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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.
October 19, 2001 -- Vol. 5 Number 42
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There can hardly be a stranger commodity in the
world than books. Printed by people who don't
understand them; sold by people who don't understand
them; bound, criticized and read by people who don't
understand them, and now even written by people who
don't understand them.
-- Georg Christoph Lichtenberg (1742-1799)
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Section 1
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Contents of this Issue (Full reports in Section 2):
1. More Evidence for Early Divergence of Neanderthals
2. Astrocytes and the Neuron Synapse
3. Cocaine Craving After Withdrawal
4. The Nicotinic Acetylcholine Receptor
5. The Permian Extinction Dead Zone
6. On the Multifunctional Hormone DHEA
7. Conditions at Earth's Core
8. Observation of the Kapitza-Dirac Effect
9. Superconductivity in a Ferromagnetic State
10. Many-Body Effects in Semiconductors
11. Transitions in Mixed-Valence Chemistry
12. Photodissociation of Simple Molecules
13. In Focus: On the Quaternary Ice Age
14. SW Archive: Science and Society: On the Teaching of
Evolution in US Schools
15. Sources
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Section 2
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1. MORE EVIDENCE FOR EARLY DIVERGENCE OF NEANDERTHALS
M.S. Ponce de Leon and C.P. Zollikofer (University of Zurich
Irchel, CH) discuss cranial *ontogeny and the *divergence of
*Neanderthals. Homo neanderthalensis has a unique combination of
craniofacial features that are distinct from fossil and extant
"anatomically modern" Homo sapiens (modern humans). Morphological
evidence, direct isotopic dates, and fossil mitochondrial DNA
from three Neanderthals have previously suggested that the
Neanderthals were a separate evolutionary lineage for at least
500,000 years. However, it is unknown when and how Neanderthal
unique derived craniofacial characters (autapomorphies) emerged
during ontogeny. The authors report the use of computerized
fossil reconstruction and *geometric morphometrics to demonstrate
that characteristic differences in cranial and mandibular shape
between Neanderthals and modern humans arose very early during
development, possibly prenatally, and were maintained throughout
postnatal ontogeny. Postnatal differences in cranial ontogeny
between the two taxa are characterized primarily by
*heterochronic modifications of a common spatial pattern of
development. The authors suggest that this evidence for early
ontogenetic divergence, together with the apparent evolutionary
stasis of taxon-specific patterns of ontogeny, is consistent with
the separation of Neanderthals and modern humans at the species
level [*Note #1].
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NAT 2001 412:534
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Notes:
... ... *ontogeny: The development of an individual from zygote
to maturity. The term "zygote" refers to the cell formed by the
union of male and female gametes (sperm and egg cells).
... ... *divergence: In this context, the acquisition of
dissimilar characteristics by related organisms in unlike
environments.
... ... *Neanderthals: The human group we call the "Neanderthals"
lived in much of Europe, part of Asia, and the Middle East
between 150,000 to probably less than 30,000 years ago.
Neanderthals were the first fossil humans to be discovered, and
they have long been the focus of anthropological investigation.
More bones of Neanderthals are known than for any other
human-related (hominine) fossil group, including 30 nearly
complete skeletons, so the preoccupation of the anthropology
community with the Neanderthals is perhaps understandable.
... ... *geometric morphometrics: In this technique, the form of
a specimen is described by the spatial configuration of a set of
3-dimensional anatomical landmarks. Size-corrected variation in
shape can then be computed in terms of between-specimen
rearrangements of landmark positions
... ... *heterochronic: The term "heterochrony" refers in general
to changes during ontogeny in the relative times of appearance
and rates of development of inherited characteristics.
... ... *Note #1: The Neanderthal samples in this study comprised
11 immature and 5 adult specimens. Ages of death of both
Neanderthals and modern human specimens were estimated with
modern standard scores of dental eruption.
