thorp (2 installations, US
ZA) now report a stable carbon isotope analysis of A. africanus
fossils from *Makapansgat Limeworks, South Africa. The authors
sampled 4 of the 14 Australopithecus africanus individuals that
have been unearthed at that location, and also analyzed the
dental enamel of associated 3-million year old animals (65
individual animals from 19 mammalian taxa) in order to place A.
africanus within a broader ecological context. The authors report
their results demonstrate that A. africanus ate not only fruits
and leaves, but they also ate large quantities of carbon-13
enriched foods such as grasses and sedges, or they ate animals
that ate these plants, or both. The authors suggest their results
indicate that early hominids such as A. africanus regularly
exploited relatively open environments such as woodlands or
grasslands for food, and that early hominids may have consumed
high-quality animal foods before the development of stone tools
and the origin of the genus Homo.
-----------
M. Sponheimer and J.A. Lee-Thorp: Isotopic evidence for the diet
of an early hominid, Australopithecus africanus.
(Science 15 Jan 99 283:368)
QY: Matt Sponheimer, Rutgers Univ. New Brunswick 908-932-8789.
-----------
Text Notes:
... ... *Laetoli: The discovery involved a 30-yard-long footprint
trail of three bipedal individuals, the trail made approximately
3.6 million years ago in a newly deposited layer of volcanic ash.
The ash layer also contains the prints of many other animal
forms. 
... ... *Note #1: The famous skeleton of "Lucy", discovered in
the 1970s, is an example of an Australopithecus afarensis fossil.
... ... *kitchen middens: In general, a "midden" is a refuse
heap. In this context, a "kitchen midden" is a mound consisting
of shells of edible molluscs and other refuse, the mound marking
the site of a prehistoric human habitation.
... ... *Makapansgat: (Makapansgaat) A cave site northeast of
Johannesburg.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 9Apr99
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ANTHROPOLOGY: DEFINING THE HUMAN GENUS
Taxonomy is the field in biology devoted to the classification of
living organisms, and the term "taxon" is used to indicate the
members of any particular group in the classification scheme:
class, family, genus, etc [*Note #1]. The general problem is how
to incorporate information about evolutionary history and
adaptation into taxonomic classification schemes, with the
problem exemplified by attempts to define our own genus, Homo.
The definition of the genus Homo has always been somewhat
contentious, not least because it is tied, implicitly or
explicitly, to the state of "being human". A series of anatomical
characteristics is found to be apparently unique in Homo -- for
example, an increase in cranial vault height and cranial
thickness, reduced lower facial prognathism (i.e., reduced
projection of the jaw), and in dentition reduction in the size of
the premolars and molars and the length of the molar row -- but
what has always been emphasized by taxonomic definitions is the
size of the Homo brain as revealed by the size of the cranium.
According to the classical scheme, to be Homo is to be a large-
brained *hominine (the apparent human-related fossil group), a
hominine presumably more technologically accomplished than the
ancestor group, the *Australopithecines. At the present time, as
is evident in this report, the taxonomic categorizations related
to the human genus are far from fixed. ... ... B. Wood and M.
Collard (2 installations, US UK) present an extensive review of
the taxonomic situation concerning the genus Homo, the authors
making the following points:
     1) The authors suggest that recent data, fresh
interpretations of the existing evidence, and the limitations of
the paleoanthropological record invalidate existing criteria for
allocating fossil species to Homo.
     2) The authors suggest that regardless of any formal
definitions, in current practice fossil hominin species are
assigned to Homo on the basis of one or more out of 4 criteria:
a) absolute brain size at least 600 cubic centimeters; b)
possession of language as inferred from *endocranial casts; c)
possession of a modern human-like precision grip involving a
well-developed and opposable thumb (pollex); d) the ability to
manufacture stone tools. The authors state: "It is now evident,
however, that none of these criteria is satisfactory."
     3) The authors present a revised definition for the genus
Homo based on criteria considered verifiable and conclude that
two species, *Homo habilis and *Homo rudolfensis, do not belong
in the genus. The authors suggest the earliest taxon to satisfy
the criteria is *Homo ergaster, or early African *Homo erectus,
which currently appears in the fossil record at about 1.9 million
years ago.
     4) The authors suggest that a fossil species should be
included in Homo only if the following can be demonstrated:
... ... a) the species is more closely related to H. sapiens than
it is to the australopiths;
... ... b) the species has an estimated body mass more similar to
that of H. sapiens than to that of the australopiths;
... ... c) the species has reconstructed body proportions that
match those of H. sapiens more closely than those of the
australopiths;
... ... d) the species has a *postcranial skeleton whose
functional morphology is consistent with modern human-like
obligate bipedalism and limited facility for climbing;
... ... e) the species is equipped with teeth and jaws that are
more similar in terms of relative size to those of modern humans
than to those of the australopiths;
... ... f) the species shows evidence for a modern human-like
extended period of childhood growth and development.
     5) The authors conclude by suggesting that the adoption of
the above criteria would mean the genus Homo would have both
phylogenetic and adaptive significance. "Researchers can then
explore whether this adaptive shift in hominin evolution
corresponds with changes in climate, analogous evolutionary
changes in other large mammal groups, particular innovations in
the hominin cultural record, substantial expansions in geographic
range, or changes in ecological tolerance as reflected in
reconstructions of hominin habitats."
-----------
B. Wood and M. Collard: The human genus.
(Science 2 Apr 99 284:65)
QY: Bernard Wood: bwood@gwu.edu
-----------
Text Notes:
... ... *Note #1: The conventional hierarchy of classification in
biology is Kingdom, Phylum, Class, Order, Family, Genus, and
Species. In the literature, organisms are usually referred to by
genus and species in binomial nomenclature, with the genus
capitalized. Human beings are genus Homo, species sapiens,
binomially Homo sapiens. The convention in binomial nomenclature
is to initialize the genus; thus: H. sapiens.
... ... *hominine: See main report.
... ... *Australopithecines: See main report.
... ... *endocranial casts: In general, an "endocast" (steinkern)
is any fossil formed after dissolution of an interior molding
substance. An "endocranial cast" is an endocast involving the
cranium. The interior of the endocast can often reveal details
concerning the absent soft interior substance (in this case, the
brain).
... ... *Homo habilis: In 1964, an early fossil hominin (1.9 to
1.6 million years before the present) was found in Olduvai,
Tanzania, the brain apparently intermediate in size between the
earliest known Homo fossil *Homo erectus and the Australopithecus
group. This new fossil was denoted as a new species by its
discoverers and named Homo habilis. The original set of H.
habilis fossils included a relatively complete hand, its
structure apparently compatible with an ability to make and use
tools. (Homo habilis literally means "handy-man") Considerable
controversy in the paleoanthropology community concerning H.
habilis has continued from 1964 until the present.
... ... *Homo rudolfensis: The original H. habilis species has
more recently been divided into H. habilis and H. rudolfensis,
after a fossil of the latter group was discovered in 1993 and
related to an earlier find in 1967, both dating at approximately
2.4 million years ago. One view is that Homo habilis/rudolfensis
evolved in Africa approximately 2 million years ago and quickly
expanded into Asia to become *Homo erectus/ergaster.
... ... *Homo ergaster: H. ergaster and *H. erectus are the two
immediate precursors of H. sapiens, with H. ergaster believed to
have originated in Africa and to have given rise to H. erectus in
Asia. But as with other hominid fossil groups, precise
evolutionary sequences and geographical loci continue to be
debated.
... ... *Homo erectus: First discovered by Eugene Dubois in 1891
in Indonesia, this fossil group is currently viewed as the
closest precursor to H. sapiens. Formerly called "Anthropithecus
erectus" and "Pithecanthropus erectus". Pithecanthropus erectus
and Sinanthropus erectus ("Peking man", discovered in 1927) were
in 1951 subsumed under the single category Homo erectus, which
was then recognized as a widespread species exhibiting
significant geographical variation.
... ... *postcranial skeleton: In general, this refers to the
skeleton behind the cranium in a quadruped and below the cranium
in a biped.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 9Jul99
For more information: http://scienceweek.com/swfr.htm

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2. MOLECULAR BIOLOGY:
ON NUTRITIONAL STATUS AND GENE ACTIVATION IN BACTERIA
     All living organisms require nutrients, and the behavior of
organisms is directly dependent on the availability of nutrients
in their local environments. This dependence has been intensively
studied in bacteria, organisms that can be maintained in large
populations in the laboratory, in experiments involving careful
manipulation of environmental nutrients.
     When abundant nutrients are suddenly provided in the
environment of bacteria, the population enters an "exponential
growth phase" involving binary fission. Then, as one or more
nutrients are exhausted, or as end products of metabolism rise to
inhibitory levels, cell division ceases and the population enters
a relatively constant "stationary phase". After a time without
sufficient nutrients, bacterial cells may begin to fall apart
(lyse) in a so-called "death phase". Some species of bacteria,
however, are capable of producing spores (endospores), dormant
entities that carry the genome and that are capable of
transforming into a new bacterial cell when the supply of ambient
nutrients is sufficient.
     Spores survive boiling, which is the reason laboratory and
clinical media and objects must be sterilized at high
temperatures (220 degC). When canning foods, it is also necessary
to achieve this level of heat to eliminate dangerous spore-
forming bacteria such as Clostridium botulinum, the cause of
botulinism. Spores can survive for years and can be stored dry.
     In the process of bacterial sporulation, the chromosome is
duplicated and the copies separated: only one copy of the
chromosome will end up in the spore. The spore is created inside
the bacterium, which then lyses and releases the spore, and such
spores are thus called "endospores". (Fungi produce spores
externally [exospores]).