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Related Background:
HAND MORPHOLOGY AND TOOL USE BY NEANDERTHALS AND EARLY MODERN
HUMANS OF THE NEAR EAST
The structure and function of the human hand distinguishes
man from all other primates, since all other primates rely on
their forelimbs and hands as major organs of locomotion. The term
"hominid" refers in general to any primate in the human family,
and although fossil evidence for the evolution of the hands of
hominids is not plentiful, an apparent important feature of the
human hand is a truly opposable thumb that rotates so that it can
oppose the other four fingers. This, and other significant
anatomical features, provide the strength and nature of the grip
of the hand. Two main grips that are recognized in man are the
power grip, used when grasping something, and the precision grip,
used with a fine instrument.
The human group we call the "Neanderthals" lived in much of
Europe, part of Asia, and the Middle East between 150,000 to
probably less than 30,000 years ago. Neanderthals were the first
fossil humans to be discovered, and they have long been the focus
of anthropological investigation. More bones of Neanderthals are
known than for any other human-related (hominine) fossil group,
including 30 nearly complete skeletons, so the preoccupation of
the anthropology community with the Neanderthals is perhaps
understandable.
The Middle East ("Near East") has been a rich source of
human fossils, the first excavations beginning in the 1930s at
the cave sites of Skhul and Tabun on Mount Carmel in Israel, with
later excavations at Kebara (Mount Carmel), Amud (near the Sea of
Galilee), and Qafzeh (near Nazareth). Until a decade ago, the
Neanderthals were believed to predate the modern human population
in the Middle East, and to be in effect the ancestors of that
human population. But the current consensus, on the basis of
dating evidence, is that the Neanderthals and modern humans were
contemporaries in the Middle East, perhaps for as much as 40,000
years. The apparent temporal overlap of the two populations in
the Middle East has been a puzzle and a source of controversy.
The term "Paleolithic" (Old Stone Age) is essentially an
archeological term applied to Eurasia with approximate time-frame
segments as follows:
Upper Paleolithic: from 40,000 to 8000 years ago.
Middle Paleolithic: from 200,000 to 40,000 years ago.
Lower Paleolithic: from 2.5 million to 200,000 years ago.
Neanderthals lived by hunting and gathering, probably in
small, nomadic groups, an existence that evidently required
extraordinary strength. Their tool technology involved the
so-called "Levallois technique" to produce flakes that were then
further worked to yield as many as 60 different implements. For
the Neanderthals, this Middle Paleolithic technology is termed
"Mousterian", after a cave at Le Moustier in France. Mousterian
flakes could be used for many purposes, including cutting flesh,
scraping hides, and working wood. The "Levallois technique" is
named after the site in France where the first examples of such
tools were found. This tool-making technique involves the
preparation of a large stone "core" with a flat upper surface and
a convex lower surface. Broad flakes are detached from the core
by striking the core sharply at an angle on an anvil. The
resulting flakes are broad and thin.
Of importance in understanding research in Paleolithic man
is that evidence comes to us from three different fields,
anthropology, paleontology, and archeology, each field with
different methods, different biases, and a different focus.
... ... Wesley A. Niewoehner (University of New Mexico, US)
presents an analysis of Middle East Paleolithic fossil hands, the
author making the following points:
1) The author points out that the Near Eastern human fossil
and archeological records present a unique paleoanthropological
situation, because two morphologically distinct but
archaeologically very similar human groups, the late archaic
Neanderthals and the early modern Skhul/Qafzeh hominids existed
at approximately the same time. Near Eastern Neanderthals are
known from a number of 50,000- to 120,000=year-old sites in
Israel, Syria, and Iraq. Neanderthals were craniofacially
distinct, highly active, and comparatively very muscular. The
fossil remains from the approximately 50,000- to 100,000-year-old
site at Skhul and the approximately 100,000-year-old-site of
Qafzeh, both in Israel, are craniofacially more modern and less
muscular than Neanderthals. Both groups are associated with
Middle Paleolithic archeological data complexes, indicating they
used typologically and technologically similar toolkits for their
subsistence activities.