     The bacterial spore consists of the normal contents of a
bacterial cell stabilized with a high concentration of calcium
dipicolinate and covered with layers of densely packed
peptidoglycan and protein (the "spore coat") that can withstand
many environmental extremes such as high heat, drying, gamma
radiation, and ultraviolet light. Bacterial spores are small,
dense, and metabolically inactive. The process of sporulation has
been well studied, but little is known concerning the reverse
process -- the transformation of a bacterial spore into a
metabolically active bacterium.
     Most bacteria can be classified into two types, depending on
the chemistry of their outer coat, which chemistry determines
whether a bacterium will admit certain dyes into the interior.
The classification, according to the differential staining
technique, is "gram-negative" vs. "gram-positive", named after
the bacteriologist H.C. Gram (1853-1938). Gram-positive bacteria
take up a crystal violet stain and turn purple, while gram-
negative bacteria exclude the crystal violet and counterstain
instead with stains such as safranin, eosin red, or brilliant
green. As might be expected, since the technique differentiates
the outer coats of bacteria, some antibiotics are effective
against one type and not the other type, and vice versa.
... ... J. Dworkin and R. Losick (Harvard University, US) present
a commentary on some new research (M. Ratnayake-Lecamwasam et al:
Genes & Development 2001 15:1093) on bacterial gene activation
and nutrient availability, the authors (Dworkin and Losick)
making the following points:
     1) The authors (Dworkin and Losick) point out that when a
biological cell is deprived of nutrients, it is difficult to
pinpoint the precise cue among the myriad alterations in
metabolic intermediates that is responsible for the ensuing
adaptive response. It is also difficult to relate this cue to the
molecular mechanisms that execute the resulting changes in gene
expression. In some cases, such as the response to growth-
limiting levels of an amino acid or of a particular carbon
source, the specific nutrient is the signal to which the cell
responds, and the mechanisms by which the cell reacts to this
signal and adapts to it are well understood. In other cases,
however, in which the general nutritional status of the cell has
been perturbed, there has been little success in linking nutrient
availability to alterations in gene expression.
     2) The authors (Dworkin and Losick) point out that the gram-
positive bacterium Bacillus subtilis displays a wide range of
adaptations to nutrient limitation, including the secretion of
enzymes capable of degrading complex carbon sources, the
production and excretion of antibiotics to ward off competing
bacteria, the import and utilization of secondary metabolites,
entry into a state allowing morphological or biosynthetic
adaptation (a state of "genetic competence"), and the elaboration
of systems for motility and chemotaxis. As a more extreme
response to nutritional limitation, B. subtilis undergoes a
profound physiological and morphological transformation that
culminates in the formation of a dormant cell type, the
endospore.
     3) Two important questions are: a) What is the specific
nature of the physiological signal or signals that triggers these
adaptive responses? and b) Precisely how does the cell respond to
this signal(s)? An important clue came from the work of E.B.
Freese et al (1979), which demonstrated that the expression of
genes involved in adaptation to nutrient limitation and
sporulation was correlated closely with, and probably caused by,
a transient but significant decrease in the cellular pools of
guanosine diphosphate (GDP) or guanosine triphosphate (GTP), and
not other purine or pyrimidine nucleotides. Freese et al also
showed that sporulation could be induced under conditions of
nutrient excess by treating cells with decoyinine, an inhibitor
of guanosine 5'-monophosphate (GMP).
     4) Now M. Ratnayake-Lecamwasam et al (Genes & Development
2001 15:1093) report that a highly conserved specific protein
(CodY) in gram-positive bacteria regulates the expression of many
B. subtilis genes which are induced as cells make the transition
from rapid exponential growth to stationary phase and
sporulation. This protein is apparently a novel guanosine
triphosphate-binding molecule that responds to the intracellular
GTP concentration as an indicator of nutritional conditions, the
protein then regulating the transcription of early-stationary-
phase and sporulation genes and thus allowing the cell to adapt
to nutrient limitation.
-----------
J. Dworkin and R. Losick: Linking nutritional status to gene
activation and development.
(Genes & Development 1 May 01 15:1051)
QY: Jonathan Dworkin: dworkin2@fas.harvard.edu
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 1Jun01
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
PALEOBIOLOGY: CULTIVATION OF A 250-MILLION YEAR OLD BACTERIUM
     The term "bacillus" (plural: bacilli) is derived from the
Latin word _bacillus_, literally meaning "a stick". Bacilli are
rod-shaped bacteria that come in a variety of forms, some
relatively short and fat, others long and slender. Often the end
of the cell is swollen to accommodate its spore (endospore). Only
certain bacterial species (including bacilli) form spores, which
are specialized cell structures that may allow survival in
extreme environments. In general, under conditions of nutritional
depletion, each bacterium forms a single internal spore that is
liberated when the mother cell undergoes destruction (autolysis).
The spore is a resting system, highly resistant to desiccation,
heat, and chemical agents. In general, bacterial spores have a
thick protective protein coat, and the cytoplasm is partially
dehydrated and mineralized, causing enzymes to become inactive,
and the DNA in the spore is stabilized. When returned to
favorable nutritional conditions and activated, the spore
germinates to produce a single bacterium.
     There have been claims of cultivatable spores found in
various ancient materials, including rocks millions of years old,
but for most of these claims there has been considerable doubt
about the origin of the cultivated bacteria. In 1995, however,
R.J. Cano and M. Barucki reported the revival and identification
of bacterial spores (bacillus species) in 25 to 40 million year
old amber, the spores obtained from a bee preserved in the amber.
Because of the care taken to avoid contamination of the sample,
this finding is currently accepted as valid.
     The term "halophilic bacteria" (halobacteria) refers to
bacteria that thrive in environments of high salt concentration.
Despite its name, the Dead Sea, which has a salt content of 28
percent, is very much alive, with various species of algae and
bacteria tolerating the extreme salinity, a salinity that would
be lethal for most other kinds of cells. The main inhabitants of
the salty brine of the Dead Sea are a special group of
halobacteria (halophilic *archaea) which have adapted to the Dead
Sea environment so thoroughly that they cannot live at salt
concentrations below 15 percent.
     Some researchers reserve the term "halophilic" for the
Archaea, and use the term "halotolerant" for all other bacteria
that can thrive in high salt concentrations. Non-archaean
bacteria (i.e., most bacteria on Earth) are often called
"eubacteria". Together, the archaea and eubacteria make up the
prokaryotes -- single-celled organisms without intracellular
membrane-bound organelles such as a nucleus.
... ... R.H. Vreeland et al (West Chester University, US) report
the isolation and growth of a previously unrecognized spore-
forming bacterium (Bacillus species, designated 2-9-3) from a
brine inclusion within a 250-million-year-old salt crystal from
the *Permian Salado Formation. Delicate crystal structures and
sedimentary features indicate the salt has not recrystallized
since formation. The authors state samples were rejected if brine
inclusions showed physical signs of possible contamination, and
surfaces of salt crystal samples were sterilized with strong
alkali and acid before extracting brines from inclusions. The
authors suggest their sterilization procedures reduce the
probability of contamination to less than 1 in 10^(9). The
authors call their discovered spore-forming Bacillus species
"halotolerant"; halophilic archaea are not known to form spores.
... ... In a commentary on this work, R. John Parkes (University
of Bristol, UK) states: "Given the care that Vreeland et al took
to avoid contamination, their results are the best evidence yet
for the extremely long-term survival of microorganisms. The
potential implications are profound. For instance, can spores
effectively be immortal? What is the biochemistry that allows
them to survive for so long? Where else on Earth, and to what
depths, might ancient bacterial life be lurking? And given this
startling example of apparent bacterial durability, do spores in
rocks even provide a mechanism for life to be transported between
planets by "*panspermia", as has been proposed?"
-----------
R.H. Vreeland et al: Isolation of a 250 million-year-old
halotolerant bacterium from a primary salt crystal.
(Nature 19 Oct 00 407:897)
QY: Russell H. Vreeland: rvreeland@wcupa.edu
-----------
R. John Parkes: A case of bacterial immortality?
(Nature 19 Oct 00 407:844)
QY: R. John Parkes: j.parkes@bristol.ac.uk
-----------
Text Notes:
... ... *archaea: The archaebacteria (also called the Archaea)
are considered to be ancient compared to other kingdoms, and
possibly the most ancient life forms and the ancestors of all
eukaryotes (see Eukarya note above). They typically exist in
extreme environments, and include the methane-producing bacteria
(methanogens), the "salt-loving" bacteria (halophilic bacteria),
and the sulfur-acid tolerant thermoacidophilic bacteria. There is
presently a controversy concerning whether the Archaea should be
classified as a kingdom separate from the Bacteria.
... ... *Permian Salado Formation: The Salado Formation is
located in southeast New Mexico (US), and it contains rock salt
and potash salts. The formation has been dated as Permian, i.e.,
in the time-frame 290 to 245 million years ago.
... ... *panspermia: In general, the term "panspermia" is the
name given to the idea that life was introduced on Earth from
elsewhere in the Universe. This is a classical notion, fostered
by the chemist Svante A. Arrhenius (1859-1927) in the early part
of the 20th century.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 24Nov00
For more information: http://scienceweek.com/swfr.htm

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3. MEDICAL BIOLOGY:
FIRST CLINICAL TOTAL TRANSPLANT OF THE HUMAN LARYNX
     The term "pharynx" refers to the throat, a tubular
passageway attached the base of the skull and extending downward
to the esophagus. The term "larynx" refers to the "voice box" a
structure that serves as a passageway for air between the pharynx
and the windpipe (trachea). The larynx lies in the middle of the
neck, and its unique structure enables it to function as a valve
guarding the entrance to the trachea and as the producer of sound
carrier waves for the voice. The epiglottis, at the upper part of
the larynx, is a flap-like projection into the throat, and as
food is swallowed, the entire larynx structure rises to the
epiglottis so that the passageway to the respiratory tract is
blocked. After food passes into the esophagus, the larynx relaxes
and resumes its natural position. In humans, the base of the
larynx is surrounded by the paired thyroid glands; the
parathyroid glands are small bodies situated on the outer
surfaces of the thyroid glands.