2) The author points out that observations that Neanderthals
were more heavily muscled, had stronger upper-limb bones, and
possessed unusual shapes and orientations of some upper-limb
joints relative to the Skhul/Qafzeh hominids, have led some
researchers to conclude that significant behavioral differences
must have been present, despite the association of the two groups
with similar Middle Paleolithic archeological data complexes.
3) The author presents a 3-dimensional morphometric analysis
of the hand remains of Skhul/Qafzeh hominids and Neanderthals.
The author reports that the Skhul/Qafzeh sample differs
significantly from the Neanderthals in many aspects of hand
functional anatomy. Given the correlations between changes in
tool technologies and functional adaptations seen in the hands of
Upper Paleolithic humans, the author concludes that the
Skhul/Qafzeh hand remains were adapted to Upper Paleolithic-like
manipulations. The author suggests these results support the
inference of significant behavioral differences between
Neanderthals and the Skhul/Qafzeh hominids and indicate that an
important shift in human manipulative behaviors was associated
with the earliest stages of the emergence of modern humans.
... ... In a commentary on this work, Steven E. Churchill (Duke
University, US) states: "Although many questions yet remain, this
study represents an important step in resolving contradictions in
our behavioral interpretations of the fossil and archeological
records of the Near Eastern Middle Paleolithic and in delineating
the adaptive characteristics of both groups of Mousterian
humans."
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PNAS 2001 98:2953,2979
SW 2001 29 Jun
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Related Background:
ANTHROPOLOGY:
ISOTOPE EVIDENCE OF NEANDERTHALS AS PREDATORS
For many years after the discovery of the first Neanderthal
fossil in 1856, Neanderthal man was considered "a dim-witted
slouching brute". But Neanderthal fossil remains, in fact,
indicate Neanderthal man had a larger brain capacity (average
volume 1500 cubic centimeters) than modern humans (average volume
1360 cubic centimeters), and considering everything known about
the Neanderthals, many anthropologists now view Neanderthal man
as merely a group at one end of the spectrum of modern human
variation.
The Neanderthals have been found only in Europe and the
Middle East in sites dated at 120,000 to 35,000 years ago, and
perhaps up to 300,000 years ago in Spain. Neanderthals were
apparently culturally advanced in many ways. They made a variety
of tools and weapons from wood, bone, and stone, including
delicate arrowheads, hand axes, scrapers for removing fat from
animal skins, and tools for engraving designs on bone and stone.
Neanderthals also apparently made clothes from animal skins, used
fire extensively, lived in caves or bone and skin shelters, and
on the basis of burial remains, apparently had religious beliefs.
The Neanderthals apparently disappeared approximately 35,000
years ago, and whether they died off as a result of environmental
changes (e.g., retraction of the northern ice sheet), were killed
off by modern man, or were interbred with modern man and lost a
separate identity is still unresolved.
... ... M.P. Richards et al (6 authors at 6 installations, UK CA
US FR HR) present new evidence concerning Neanderthal diet, the
authors making the following points:
1) The authors point out that understanding Neanderthal diet
has implications for understanding Neanderthal land use, social
organization, and behavioral complexity. Yet despite the abundant
evidence for successful hunting techniques across Neanderthal
Eurasia, animal remains can indicate only hunting or scavenging
episodes; such remains cannot reveal predominant foods in the
diet over the long term. In contrast, the measurement of ratios
of the stable isotopes of carbon (C-13) and nitrogen (N-15) in
mammal bone *collagen provides an indication of aspects of diet
over the last few years of life, and such isotope evidence can
therefore provide direct information on Neanderthal diet.