     Exhalation of air through the larynx is controlled by
voluntary muscles, and this enables the larynx to serve as the
organ of voice. The central portion of the larynx is reduced to
slit-like openings at two sites, both sites representing large
folds in the mucous membrane lining the larynx. There are two
paired folds: "false vocal cords" (ventricular folds) that do not
function in the production of sound, and "true vocal cords"
(vocal folds) that do function in the production of sound.
Muscles attached directly and indirectly to the vocal cords
permit the opening and closing of the folds. Essentially, speech
is normally produced when air expelled from the lungs moves up
the trachea and strikes the underside of the vocal cords,
producing vibrations as the air passes through the cords. Raw
sound emerges from the larynx and passes to the upper cavities,
which act as resonating chambers (or as sound modulators in some
languages), and then sound passes through the mouth for
articulation into speech by the tongue, teeth, hard and soft
palates, and lips. If the larynx is removed or destroyed, the
esophagus can function as the source for sound (esophageal
speech), but the control of pitch and volume is essentially lost.
An alternative substitute is an "electronic larynx", which in
general consists of an external electronic sound source applied
to the throat to allow the sound to pass into the pharynx and
mouth for modulation and articulation.
     This report concerns the first total transplantation of the
human larynx.
... ... M. Strome et al (9 authors at Cleveland Clinic
Foundation, US) report the first human total laryngeal
transplant, the authors making the following points:
     1) The authors report that in 1987 they initiated a program
to explore the potential of laryngeal transplantation, the
program addressing four issues critical to successful
transplantation: revascularization, reinnervation, tissue
rejection, and the ethics of transplanting an organ considered by
some people to be nonvital. In rats, the authors found the rate
of success of laryngeal transplantation was nearly 100 percent.
In their animal studies, the authors classified the histologic
features of laryngeal rejection, determined the maximum tolerable
period of absence of tissue blood supply (period of ischemia),
evaluated preservative solutions, and determined doses of various
drugs necessary or helpful in a transplantation procedure.
     2) In 1998, the authors performed a total laryngeal
transplantation in a man who had sustained a severe traumatic
injury to the larynx and pharynx. The patient was a 40-year-old
man who had been in a motorcycle accident 20 years earlier. His
larynx and pharynx had been crushed, leaving him with a loss of
voice (aphonia). A donor larynx was found after a 6-month search,
the donor a 40-year-old man who had died from a ruptured cerebral
artery (a cerebral aneurysm) but who had no coexisting illnesses
and who had not smoked. In addition, appropriate serological
tests determined that rejection of tissue from this donor by the
host patient would be minimal. The entire pharyngolaryngeal
complex, including six tracheal rings and the thyroid and
parathyroid glands, was removed and stored in solution and ice
until transplantation of all of this complex 10 hours later.
     3) The authors report the following results of this
transplantation procedure: On the 3rd postoperative day, the
patient uttered his first laryngeal speech in 20 years (the word
"hello"). At one month, both vocal folds had assumed positions to
create a "breathy" voice, and subsequent electromyography
confirmed that both vocal folds and neighboring muscles had been
reinnervated. The patient's voice improved steadily and became
stable at normal or near-normal levels 16 months after
transplantation. Voice measures included pitch, vocal quality
("jitter"), intensity, maximal phonation time, respiratory
volume, and airflow. At 16 months, the pitch of the patient's
voice changed, increasing its natural quality, as confirmed by a
trained listener.  At 36 months, the characteristics of the
speech of the patient were all within the normal range. His voice
sounded normal, and it served all his verbal communication needs.
Unemployed before the transplantation, the patient has since
become a motivational speaker.
-----------
M. Strome et al: Laryngeal transplantation and 40-month follow-
up.
(New England J. Med. 31 May 01 344:1676)
QY: Marshall Strome: Dept. of Otolaryngology, Cleveland Clinic
Foundation, 9500 Euclid Avenue, Cleveland, OH 44195 (US).
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 1Jun01
For more information: http://scienceweek.com/swfr.htm

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4. GEOPHYSICS: ON THE CLIMATE OF EARLY EARTH
     Any hypothesis concerning the origin of life on Earth must
be subject to the constraints of the climate of early Earth, and
these constraints are best derived from geophysical analysis.
Certainly, geophysical analyses of the climate of early Earth
always involve assumptions that may later prove to be untenable, 
but the incorporation of such analyses into origin-of-life
formulations is both valuable and necessary.
     In general, bacteria have adapted to a wide range of
temperatures, with the range of temperature over which optimal
growth can occur in any one species spanning approximately 20
degC; the range over which any growth at all takes place usually
spans approximately 40 to 50 degC.
     The term "mesophilic bacteria" refers to those bacteria in
which optimum growth occurs between 20 and 45 degC; such bacteria
can usually grow in or survive temperatures between 10 and 50
degC, and all animal pathogens are in this group.
     So-called "thermophilic bacteria" are the only organisms
that can grow at temperatures higher than 60 degC. Such
temperatures are encountered in rotting compost piles, hot
springs, and oceanic geothermal vents. In the runoff of a hot
spring, various thermophiles are found near the source where the
temperature has fallen to approximately 70 degC. An example is
the species Thermus aquaticus, which has an optimum temperature
for growth of 70 degC, and a maximum temperature for growth of 79
degC.
     In the mid-1980s, researchers discovered bacteria in
nutrient-rich, extremely hot hydrothermal vents in the deep sea
floor. For example, the bacteria in the genus Pyrodictium thrive
in the temperature range 80 to 110 degC, temperatures at which
the water remains liquid only because of the extremely high
pressure.
     In this context, the term "geothermal" refers to the escape
of heat from the interior of the Earth into the Earth's surface,
and the term "hydrothermal" refers to hot solutions rising from 
cooling molten rock (magma). "Hydrothermal vents" are hot springs
occurring in volcanic regions of the ocean floor.
... ... N.H. Sleep et al (3 authors at 2 installations, US)
present a report of an analysis of surface climate conditions on
the earliest Earth, the authors making the following points:
     1) The authors point out that the present Earth-Moon system
is generally believed to have formed in the aftermath of a
collision between two planets 4.45 to 4.5 billion years ago. If
the gravitational potential and kinetic energy of the impact all
were converted into heat, the energy would have been sufficient 
to vaporize much of the Earth and exterminate any life present on
either body. In practice, vaporization and even melting are
expected to be incomplete, so that some rocks would survive the
initial impact and some of the survivors enter into orbit around
the Sun. In principle, life also might survive in such rocks to
be returned later once Earth became habitable. The authors
suggest that it is obvious that an initially hot Earth implies
that the surface of the Earth passed through those conditions
associated with modern thermophilic microbes (approximately 100
degC) before becoming milder. The authors suggest this is
interesting because the proposed thermophilic root of a tree of
life is compatible with the origin of life while the surface was
near 100 degC.
     2) The authors focus on the hypothesis that Earth was hot
everywhere when life began ("hot Earth scenario"). Alternative
explanations for such a thermophilic origin of life are: a)
origin of life within hydrothermal systems; b) extinction of all
early mesophiles by large asteroid impacts at later times. In the
hot Earth scenario, the time scale and mechanism by which the
Earth reached clement conditions and the time interval over which
surface temperatures lingered near 100 degC are important
parameters. Also relevant is the gross chemistry of seawater at
that time. The authors suggest it is convenient to divide the
time after the Moon-forming impact into two epochs distinguished
by different controlling physics. In the first epoch, surface
temperatures were directly maintained by heat vented from the
interior of the Earth. In the second epoch, surface temperatures
were maintained by a solar-heated *greenhouse as on the present
Earth. The authors consider both possibilities and find that the
implications of both histories for biology are similar.
     3) The authors report they investigated the climate on
earliest Earth to see how long temperatures of approximately 100
degC persisted as the surface and interior cooled after the Moon-
forming impact. Only gross constraints such as conservation of
energy in the planetary interior, chemical equilibrium between
volatiles and rock, and heat transfer through a massive carbon
dioxide atmosphere were considered in the analysis "to obtain the
essence of the result without obscuration by detail." The authors
report that the time of 100 degC temperature was geologically
brief but not instantaneous. The available internal heat within
the Earth could maintain approximately 100 degC surface
conditions for at most a few million years under contrived
circumstances and more likely for much less than a million years.
The lifetime of a massive carbon dioxide greenhouse would also be
determined by internal conditions in the planet, mainly the
relative efficiencies with which carbon dioxide was vented to the
surface and with which it was subducted into the deep interior.
The authors point out that carbon dioxide reacts with *basalt to
form carbonates under reasonable greenhouse conditions, and the
greenhouse climate would gradually evolve to lower temperatures
as the deep interior cooled and made the deep interior a more
effective carbon dioxide reservoir. Surface temperatures of
approximately 100 degC might have been maintained in this way for
at most 20 million years, with a best estimate on the order of 1
million years. Rock-water reactions within the sea-floor would
maintain Earth-like ocean chemistry.
     4) The authors conclude: "The biological implications of a
brief but finite period with a surface near 100 degC are evident.
Thermophilic life might have originated during that epoch, but a
climate in the range preferred by thermophile organisms did not
exist for most of the early history of the Earth. This leaves
other hypotheses on the table for a tree of life rooting in a
thermophilic ancestor, as hydrothermal environments have existed
throughout the rest of geologic time. For example, subsurface
high-temperature organisms are the likely survivors of an ocean-
boiling asteroid impact that otherwise sterilized the planet."
-----------
N.H. Sleep et al: Initiation of clement surface conditions on the
earliest Earth.
(Proc. Natl. Acad. Sci. US 27 Mar 01 98:3666)
QY: N.H. Sleep: norm@pangea.stanford.edu
-----------
Text Notes:
... ... *greenhouse: The physical basis of the so-called
"greenhouse effect" is as follows: carbon dioxide gas is
transparent to visible light but relatively opaque to infrared
radiation. The same is true of glass. Relatively high-energy
visible light radiation from the sun passes inward through the
atmosphere, warms the surface of the Earth, which then radiates
lower energy in the form of infrared radiation (heat) back to the
atmosphere. But if the atmosphere has a concentration of infrared
impenetrable gases such as carbon dioxide, the infrared radiation
cannot pass out, and the surface of the Earth underlying the
atmosphere cannot cool, and the surface of the Earth thus will
continue to grow hotter.
... ... *basalt: Igneous rocks are rocks that have congealed from
a molten mass. Basalt is a dark gray to black igneous rock of
volcanic origin that cools rapidly. It is found as basement
rock on land, and on sea floor spreading from mid-ocean ridges.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 1Jun01
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ORIGIN OF LIFE: THE HABITAT AND NATURE OF EARLY LIFE
Central to many scientific and philosophical problems, and to any
consideration of extraterrestrial life, is the question of the
origin of life on Earth. Of particular significance is our
understanding of the early history of Earth and of the time
during which life apparently first appeared on the planet.
... ... E.G. Nisbet and N.H. Sleep (2 installations, UK US)
present a review of current research on the habitat and nature of
early life on Earth, the authors making the following points:
     1) The current consensus view is that the Solar System began
after one or more local *supernova explosions approximately 4.6
billion years ago. In one generally accepted scenario of the
later stages of accretion of the Solar System, it is proposed
that there were approximately 500 *planetesimals, bodies
approximately the size of the Moon, in the region now occupied by
Mercury, Venus, Earth, and Mars (the "inner planets"). Venus,
Earth, and Mars all received during their formation water vapor
and carbon, perhaps with early oceans on all three. But other
models are also possible. The fate of the volatiles associated
with each planet was completely different: a) Venus is dry, with
a surface now at approximately 500 degC under 90 *bar of carbon
dioxide. b) Mars is in *permafrost. c) Earth has approximately
the same external inventory of carbon dioxide as Venus, and both
planets radiate heat to space at very similar "effective"
temperatures, but for Earth most of the carbon dioxide is in the
form of carbonate minerals (e.g., limestone). The carbon dioxide
blanket on Earth is less dense, and so the oceans can exist. 
     2) Water is a strong *greenhouse gas, and at some stages
early in the history of Venus and Earth water vapor was probably
present high in the atmosphere. Such water vapor would have been
*photolyzed into hydrogen and oxygen, and the hydrogen present in
the upper atmosphere would have been lost rapidly to space.
Deuterium would have been lost also, but being more massive,
would have been lost more slowly. In comparison to the deuterium
content that is thought to have been originally in the planetary
mix, the atmosphere of Venus has a strong deuterium enrichment,
and the simplest explanation is that Venus lost its water early
in its history when a runaway greenhouse developed. In this
model, Venus initially had oceans and a warm surface of
approximately 75 degC. Water was partitioned into the high
atmosphere, there photolyzed, hydrogen was lost and the planet
dehydrated and left more oxidized. Alternately, if Venus has or
had a molten *magma ocean in its *mantle, it may there too have
sequestered deuterium-poor hydrogen ("light hydrogen") as OH.
Mantle minerals are typically light, or depleted in deuterium
relative to sea water.
     3) The authors summarize their view of the early history of
Earth: Our planet is more than 4.5 billion years old. Massive
bombardment of the planet occurred in the first 500 to 700
million years, and the largest impacts would have been capable of
sterilizing the planet. Probably until 4 billion years ago or
later, occasional impacts might have heated the ocean over 100
degC. Life on Earth dates from before approximately 3.8 billion
years ago, and is likely to have experienced one or more
hot-ocean bottlenecks. Only organisms optimally living in water
at 80 to 110 degC (hyperthermophile organisms) would have
survived. It is possible that early life diversified near
*hydrothermal vents, but hypotheses that life first occupied
other pre-bottleneck habitats are tenable (including transfer
from Mars on ejecta from impacts there). Early hyperthermophile
life, probably near hydrothermal systems, may have been
*non-photosynthetic, and many *housekeeping proteins and
biochemical processes may have an original hydrothermal heritage.
The development of anoxygenic and then oxygenic photosynthesis
would have allowed life to escape the hydrothermal setting. By
approximately 3.5 million years ago, most of the principal
biochemical pathways that sustain the modern biosphere had
evolved, and were global in scope.
-----------
E.G. Nisbet and N.H. Sleep: The habitat and nature of early life.
(Nature 22 Feb 01 409:1083)
QY: E.G. Nisbet: Department of Geology, University of London, UK.
-----------
Text Notes:
... ... *supernova: A violent explosion in which certain stars
end their lives. The star may become more than 10^(9) times as
bright as the Sun and may outshine its host galaxy for weeks.
... ... *planetesimals: Planetesimals are bodies with dimensions
of 10^(-3) to 10^(3) meters that are believed to form planets by
a process of accretion. The term "accretion" refers to an
aggregation, an increase in the mass of a body by the addition of
smaller bodies that collide and adhere to it, provided the
relative velocities are low enough for coalescence.
... ... *bar: 1 bar equals 0.99 atmosphere.
... ... *permafrost: In general, permanently frozen ground.
... ... *greenhouse gas: See main report.
... ... *photolyzed: In general, any chemical reaction produced
by exposure to light or ultraviolet radiation.
... ... *magma: In general, any molten mass of rock.
... ... *mantle: Seismic studies indicate the interior of the
Earth consists of three parts: a metallic core, a dense rocky
mantle, and a thin low-density crust. In this context, the
assumption is that Venus, like Earth, has a mantle layer between
its core and its crust.
... ... *hydrothermal vents: See main report.
... ... *non-photosynthetic: In general, photosynthesis is the
utilization of light energy to power biosynthesis.
... ... *housekeeping proteins: So-called "housekeeping" proteins
are proteins involved in essential functions such as metabolic
cycles.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 16Mar01
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ON THE ORIGIN OF THE EARTH AND THE MOON
Two of the central questions in planetary and Earth science
concern the origin of the Earth and Moon. How did these two
bodies form and what forces defined their basic physical
structures? ... ... A.N. Halliday and M.J. Drake (2
installations, CH US) present a short review of current research
in this area, the authors making the following points:
     1) Advances in this field have come mainly with progress in
simulating the dynamics of planetary accretion, in measuring
isotopes that act as chronometers for early Solar System
processes, in analysis of noble gas isotopes that yield clues
about the early atmosphere, and in melting experiments at
previously unattainable pressures and temperatures. Although a
general picture may be emerging, many issues remain hotly
debated.
     2) Planet formation is believed to begin with sticking and
frictional coagulation of dust particles in a gaseous nebula that
persists in the *circumstellar disk. The particles grow in size
until there is substantial gravitational attraction between
kilometer-sized bodies, and these coalesce further. Major
collisions between small proto-planets eventually result in
objects the size of Earth.
     3) The energy of late-stage planet-building impacts would be
colossal, sufficient to melt the entire planet. *Magma oceans
would be formed, and some volatile elements would escape into
space. The most widely accepted theory for the origin of the Moon
is that it coalesced from a ring of debris produced by such a
late-stage collision between two Earth-forming proto-planets.
This "Giant Impact Theory", established over a decade ago,
explains the rotational speed of the Earth-Moon system, a
critical feature that must be reproduced by any satisfactory
model. But in spite of a growing consensus, some researchers are
still opposed to the Giant Impact Theory on both dynamical and
geochemical grounds.
     4) All isotopic data are consistent with Earth being fully
formed within 50 to 100 million years after the start of the
Solar System. The isotopic record from Moon rocks is consistent
with the formation of the Moon at about the same time.
     5) The authors conclude: "We have recently come a long way
in obtaining hard constraints on the origin of Earth and the
Moon. The issues have changed from discussion of whether or not
there was a giant Moon-forming impact to debate about the
accretion rates of the Earth and the chemical, isotopic, and
physical effects of such catastrophic accretionary scenarios."
... ... In a contiguous short review of the same research area,
Frank A. Podosek (Washington University St. Louis, US) makes the
following points:
     1) The age of the Solar System as a whole is easier to
determine than the age of Earth. The age of the Solar System is
reliably inferred from the age of *refractory element-rich
inclusions in meteorites to be approximately 4.57 billion years,
thus providing an upper limit to the age of Earth. These
inclusions are the oldest known objects in the Solar System, and
their content indicates that the Solar System did not exist for
more than approximately 1 million years before the inclusions
formed.
     2) In contrast to these ancient extraterrestrial objects,
there are no known terrestrial rocks or minerals whose formation
essentially coincides with the formation of Earth, and therefore
the age of Earth must be inferred indirectly. Several independent
approaches indicate that Earth formed approximately 100 million
years later than the Solar System as a whole.
     3) All the various isotopic chronometers are intrinsically
capable of considerably higher precision, but this precision
cannot yet be realized. It is not even clear whether the
chronometers are consistent or in conflict with each other. All
methods rely on models of varying complexity involving
assumptions difficult to verify and parameters difficult to
measure.