2) The authors report that isotope analysis of two
Neanderthal fossils and associated animal fossils from Vindija
Cave, Croatia, indicate that the bulk of the dietary protein of
the Neanderthals came from animal sources. Comparison with animal
remains from this and other sites of similar age indicates that
the Vindija Neanderthal isotope values are similar to those of
other carnivores, and these results are very close to the results
for the earlier Late Pleistocene Neanderthals from France and
Belgium. Therefore, the authors suggest, the emerging picture of
the European Neanderthal diet indicates that although
physiologically the Neanderthals were presumably omnivores, they
behaved as carnivores, with animal protein the main source of
dietary protein. The authors suggest "this finding is in
agreement with the indirect archeological evidence and strongly
points to the Neanderthals having been active predators."
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PNAS 2000 97:7663
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Notes:
... ... *collagen: The term "collagen" refers to a group
of fibrous proteins of very high tensile strength that form the
main component of connective tissue, cartilage, and bones in
animals.
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SW 2000 18 Aug
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Related Background:
ON THE NEANDERTHALS IN HUMAN EVOLUTION
About 10 kilometers east of Dusseldorf in Germany, in the valley
of the Dussel, there is a little town called Neander. One hundred
and forty-three years ago, in the summer of 1856, some workmen
broke into a cave to get at the limestone inside and discovered a
set of ancient bones. Most of the bones were smashed to bits by
the workmen, but some of the bones, including part of the skull,
survived, and the skeleton was soon recognized by anthropologists
as belonging to an ancient race of men who came to be known as
the Neanderthals. A Neanderthal fossil had actually been
discovered some years earlier in Gibraltar, but not recognized as
such. Neanderthal-like fossils have also been found in France,
Spain, Italy, Yugoslavia, Iraq, China, Java, and Israel. For more
than a century, one of the central questions in paleoanthropology
has been whether modern man evolved from this race -- or was the
Neanderthal a separate branch that became extinct?
... ... I. Tattersall and J.H. Schwartz present a review of
recent research in this area, the authors making the following
points:
1) Although many paleoanthropologists have lately begun to
look favorably on the view that Neanderthals merit species
recognition in their own right as Homo neanderthalensis, at least
as many paleoanthropologists still regard the Neanderthals as no
more than a strange variant of our own species, Homo sapiens.
This difference in viewpoint represents far more than a simple
matter of taxonomic hair-splitting, since if the Neanderthals are
considered as a single species they must be analyzed and
understood on their own terms. In contrast, if the Neanderthals
are merely subspecies variants of ourselves, they can be
dismissed as little more than an evolutionary epiphenomenon, a
minor and ephemeral appendage to the history of Homo sapiens.
2) Recently, Duarte et al (1999) proposed that the skeleton
of a 4-year-old child, unearthed in late 1998 at the 24,500-year-
old site of Lagar Velho in Portugal, represents not merely a
casual result of a Neanderthal/modern human mating, but rather is
the product of several millennia of hybridization among members
of the resident Neanderthal population and the invading Homo
sapiens.
3) In general, "Neanderthals" is the informal designation of
a morphologically distinctive group of large-brained *hominids
who inhabited Europe and western Asia between approximately
200,000 and less than 30,000 years ago. They are sharply
distinguished from modern humans by a wide range of cranial and
*postcranial characteristics, although they do share a number of
derived bony features with other members of the European/western
Asian hominid *clade that diversified in this part of the world
after approximately 500,000 years ago. Subsequent to
approximately 150,000 years ago, the Neanderthals appear to have
been the sole surviving species of this clade.
4) The Neanderthals were apparently highly successful over a
large region for a substantial period of time, but this situation
changed dramatically with the arrival in Europe of the first
modern humans, Homo sapiens. The evidence is that these "Cro-
Magnons" had begun to arrive both in eastern Europe and in the
far northeast of the Iberian Peninsula by approximately 40,000
years ago, and within little more than 10,000 years, the
Neanderthals were gone. The mechanism of their eviction has long
been debated, but there are four main possibilities. The first
and second of these possibilities, that the Neanderthals were
eliminated by the moderns in direct conflict or by indirect
economic competition, both imply the separate species status of
the Neanderthals, as does any combination of these two
possibilities. The alternative possibilities, that the
Neanderthals had simply evolved rapidly into moderns, or that the
genes of the invading moderns simply "swamped" those of the
Neanderthals, both imply some form of species continuity.