     4) The author concludes: "For testing the giant impact
scenario in particular, it would be useful to have a quantitative
theory for whether a preexisting atmosphere is lost in the
impact, whether preexisting planetary structures (*core, mantle,
and crust) are re-equilibrated after such an impact, and how much
of the Moon comes from the impactor and how much comes from the
target."
-----------
A.N. Halliday and M.J. Drake: Colliding theories.
(Science 19 Mar 99 283:1861)
QY: A.N. Halliday: halliday@erdw.ethz.ch
-----------
Frank A. Podosek: A couple of uncertain age.
(Science 19 Mar 99 283:1863)
QY: Frank A. Podosek: fap@levee.wustl.edu
-----------
Text Notes:
... ... *circumstellar disk: One of the important discoveries of
the 1980s was the existence of circumstellar disks of dust around
some stars, the disks apparently replenished by unseen parent
bodies such as comets and asteroids.
... ... *Magma: In general, any mass of molten rock.
... ... *refractory: Refractory materials are materials resistant
to decomposition by heat, pressure, or chemical attack. The term
is most commonly applied to heat resistance.
... ... *core, mantle, and crust: Seismic studies indicate the
interior of the Earth consists of three parts: a metallic core, a
dense rocky mantle, and a thin low-density crust. The central
part of the core is solid, but the outer part of the core is
evidently liquid.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 4Jun99
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
AGE AND ORIGIN OF EARTH'S MOON
The most widely accepted theory for the origin of the Earth's
moon is that during the late stages of the Earth's accretion an
impact with another planet at least the size of Mars occurred,
and the impact generated both the hot debris that formed the moon
and the angular momentum of the Earth-moon system. In geology,
the mantle of a planet or moon is the layer that lies between the
crust and the core. Chondrites are a type of stony meteorite
consisting of an agglomeration of millimeter-sized globules
(chondrules) that are thought to be unchanged since the original
condensation out of the nebula from which the sun and solar
system formed, and "chondritic" is the term used to describe a
rock composition similar to that of chondrites, which implies an
age of 4.2 to 4.5 billion years. The term "radiogenic", on the
other hand, is used to describe a rock composition apparently
resulting from varying isotope decays, and the oldest radiogenic
compositions on Earth have been dated at 3.6 to 3.8 billion
years. A hafnium-tungsten chronometer is not an actual instrument
but a method of radiometric age determination using the isotope
ratios of the elements hafnium and tungsten. Hafnium is
lithophilic (silicate-loving), which means it tends to associate
with chondritic materials, while tungsten is siderophilic (metal-
loving), which means it tends to associate with metal cores, and
using these differing affinities of these elements, one can
attempt a construction of the age and origin of the moon by
analysis of moon rock samples and comparisons with Earth rocks.
Lee et al (4 authors at 2 installations, US) report a study of
the age and origin of the moon with the hafnium-tungsten
chronometric method. The tungsten isotopic compositions of 21
lunar samples were found to range from chondritic to slightly
radiogenic. The authors suggest this heterogeneity is probably
the result of late radioactive decay within the moon itself, and
that the moon formed 4.52 to 4.50 billion years ago and its
mantle has since remained poorly mixed.
QY: Der-Chuen Lee: dclee@umich.edu
(Science 7 Nov 97) (Science-Week 28 Nov 97)
-------------------
Related Background:
THE ORIGIN OF EARTH'S MOON
The large impact hypothesis of the origin of the Earth's moon is
the current consensus view. The essential idea is that the moon
formed from debris ejected into a disk around Earth by the impact
of a large body. A version of this is that Earth and its moon
were created more or less simultaneously by the collision of two
large planetesimals, the resultant large body becoming Earth, and
the ejected debris formed the moon. What is accepted by nearly
everyone is that an accretion disk of debris was the first stage
of the moon's formation. Shigeru Ida et al (Tokyo Institute of
Technology, JP; University of Colorado Boulder, US) have
evidently now provided the most detailed simulation calculations
of lunar growth in an impact-generated accretion disk. Using
direct N-body simulations, they show that a single dominant moon
can grow from such a disk within a year, but to satisfy the
present angular momentum and mass constraints on the analysis,
the impacting body must have been at least twice as massive as
Mars, and had to provide the resultant system with a few times
more angular momentum than it now possesses. There is presently
no explanation for the subsequent loss of angular momentum, and
the required massive size of the impacting object is also
puzzling. Although this is apparently the best set of simulation
calculations to date, the authors emphasize that further
simulation modeling is needed [*Note #1].
QY: S. Ida: ida@geo.titech.ac.jp
(Nature 25 Sep) (Science-Week 10 Oct 97)
-----------
Text Notes:
... ... *Note #1: Accretion is considered an important factor in
the evolution of stars, planets, and comets. The essential idea
is the coalescence of small particles in space as a result of
collisions, and the gradual formation of larger bodies from
smaller ones as a result of gravitational attraction. An
accretion disk is a disk of gas or particles in orbit around an
object, the disk formed by inflowing matter. A simulation of the
sort mentioned in the report involves computational solutions of
the dynamical equations for the history of a chosen mass of
particulate matter initially ejected from a larger body. By
solving the equations for the mathematical model, one can follow
the evolution of the accretion disk and the agglomeration that
forms the final orbiting satellite. The study mentioned here was
first presented at a meeting of the American Astronomical Society
in July, and here is part of the related SCIENCE-WEEK (1 Aug 97)
report: Until the 1980s, there were three extant theories, with
no data available to support or refute any of them. The Fission
Hypothesis proposed that the moon broke away from a rapidly
spinning proto-Earth after the proto-Earth's differentiation, the
moon forming from iron-poor crust. But the moon rocks in hand
have been found to differ chemically from those of Earth. Also,
if the proto-Earth had been spinning fast enough to break up, the
present Earth-moon system should contain a great deal more
angular momentum than is observed. The Fission Hypothesis
therefore had to be abandoned. The Condensation Hypothesis was
based on the idea that the Earth and the moon condensed
simultaneously from the same cloud of material in the solar
nebula. This hypothesis did not survive because analysis of moon
rocks has shown the Earth and the moon have greatly different
densities and compositions. The Capture Hypothesis proposed that
the moon was formed elsewhere in the solar system and later
"captured" by Earth. This hypothesis was always the least popular
because it required too many coincidental events. Thus, after the
mid-1980s, there was no satisfactory theory of the moon's origin.
During the past decade, a new idea gradually developed, the
Large-Impact Hypothesis, the idea of which is that the moon
formed from debris ejected into a disk around the Earth after a
major collision of the Earth with another large body about 4.5
billion years ago, the other body a planet perhaps as large as
Mars. The Large-Impact Hypothesis is at present the consensus
theory in planetary science. 
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 10Oct97
For more information: http://scienceweek.com/swfr.htm

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

5. ASTROPHYSICS: A BROWN DWARF RADIO STAR
     Brown dwarf stars are formed by the contraction of a lump of
gas with a mass too small (less than approximately 0.08 the mass
of the Sun) for nuclear reactions to begin in the core. Such a
star has a relatively short-lived luminosity (approximately 100
million years to several billion years) as the result of
conversion of gravitational energy to radiation. The surface
temperature of a brown dwarf is below 2500 kelvins. As recently
as 1994, brown dwarfs were "theoretical" stars, with no brown
dwarfs considered to be unambiguously identified. In the past few
years, however, a number of stars have been recognized as brown
dwarfs, and they are under intensive study.
     In this context, the term "gas giant" (giant planet) refers
to a planet of much larger mass and diameter than the Earth, and
which consists mostly of gas. In our own Solar System, Jupiter,
Saturn, Uranus, and Neptune are gas-giant planets. 
     A synchrotron is a device for accelerating electrons or
protons in closed orbits in which the frequency of the
accelerating voltage and the strength of an applied magnetic
field are simultaneously varied to keep the orbit radius
constant, and laboratory synchrotron radiation is electromagnetic
radiation generated by the acceleration of charged relativistic
particles in a synchrotron (or in any magnetic field). In
astronomy, the term "synchrotron emission" refers to
electromagnetic radiation emitted by charged particles moving in
a magnetic field at a velocity close to that of light, and
emissions from various types of astronomical objects are
apparently synchrotron emissions. In general, relativistic
electrons gyrating in a magnetic field emit radio waves at high
harmonics of the electron gyrofrequency, so emission of radio
waves can be an indication of synchrotron emission by
astronomical objects.
     The term "radio waves" refers to electromagnetic radiation
of wavelength longer than approximately 1 millimeter (30
gigahertz). The longest radio waves observable in astronomy have
a wavelength of approximately 30 meters. The shortest radio
wavelengths, from approximately 1 millimeter to 30 centimeters,
are known as "microwaves". In this context, the term "radio
emission" refers to radio wave emission from an astronomical
source. Astronomical radio sources produce either continuum
radiation or line radiation. Line radio radiation is emitted at
only one specific wavelength and is equivalent to an optical
spectral line. The most important of such lines is the 21-
centimeter line emitted by neutral hydrogen atoms. Of continuum
radio radiation there are two kinds: a) thermal radio radiation
is electromagnetic energy emitted by hot ionized interstellar
gases; b) non-thermal radio radiation is a result of a process of
synchrotron emission, the release of radiation by electrons
spiraling in magnetic fields at speeds near the speed of light.
     In this context, the term "x-ray emission" refers to x-rays
emitted from various astronomical sources. Most stars emit only
an extremely small fraction of their energy as x-rays, with young
massive stars the most powerful x-ray emitters. In general, gases
heated to temperatures above 10 million kelvins will emit x-rays.
     An "M star" is a star of spectral type "M", i.e., with a
very cool surface (below 3900 kelvins), appearing reddish in
color and emitting most of its radiation in the infrared. M-type
dwarf stars ("red dwarfs") have masses below 0.5 solar-masses and
potential lifetimes longer than the present age of the Universe.
     In this context, the term "corona" refers to the hot 
outer atmosphere of certain stars, where the temperature can be 2
million kelvins or more. The term "flare" refers to a sudden
release of corona energy.
... ... E. Berger et al (14 authors at 14 installations, US)
report the discovery of radio emission from a brown dwarf star,
the authors making the following points:
     1) The authors point out that brown dwarf stars are not
massive enough to sustain thermonuclear fusion of hydrogen at
their centers, but they are distinguished from gas-giant planets
by their ability to burn deuterium. Brown dwarf stars older than
approximately 10 million years are expected to have short-lived
magnetic fields and to emit only weak radio waves and x-rays from
their coronas. An x-ray flare was recently detected on the brown
dwarf star LP944-20.
     2) The authors report the discovery of both quiescent and
flaring radio emission from LP944-20, with luminosities several
orders of magnitude larger than that predicted by the empirical
relation between the x-ray and radio luminosities that has been
found for many types of stars. An analysis of the radio data
within the context of synchrotron emission indicates that the
brown dwarf star LP944-20 has an unusually weak magnetic field in
comparison to M-dwarf stars.
... ... In a commentary on the above work, Arnold O. Benz (ETH-
Zentrum Zurich, CH) makes the following points:
     1) The author (Benz) points out that this is the first time
that radio emission has been recorded from a bona fide brown
dwarf star. Although brown dwarf stars are too small to burn
hydrogen, they can fuse deuterium in their cores, provided they
are at least 12 times the mass of Jupiter. But this fusion of
deuterium is possible only when the brown dwarf star is young, in
the first 10 million years or so. The brown dwarf star LP944-20,
however, is apparently 500 million years old and should have no
deuterium source left in its core. This suggests that the radio
emission of LP944-20 may be synchrotron emission associated with
a hot corona. However, the apparently weak magnetic field of this
star makes this explanation for the radio emission tenuous. The
author (Benz) concludes: "The whistling brown dwarf is a new
mystery for astronomers to puzzle over, and may indicate that
these dusky little objects have more surprises in store."
-----------
E. Berger et al: Discovery of radio emission from the brown dwarf
LP944-20.
(Nature 15 Mar 01 410:338)
QY: E. Berger: ejb@astro.caltech.edu
-----------
Arnold O. Benz: Brown dwarf is a radio star.
(Nature 15 Mar 01 410:310)
QY: Arnold O. Benz: benz@astro.phys.ethz.ch
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 1Jun01
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ON BROWN DWARF STARS
Brown dwarf stars are formed by the contraction of a lump of gas
with a mass too small for nuclear reactions to begin in the core
[*Note #1]... The surface temperature of a brown dwarf is
estimated to range from below 2500 kelvins to less than 1000
kelvins...
... ... C.G. Tinney (Anglo-Australian Observatory Epping, AU)
presents a review of recent observations of brown dwarf stars,
the author making the following points: 1) Most stars spend most
of their lives in a state of pressure balance maintained between
gravitational contraction and the energy generated by nuclear
reactions. In 1963, Kumar suggested there may exist a class of
star-like bodies with masses too low to create the central
temperature and densities required to ignite nuclear fusion
reactions. These "failed stars" became known to astronomers as
"brown dwarfs". 2) The lowest-mass ordinary stars can
theoretically maintain a quasi-equilibrium luminosity for almost
6000 billion years. Brown dwarf stars, in contrast, are expected
to fade throughout their lifetime, cooling to temperatures below
1000 kelvins and becoming undetectable by direct observation
after just a few billion years. This has engendered considerable
interest in brown dwarf stars as possible candidates for the
*dark matter which apparently composes more than 90 percent of
the mass our Galaxy. 3) The past 4 years have seen success
finally achieved in the hunt for brown dwarf stars. These
detections have confirmed predictions that both methane and dust
play an important role in determining the spectral behavior of
these objects. But the detection of brown dwarf stars in
significant numbers, when combined with results for the space
density of low-mass stars and *gravitational microlensing
results, allows us to conclude that brown dwarf stars do not make
a significant contribution to the dark matter of our Galaxy. The
author concludes: "No matter how nicely brown dwarfs would solve
the *baryonic dark matter problem, it appears we must look
elsewhere for a solution to this long-standing astronomical
quandary."
-----------
C.G. Tinney: Brown Dwarfs: The stars that failed.
(Nature 7 Jan 99 397:37)
QY: C.G. Tinney, Anglo-Australian Observatory, PO Box 296, Epping
NSW 1710, AU.
-----------
Text Notes:
... ... *Note #1: Present theoretical models predict a lower
mass-limit for fusion burning stars with the same element mix as
the Sun of 0.07 solar-mass, equivalent to 74 times the mass of
Jupiter.
... ... *dark matter: In general, in this context, the term "dark
matter" refers to material whose presence can be inferred from
its effects on the motions of stars and galaxies, but which
cannot be seen directly because it emits little or no radiation.
It is believed that at least 90 percent of the mass in the
Universe exists as some form or dark matter.
... ... *gravitational microlensing: Gravitational lensing is the
bending of light and other radiation by a massive gravitational
entity such as a star, a black hole, a galaxy, or a cluster of
galaxies. The effect is predicted by Einstein's theory of
relativity and was first detected during a total solar eclipse by
Eddington in 1919. Large-scale gravitational lensing causes
multiple images of an object, the type and arrangement of the
images determined by the specifics of the lensing entity.
Gravitational "microlensing" is a small-scale lensing effect, the
gravitational field of the lensing object not strong enough to
form distinct images of the background source, but instead
causing an apparent brightening of the source. Stars are expected
to vary in brightness in a characteristic manner if low-mass
stars pass in front of them.
... ... *baryonic dark matter: Ordinary matter too dim to be
observed. A baryon is a nuclear particle, e.g., a proton, built
from 3 quarks (fundamental particles that combine to make up
protons, neutrons, and mesons).
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 19Mar99
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
THE DUSTY ATMOSPHERE OF A BROWN DWARF STAR
... C.A. Griffith et al (3 authors at 3 installations, US) now
report observations of the brown dwarf star Gliese 229B, which
exhibits certain unique characteristics. At 900 kelvins, the
atmosphere of this object is too warm to contain ice clouds
like those on Jupiter and too cool to contain silicate clouds
like those on low-mass stars. These unique conditions (high
gravity and the lack of high clouds) permit spectroscopic
visibility of the atmosphere down to higher pressures (i.e.,
closer to the surface) than possible in cool stars or planets.
The authors investigated the structure of the atmosphere of
Gliese 229B by analyzing its optical spectrum in the interval
0.85 to 1.0 micron, the spectrum obtained at the *Keck 1
telescope. The authors report that the spectrum of Gliese 229B
indicates deep-atmosphere particulate matter with the optical
properties of neither ice nor silicates. The authors suggest the
reddish color of the particles indicates an organic composition
characteristic of aerosols in planetary stratospheres, and that
the *mass fraction of the particles agrees with a photochemical
origin involving incident radiation from its companion primary
star (Gliese 229A).
-----------
C.A. Griffith et al: The dusty atmosphere of the brown dwarf
Gliese 229B.
(Science 11 Dec 98 282:2063)
QY: Caitlin A. Griffith, Northern Arizona University 520-523-5511
-----------
Text Notes:
... ... *Keck 1 telescope: The Keck telescopes are a pair of twin
telescopes at the W. M. Keck Observatory on Mauna Kea, HI US,
each with a 10 meter mirror, the pair constructed 1992-1996. The
installation is managed by the University of California (US) and
the California Institute of Technology (US).
... ... *mass fraction: The mass fraction of aerosols is related
to the *eddy diffusion coefficient k, the mass density of the
atmosphere d, the net mass flux f, and the scale height of the
atmosphere h according to F = fh/kd.
... ... *eddy diffusion coefficient: (turbulent diffusion
coefficient) The exchange coefficient for the diffusion of a
conserved property by eddies in a turbulent flow. In general, an
"eddy" is a vortex-like motion of fluid running contrary to the
main current.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 12Feb99
For more information: http://scienceweek.com/swfr.htm

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

6. CHEMICAL PHYSICS:
MEASUREMENT OF SHORT-RANGE CHEMICAL BONDING FORCES
     In atomic and molecular systems, forces can be categorized
as either long-range or short-range, depending on the relative
distances over which the forces are effective. For example, the
Coulomb force, which falls off with the inverse square of the
distance, is long-range; van der Waals forces, which fall off
with the inverse 6th power of the distance, are short-range, as
are the interatomic forces responsible for the various types of
chemical bonds. In general, forces that fall off less rapidly
than the inverse 4th power are called "long-range".
     In an increasing number of laboratories, the technique of
atomic force microscopy is being used to examine the interaction
between atoms and surfaces at angstrom-scale distances. In
general, in atomic force microscopy, a tip is fixed to a
cantilever whose position is monitored while the tip scans a
surface. The force between the tip and the surface determines the
position of the cantilever, and when recorded in atomic
resolution, one can obtain an image that represents a map of
atomic forces at the surface. But the technique has also been
used to investigate the behavior of single atoms and molecules at
surfaces, and it is beginning to provide important insights into
the physical chemistry of adsorption phenomena.
     In this context, the term "adatom" refers to an atom
adsorbed on a surface.