5) The authors suggest that the analysis by Duarte et al of
the Lagar Velho child's skeleton is "a brave and imaginative
interpretation" which the majority of paleoanthropologists will
consider unproven. The archeological context of Lagar Velho is
that of a typical *Gravettian burial, with no sign of *Mousterian
cultural influence, and the specimen itself lacks not only
derived Neanderthal characteristics, but also lacks any
suggestion of Neanderthal morphology.
6) the authors conclude: "The probability must thus remain
that this is simply a chunky Gravettian child, a descendant of
the modern invaders who had evicted the Neanderthals from Iberia
several millennia earlier. However, in this contentious and
poorly documented field, any new data are eagerly sought, and
Duarte et al's courageous speculations will doubtless spur much-
needed new research."
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PNAS 1999 96:7117
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Notes:
... ... *hominids: In general, any primate in the human family.
... ... *postcranial: In general, this refers to the skeleton
behind the cranium in a quadruped and below the cranium in a
biped.
... ... *clade: A "clade" is a cluster of taxa derived from a
single common ancestor.
... ... *Gravettian: A paleolithic culture in Europe extending
from approximately 30,000 to approximately 22,000 years ago.
... ... *Mousterian: See reports above.
... ... *Levallois technique: See reports above.
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SW 1999 3 Sep
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Related Background:
ON MODERN HUMAN ORIGINS
The study of human origins, the field called paleoanthropology,
has intrinsic difficulties because of the relative scarcity of
data, but these difficulties are magnified enormously by the
simple fact that paleoanthropology, in essence, represents a
species attempting to reconstruct its own early history. As might
be expected, an objective reconstruction, one without biases and
preconceptions, is far from easy... One of the important
questions concerning the Neanderthals is what happened to them?
Hypotheses have shifted back and forth since the first discovery
in 1856 of Neanderthal bones, with two major views. One view is
that the Neanderthals were the direct ancestors of modern
Europeans. The other view regards the Neanderthals as a side
branch of human evolution, with extinction as their fate. This
latter view is apparently the majority view in the
paleoanthropology community
.... ... G.A. Clark (Arizona State University, US) presents a
review of current research controversies and methods concerning
the transition from early humans to modern humans that apparently
occurred during the period from 50,000 to 10,000 years ago
(Middle-Upper Paleolithic transition). A central question is
whether the transition occurred abruptly or gradually. The author
makes the following points:
1) Insufficient data is only part of the reason the question
of human origins remains unresolved. Researchers in this area
come from various research traditions, and in each of these
traditions different assumptions about the remote human past
determine what is considered relevant data, which questions are
asked of the data, and how the data are interpreted. More data do
not remove the paradigmatic bias implicit within each research
tradition, and in consequence people from the different relevant
fields fail to communicate effectively.
2) The disciplines that contribute to the field (archeology,
human paleontology, and molecular biology) tend to be discovery-
driven and focused on methodology. The result is a common absence
of concern for the logic of inference underlying claims of
knowledge. European archeological studies of modern human origins
are a particularly good example of such epistemological naivete.
These studies are based on a century-old typological systematics
that emphasizes retouched stone tools, coupled with a set of
biases and preconceptions concerning the relationships between
developments in tool-making and developments of cultures.
3) On the surface, the voluminous literature produced by the
debate concerning modern human origins suggests an informed and
sophisticated interdisciplinary research in which data are
absorbed and digested, arguments assimilated, and methodologies
understood, compared, and evaluated. The author suggests "this is
a gross simplification of a much more complex reality."
4) The author concludes: "We are, in effect, consumers of
one another's research conclusions, but we select among
alternative sets of research conclusions in accordance with our
biases and preconceptions. These biases and preconceptions must
be subjected to critical scrutiny. As long as there is no
explicit concern with the logic of inference -- how we know what
we think we know about the past -- there can be no consensus."
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SCI 1999 283:2029
SW 1999 28 May
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Related Background:
HUMAN EVOLUTION: THE FATE OF THE NEANDERTHALS
The current consensus in paleoanthropology is that the
Neanderthals were an extinct side-line of human evolution.
European Neanderthals are thought to have diverged from the
lineage that gave rise to modern humans at least 500,000 years
ago. The current view is that approximately 30,000 to 40,000
years ago the Neanderthals were replaced by modern populations,
probably from an ultimately African source. A present debate
concerns how this population replacement occurred.
... ... Paul Mellars, in a short review of a recent conference
(28-30 Aug 1998, Gibraltar, UK) on the Neanderthals, makes the
following points: 1) The current consensus is that in the
southern part of the Spanish peninsula, roughly to the south of
the Ebro valley, the local Neanderthals survived for at least
5000 to 10,000 years after the arrival of modern populations in
the adjacent parts of northern Spain and the Mediterranean coast.
2) The most likely explanation for the prolonged coexistence of
these two populations lies in the ecological differences between
the northern and southern parts of the Iberian peninsula. 3)
Studies of Neanderthal skeletal remains reinforce the conclusion
that the Neanderthals were a divergent lineage that probably made
no contribution to the evolution of anatomically modern humans.
This is consistent with the DNA evidence that the two lineages
separated at least 500,000 years ago, and even longer divergence
times are favored by some researchers. 4) The impression at the
end of the conference was that the Neanderthals were really quite
different from humans -- well adapted to survive in the harsh
glacial environments of Europe, but with distinct anatomical and
behavioral patterns different from their modern human successors.
The author concludes: "The eagerness of some scientists to claim
close kinship with the Neanderthals could come close to denying
that human evolution actually took place."
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NAT 1998 395:539
SW 1998 6 Nov
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2. ASTROCYTES AND THE NEURON SYNAPSE
V. Gallo and R. Chittajallu (National Institutes of Health, US)
discuss the involvement of *glial cells in *synaptic
transmission. The *central nervous system contains two main types
of cells -- neurons, which transmit information from one part of
the nervous system to another part in the form of electrical
signals, and glial cells, which classically were considered to
fulfill only a supporting function. Recent data, however,
suggests that the major type of glial cell, the astrocyte, is
actually required for synapse formation and maintenance, and for
synaptic efficacy. The long thin processes of astrocytes
ensheathe the synaptic connections between neurons and are
therefore well-positioned anatomically to contribute to synaptic
transmission. Furthermore, these glial cells express a wide
variety of *neurotransmitter receptors and *voltage-gated ion
channels, which are important in receiving and integrating
neuronal signals. Glutamate receptors called AMPA receptors,
triggered by the neurotransmitter glutamate, mediate the majority
of fast synaptic transmission in the central nervous system. New
studies suggest a functional role of these receptors,
demonstrating that AMPA receptor-mediated calcium ion influx is
important in generating and maintaining the appropriate
structural and functional association between the neuronal
elements of certain glutamatergic synapses in the *cerebellum.
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SCI 2001 292:872,923,926
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Notes:
... ... *glial cells: Glial cells are cells of the central and
peripheral nervous system that metabolically support neurons.
Such cells also produce the multiple membrane layers called
myelin and enfold nerve cell axons with it. Glial cells are found
everywhere in the brain and spinal cord.
... ... *synaptic transmission: This is a general term referring
to the events mediating the membrane-to-membrane interaction
between a neuron and another neuron, or a neuron and a muscle or
gland cell, or a neuron and a sensory receptor cell. The junction
is called a "synapse", and in many cases junction transmission
involves release and uptake of "transmitter" substances.