... ... M.A. Lantz et al (8 authors at University of Basel, CH)
report quantitative measurements of short-range chemical bonding
forces, the authors making the following points:
     1) The authors point out that although the atomic force
microscope was originally intended to be used as a tool capable
of measuring the forces acting between a single pair of atoms, it
has evolved into an instrument capable of producing atomically
resolved images of surfaces. Such atomic-scale images are
generally interpreted as resulting from the short-range chemical
interaction between an atomically sharp atomic-force-microscope
tip and the nearest atoms on the surface of the sample. In
principle, it should therefore be possible to map the chemical
bonding potential between the foremost atom on an atomic force
microscope tip and a specific atom on the sample.
     2) The authors report quantitative measurements of the
short-range chemical bonding force between the apex of a silicon
atomic force microscope tip and specific atomic sites on a
silicon sample. The force is measured over a large range of tip-
sample distances using a technique developed by the authors for
resolving force-distance characteristics. The short-range and
long-range forces are accurately separated by compensating the
electrostatic force and measuring the van der Waals force above
nonreactive sites. The authors report that the magnitude and
range of the measured short-range force are in good agreement
with first=principles calculations designed to model the same
situation. In addition, the authors report they have demonstrated
that the measured short-range force results from the formation of
a single bond.
     3) The authors suggest these measurements may provide
insight into covalent bond formation at surfaces. The authors
conclude: "The measurements presented here demonstrate the
possibility of directly measuring and quantifying local surface
reactivity with the atomic force microscope. The range of systems
that can be studied by such measurements could be extended by
depositing different materials or even single atoms on the tip.
The resolution of our measurements was sufficient to reveal
differences in the interaction potential between inequivalent
silicon adatoms; hence, it may eventually be possible to use
similar methods to distinguish between different atomic
species..."
-----------
M.A. Lantz et al: Quantitative measurement of short-range
chemical bonding forces.
(Science 30 Mar 01 291:2580)
QY: M.A. Lantz: mark.lantz@unibas.ch
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 1Jun01
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
CHEMISTRY: PROBE CHARACTERIZATION OF SINGLE BOND FORMATION
In this context, "tunneling" is a quantum mechanical phenomenon
involving an effective penetration of an energy barrier by a
particle resulting from the width of the barrier being less than
the wavelength of the particle. First available in the early
1980s, the technique of scanning tunneling microscopy involves an
atomically sharp metal tip brought in atomic proximity (e.g., 0.5
to 1 nanometer) to a flat surface so that electrons can tunnel
between the two systems. Recording the atomic modulation of the
atomic structure while scanning the tip across the surface allows
one to image adsorbed species and surface morphologies.
... ... H.J. Lee and W. Ho (Cornell University, US) now report
the use of a scanning tunneling microscope to manipulate the
bonding of a carbon monoxide (CO) molecule and to analyze the
structure and vibrational properties of individual products.
Individual iron (Fe) atoms were evaporated and coadsorbed with CO
molecules on a silver surface [Ag(110)] at 13 kelvins in
ultra-high vacuum. A CO molecule was transferred from the surface
to the scanning tunneling microscope tip and bonded with an Fe
atom to form Fe(CO). A second CO molecule was similarly
transferred and bonded with Fe(CO) to form Fe(CO)(sub2). The
authors suggest this controlled bond formation and
characterization at the single-bond level is effectively probing
chemistry at the spatial limit. The authors conclude: "The
binding of a diatomic molecule to an atom constitutes one of the
simplest chemical transformations involving a molecule. By
combining the present manipulation approach with other mechanisms
such as 'sliding', 'pulling', and 'pushing', extension of
spatially controlled bond formation to other atoms and molecules
is envisioned. The ability to control step-by-step bond formation
of adsorbed chemical species at the single-molecule level
provides a real-space understanding and direct visualization of
the nature of the chemical bond."
-----------
H.J. Lee and W. Ho: Single-bond formation and characterization
with a scanning tunneling microscope.
(Science 26 Nov 99 286:1719)
QY: Wilson Ho: wilsonho@ccmr.cornell.edu
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 18Feb00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
MEASUREMENT OF THE RUPTURE FORCE OF SINGLE COVALENT BONDS
From an elementary standpoint, what we call a "chemical bond" is
something that ties two or more atoms together to form a
molecular entity with sufficient stability for us to measure or
describe some of the entity's properties. The idea of the
chemical bond, including the notion of valence and the way we
draw the connections between atoms in a molecule, is usually
ascribed to Friedrich Kekule (1829-1896), but in fact it was
Edward Frankland (1825-1899) who first suggested the idea of
valence, and Archibald Couper (1831-1892) who first suggested the
depiction of the bond between 2 atoms by a dash. The use of
Couper's dashes to depict bonds was made popular by a chemist
well-known at the time for various discoveries in synthetic
chemistry, Richard Erlenmeyer (1825-1909). So today we have the
irony that Erlenmeyer is known primarily for the name of a glass
flask that he designed, Cooper and Frankland are hardly known at
all, and Kekule is often described to students as having
concocted these beginnings of structural chemistry in a dream.
Kekule may have indeed dreamt of the resonating benzene ring, but
his dream had the intimate help of his generation of chemists.
Putting aside the personal history of the concept of the chemical
bond, during its relatively short 150-year-old existence, this
concept has without any doubt been one of the most important
ideas in modern science. During this century, the focus has been
to understand the chemical bond in terms of quantum physics, but
certain classical aspects remain of great practical interest,
particularly the question of the relative strengths of the bonds
between different types of atoms. There are various ways to
approach this question of "bond strength", including actual
determinations of the mechanical force necessary to rupture
bonds. The mechanical stabilities of covalent bonds, bonds that
involve sharing of electrons, have in the past been investigated 
indirectly in ensemble measurements or by flow-induced chain
fracture in liquids. The recent development of nanoscale
manipulation techniques has made it possible to directly address
single atoms or molecules and probe their mechanical properties.
... ... M. Grandbois et al (5 authors at 3 installations, DE US)
now report a study of the rupture force of single covalent bonds
under an external load measured with an *atomic force microscope.
Single polysaccharide molecules were covalently anchored between
a surface and an atomic force microscope tip and then stretched
until the molecule became detached. The authors report that by
using different surface chemistries for the attachment, it was
found that the silicon-carbon bond ruptured at 2.0 +- 0.3
nanonewtons, whereas the sulfur-gold anchor ruptured at 1.4 +-
0.3 nanonewtons, at force-loading rates of 10 nanonewtons per
second. The authors report these results agree with bond rupture
probability calculations based on *density functional theory. The
authors conclude: "Although chemical compounds play a dominant
role in material sciences, the forces that chemical bonds can
withstand could previously not be directly measured in
experiments. The experiments reported here demonstrate that the
individual chemical bonds can be probed in mechanical
experiments. An important feature of such experiments is the
mechanical activation of chemical bonds (here in the simplest
form as bond rupture), which can now be studied on an individual
basis."
-----------
M. Grandbois: How strong is a chemical bond?
(Science 12 Mar 99 283:1727)
QY: Hermann E. Gaug, Lehrstuhl fur Angewandte Physik, Ludwig-
Maximillians-Universitat, Amalienstrasse 54, D-80799 Munich, DE.
-----------
Text Notes:
... ... *atomic force microscope: In general, in atomic force
microscopy, a tip is fixed to a cantilever whose position is
monitored while the tip scans a surface. The force between the
tip and the surface determines the position of the cantilever.
When recorded in atomic resolution, the image represents a map of
atomic forces at the surface. In the present study, the active
polymer molecule was first coupled to the substrate surface. The
tip was then slowly brought into contact with the surface,
allowing the polymer to bind to the tip (which occurred in
approximately 30 percent of the cases). The tip and the substrate
where then gradually separated while the force was recorded. The
polymer was repeatedly stretched and relaxed through one or more
conformational transitions. After analysis confirmed that a
single molecule was bound, the force was gradually increased
until the molecular bridge ruptured.
... ... *density functional theory: For atomic force calculations
on solids, the current method of choice is density functional
theory, due to Kohn, Hohenberg, and Sham. Its name comes from its
predicted connection between the total ground state electronic
energy of a system and the electronic charge density. The theory
was first proposed in 1964, and has since been useful as a
simplifying alternative to more rigorous but intractable many-
electron wavefunction calculations. In general, in density
functional theory, it is the electron density which is the
fundamental variable: the ground state of a system is defined by
that electron density distribution which minimizes the total
energy. In this approach, once the ground state electron density
is known, all other ground state properties (lattice constants,
cohesive energies, etc.) follow, at least in principle. In
mathematics, a "functional" is a function whose value depends on
the set of all values of another function. In density functional
theory, the ground state properties of a system are functionals
of the ground state electron density function.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 4Jun99
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ON SINGLE MOLECULE PHYSICS AND CHEMISTRY
Only a few decades ago, most scientists believed that individual
molecules would not come within the domain of experimental
observations within their lifetime, if ever, and that the
statistical ensemble properties of molecules were therefore the
only properties of relevance. That view has now undergone a
dramatic alteration as a consequence of technological advances,
and there is much excitement evident in many laboratories over
the prospects of single-molecule explorations in physics,
chemistry, and biology.
... ... C. Bai et al (4 authors at 3 installations, CN US)
present a short review of recent work in single-molecule physics
and chemistry, the authors making the following points:
     1) The authors point out that when Richard Feynman (1918-
1988) was bothered while looking through one of the first
*scanning tunneling microscopes, he was upset to have been
interrupted because seeing the images of singe atoms was a
"religious experience". For many generations of scientists, the
molecule was both the concrete ultimate entity upon which our
understanding of the everyday world was based, and at the same
time an elusive intellectual construct whose very existence could
only be inferred circumstantially by experiments on macroscopic
samples. Thus, seeing an individual atom or molecule in motion
brings immediate emotional impact to this central concept of
modern thought.
     2) The authors ask: "When is molecular individuality
important?" The new possibility of studying single molecules is
important because molecular individuality does finally come into
play when the molecule is a complex entity. This may occur
because the molecule itself may have an intricate internal
structure -- e.g., a biomolecule -- resulting in a complex energy
landscape. Alternatively, the molecule may be part of a complex
environment that substantially changes the behavior of the
molecule. Here, distinguishing different molecules at different
locales is crucial for understanding the system as a whole.
Biomolecules in living cells are examples of this. Even simple
inorganic molecules on structured surfaces or in disordered
systems such as viscous liquids or glasses provide situations in
which molecular individuality matters. In all of these cases, the
capability of studying an individual molecule over time can
provide new insights unavailable by straightforward experiments
on macroscopic populations of molecules.
     3) With the aid of *scanning probe microscopy, direct
observations of entire arrays of atoms, molecules, and the fine
structures of molecular aggregates have become possible. The
ability to precisely control probes permits the study of long-
range structures made by molecules lying on surfaces. However,
although pretty pictures of such systems are easy to construct,
obtaining quantitative characteristics of surface-bound molecules
is not entirely straightforward, and the rigorous interpretation
of scanning probe microscopy images requires substantial
theoretical as well as experimental effort.
     4) The authors conclude: "We are only at the beginning, but
it is clear there is much to be discovered of a fundamental
nature about complex molecules viewed as individuals. Perhaps
equally important will be the idea of single molecule control.
Now that experiments interact with molecules at an individual
level, we can try to control them as individuals, not as
populations. A molecule under active control by an adaptive
environment will be a new beast. Such tamed molecules may well
resemble much more the elegant engineered machinery of everyday
experience than the unruly, wild molecules we are used to
studying today."
-----------
C. Bai et al: Single molecule physics and chemistry.
(Proc. Natl. Acad. Sci. US 28 Sep 99 96:11075)
QY: Chunli Bai, Institute of Chemistry, The Chinese Academy of
Sciences, Beijing 100080 CN.
-----------
Text Notes:
... ... *scanning tunneling microscopes: The general approach in
scanning probe microscopy research is illustrated by
consideration of two major techniques, scanning *tunneling
microscopy (STM) and atomic force microscopy (AFM). In scanning
tunneling microscopy, an atomically sharp metal tip is brought in
atomic proximity (e.g., 0.5 to 1 nanometer) to a flat surface so
that electrons can *tunnel between the two systems. The probe is
slowly moved across the surface and raised and lowered so as to
keep the tunneling current constant. A computer-generated contour
map of the surface is thus produced. The technique can resolve
individual atoms, but requires electrically conducting materials.
In atomic force microscopy, a tip is fixed to a cantilever whose
position is monitored while the tip scans the surface. The force
between the tip and the surface determines the position of the
cantilever. When recorded in atomic resolution, the image
represents a map of atomic forces at the surface. The advantage
of atomic force microscopy is that the probed surface does not
need to be electrically conducting.
... ... *tunneling: "Tunneling" is a quantum mechanical
phenomenon involving an effective penetration of an energy
barrier by a particle resulting from the width of the barrier
being less than the wavelength of the particle.
... ... *scanning probe microscopy: A general term comprising all
atomic-level probe techniques.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 5Dec99
For more information: http://scienceweek.com/swfr.htm

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7. IN FOCUS: ON QUANTUM WEIRDNESS
"In the realm of subatomic particles (the quantum realm),
individual subatomic particles don't acquire some of their
characteristics (e.g., position and velocity) until they're
observed. In other words, subatomic particles do not seem to
exist in a definite form until observers measure them! Bizarre as
this quantum weirdness may sound, it has been confirmed
repeatedly in rigorous tests. Some people have misinterpreted
this finding. They reason that since normal objects are
ultimately made of subatomic particles, ordinary things also must
be observed in order to exist. This conclusion is a quantum leap
in the wrong direction because the properties of the whole are
not the same as the properties of its parts. Quantum theory is
concerned only with what happens to _individual_ subatomic
particles. Quantum effects at the level of individual subatomic
events are averaged out on the macroscopic scale. The Moon
continues to orbit the Earth even when no one observes it.
Science continues to have a claim on objective reality. An
additional misinterpretation of the finding that particles don't
acquire some of their characteristics until they're observed by
someone is that 'ultimate reality is in the mind of the observer'
or 'thoughts can make anything happen'. Neither of these ideas
can be derived from quantum theory. This theory says _nothing_
about the role of human consciousness or mental processes in the
physical world."
-----------
C.M. Wynn and A.W. Wiggins: _Quantum Leaps in the Wrong
Direction: Were Real Science Ends and Pseudoscience Begins_
(Joseph Henry Press, Washington 2001, p.24)
[Authors: Charles M. Wynn is Professor of Chemistry at Eastern
Connecticut State University; Arthur W. Wiggins is Professor of
Physics at Oakland Community College Michigan]
http://www.amazon.com/exec/obidos/ASIN/030907309X/scienceweek
-------------------
SCIENCE-WEEK http://scienceweek.com 1Jun01

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8. FROM THE SCIENCEWEEK ARCHIVE:
PUBLIC HEALTH: EXPECTED CONSEQUENCES OF GLOBAL WARMING
Climate change produced by global warming is expected to result
in melting ice caps, rising sea levels, torrential floods,
devastating droughts, and severe harvest failures. What are often
not considered in discussions of global warming are the effects
of global warming on public health.
... ... Pim Martens (University of Maastricht, NL) presents a
review of the expected effects of global warming on public
health, the author making the following points:
     1) Concerning heat stress: The author suggests that perhaps
the most immediate consequence of increasing global temperatures
will be a rise in the number of heat waves and heat-related
illnesses. Such temperature extremes can, for example, increase
the sensitivity of asthmatics to their condition. There will also
be an increasing number of deaths from heat stress brought about
by high ambient temperatures lasting days on end. On the other
hand, the milder winters associated with global warming will
offer a better chance of survival for at-risk groups such as the
elderly during the coldest months. Research into the effect of a
gradual temperature increase has revealed that we can expect a
decline in mortality from cardiovascular and pulmonary disease in
the winter. Whether the milder winters could offset the mortality
during the summer heat waves is not clear.
     2) Concerning malaria: The spread of this disease is limited
by conditions that favor the disease vector (the malarial
mosquito Anopheles) and the protozoan parasite (Plasmodium). The
malarial mosquito is most comfortable at temperatures of
approximately 20 to 30 degrees centigrade and at a relative
humidity of at least 60 percent. Also, the malaria parasite
develops more rapidly inside the mosquito as the temperature
rises, and the development ceases entirely below approximately 15
degrees centigrade. Increased rainfall and increased surface
water, expected to result from global warming, will produce more
breeding grounds for the mosquito. Malaria currently kills 1 to 2
million people each year.
     3) Concerning schistosomiasis (bilharzia): The enormous
expanse of irrigation systems in many tropical countries has
doubled the incidence of this disease in the past 50 years. There
are some estimates that nearly 200 million people are infected
worldwide. The disease is caused by a parasitic worm (a
trematode; also called a "fluke"; a type of flatworm) whose eggs
enter the water supply by way of human urine or feces. Infected
water snails serve as hosts for the parasites while they develop
into free-swimming "mini-worms" (larvae; cercaria). The circle
closes when a larva penetrates the skin of a human who comes in
contact with the contaminated water. The development of the
parasite and the population of the host snails are both governed
by the ambient temperature, with warm waters favoring their
growth. Also, the warmer the ambient temperature, the more often
people come into contact with water. In places where the disease
is endemic, it is known that the number of infected snails
declines sharply during the winter months. A temperature rise of
only a few degrees will ensure that this disease is transmitted
throughout the year. It is estimated that currently worldwide
approximately 500 million people are at risk of infection by this
pathogen.
     4) Concerning dengue: Like malaria, this disease is
transmitted by mosquitoes (Aedes aegypti, which also transmits
yellow fever), but the pathogen is a virus (dengue virus, a
flavivirus). The dengue virus is currently restricted to the
tropics, approximately between latitudes 30 degrees south and 20
degrees north. Temperature affects the development of both the
mosquito and the virus as well as the frequency of mosquito
bites. A warmer climate may increase not only the elevations
above sea level at which the disease occurs, but also its
northern and southern ranges. Dengue hemorrhagic fever, a severe
form of the disease, has a mortality of 6 to 30 percent, with
most deaths occurring in infants less than 1 year old.
     5) Concerning various water-borne diseases: Changes in the
amount of precipitation will accompany the temperature changes to
a warmer Earth. Many disease-causing organisms require water for
survival, and increases in rainfall and flooding will encourage
the wider distribution of such pathogens, with higher
temperatures increasing the chances of pathogen survival. Various
bacteria (e.g., Salmonella and Shigella), viruses (e.g.,
rotavirus), protozoa (e.g., Giardia and Cryptosporidium) can
cause diarrhea, which kills more than 3 million children every
year.
     6) In general, many factors will interact with a changing
climate in a nonlinear way, so their effects on human health are
extremely difficult to quantify. Despite the uncertainties, there
are increasing indications that a changed global climate may be a
major factor in the global distribution of many diseases.
-----------
Pim Martens: How will climate change affect human health?
(American Scientist Nov/Dec 1999 87:534)
QY: Pim Martens [p.martens@icis.unimaas.nl]
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 21Jan00
For more information: http://scienceweek.com/swfr.htm


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