... ... *central nervous system: In vertebrates, the spinal cord
is continuous with the brain, and the two together constitute
what is called the "central nervous system".
... ... *neurotransmitter: Neurotransmitters are chemical
substances released at the terminals of nerve axons in response
to the propagation of an impulse to the end of that axon. The
neurotransmitter substance diffuses into the synapse, the
junction between the presynaptic nerve ending and the
postsynaptic neuron, and at the membrane of the postsynaptic
neuron the transmitter substance interacts with a receptor.
Depending on the type of receptor, the result may be an
excitatory or an inhibitory effect on the postsynaptic nerve
cell.
... ... *voltage-gated ion channels: Refers to opening or closing
of an ion channel by changes in the electrical potential across
the membrane.
... ... *cerebellum: The human cerebellum is about the size of a
large apple, is placed at the lower back of the head under the
optic lobes of the cerebrum, and is apparently involved in the
input-output control of automatic sensorimotor functions. If you
are sitting at your breakfast table, holding a newspaper in one
hand, and using the other hand to routinely and repetitively dip
a spoon into cold cereal and bring the cold cereal to your mouth
while you read the newspaper, it is the cerebellum which is
governing the automatic feeding movements while your cerebral
cortex processes the information that you read.
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3. COCAINE CRAVING AFTER WITHDRAWAL
J.W. Grimm et al (National Institutes of Health, US) discuss the
physiology of *cocaine craving after withdrawal from the drug.
Relapse to *cocaine addiction is frequently associated with
subjective reports of craving, a poorly understood state that
precedes and accompanies cocaine-seeking behaviors. It has been
suggested that over the first few weeks of withdrawal from
cocaine, human addicts become sensitized to drug-associated
environmental cues that act as external stimuli for craving,
although the evidence for this is inconsistent. The authors
provide behavioral evidence from laboratory animals suggesting
that the onset of craving does not decay, but instead increases
progressively over a 2-month withdrawal period. The authors
suggest their results are consistent with clinical observations
in humans, indicating that a delayed-onset craving syndrome
develops or "incubates" during the first 2 months of cocaine
abstinence, and probably lasts longer than that. Although the
mechanisms responsible for this incubation are not known, the
intensification of cocaine seeking develops over a period when
most of the neuro-adaptations that accompany withdrawal from
chronic cocaine addiction are in progressive decline. The authors
suggest that whatever the mechanism by which craving is
incubated, their results are inconsistent with the view that
cocaine craving decays progressively after cessation of drug use,
and the results suggest instead that the individual is most
vulnerable to relapse at times well beyond the acute phase of
drug withdrawal.
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NAT 2001 412:141
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Notes:
... ... *cocaine: Cocaine is an alkaloid extracted from leaves of
the plant Erythroxylan coca, native to South America. Once
absorbed, cocaine is rapidly broken down by esterases (including
cholinesterases) released simultaneously with dopamine in the
brain. The drug binds with high affinity to dopamine,
norepinephrine, and serotonin uptake sites, preventing the
reuptake of these neurotransmitters from the synaptic cleft and
leading to their increased concentration in the synapse.
... ... *cocaine addiction: Compulsive cocaine abuse occurs even
when tolerance to the drug has developed. There are distinct
phases in the compulsive cocaine abuse cycle, including euphoria,
crash, and craving. Repeated use of cocaine depresses dopamine
synthesis, and in time toxic dopamine metabolites kill cells in
the midbrain. There is no evidence that such toxic effects are
reversible.
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4. THE NICOTINIC ACETYLCHOLINE RECEPTOR
D.A. Dougherty and H.A. Lester (California Institute of
Technology, US) discuss the nicotinic acetylcholine receptor.
Communication between neurons occurs at junctions known as
"synapses". When stimulated, the presynaptic neuron releases a
neurotransmitter, for example, the release of acetylcholine at
"cholinergic" synapses. The neurotransmitter diffuses from the
presynaptic terminal, some of it binding to postsynaptic
receptors, which are large proteins in the membrane of the
postsynaptic neuron. The so-called "nicotinic acetylcholine
receptor" not only has a binding site for the neurotransmitter,
but also has a channel portion: when the receptor binds
acetylcholine or other "agonists" (including nicotine), the
channel opens to allow ions to flow. The nicotinic acetylcholine
receptor belongs to a receptor superfamily that also contains
receptors for the neurotransmitters serotonin, gamma-aminobutyric
acid, glycine, and glutamate. The terminal 230 or so amino acids
of the receptor form the agonist-binding domain. This region, as
well as binding neurotransmitters, also binds molecules of
medical importance: muscle relaxants, ganglionic blockers,
possible painkillers, possible cognition enhancers, etc. In
addition, in myasthenia gravis (a human autoimmune disease),
antibodies bind to the main immunogenic region of the nicotinic
acetylcholine receptor. Finally, some naturally occurring
mutations in the binding domain prolong the open state of the
human nicotinic acetylcholine receptor in muscle. This leads to
excessive activation, which damages the synapse and muscle fiber,
causing a muscular weakness termed "slow-channel myasthenic
syndrome."
-----------
NAT 2001 411:252
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5. THE PERMIAN EXTINCTION DEAD ZONE
C.V. Looy et al (Utrecht University, NL) discuss the *Permian
extinction. The irreversible loss of species associated with
extinction is perhaps the most alarming symptom of the ongoing
biodiversity crisis. In an attempt to understand the long-term
consequences of the present biodiversity decline, past episodes
of dramatic *mass extinction are investigated for consistent
patterns of species responses to global environmental
deterioration. At present, conceptual models linking patterns of
mass extinction, survival, and recovery mainly involve
qualitative generalizations of temporal distributions of marine
invertebrate lineages, and focus is commonly on the profound
macroevolutionary consequences of mass extinctions. The fossil
record of land plants is an obvious source of information
concerning the dynamics of mass extinctions in the geological
past. In conjunction with the end-Permian ecological crisis that
occurred approximately 250 million years ago, *palynological data
from East Greenland reveal some unanticipated patterns. The
authors document the significant time lag (0.5 to 0.6 million
years) between terrestrial ecosystem collapse and selective
extinction among characteristic Late Permian plants. Furthermore,
ecological crisis apparently resulted in an initial increase in
plant diversity, instead of a decrease. Paradoxically, these
floral patterns correspond to a "dead zone" in the end-Permian
faunal record, the dead zone characterized by a paucity of marine
invertebrate large fossils ("megafossils"). The time-delayed end-
Permian plant extinctions resemble modeled "*extinction debt"
responses of multispecies metapopulations to progressive habitat
destruction.
-----------
PNAS 2001 98:7879
-----------
Notes:
... ... *Permian extinction: The "Permian period" comprises the
approximate time-frame 286 to 248 million years ago, and the
"Triassic period" comprises the approximate time-frame 248 to 213
million years ago. At the end of the Permian period, many groups
of animals and plants apparently vanished in the greatest known
crisis in the history of life on Earth. This Permian/Triassic
mass extinction has traditionally been considered to have had a
slow time course, but recent evidence has suggested this event
was more abrupt than previously recognized.
... ... *mass extinction: In the fossils of the past 530 million
years, there is evidence of many mass extinctions, but evidence
only of 5 extinctions that killed more than half the extant
species. The best known extinction is the Cretaceous-Tertiary
(also called K/T) event of 65 million years ago, apparently
caused by the impact of a comet or an asteroid, the event
characterized by the extinction of the dinosaurs. But the K/T
event destroyed only about 50 percent of the species on
Earth, which means it was a much less extensive extinction than
the Permian.
... ... *palynological data: Palynology is the study of fossil
spores and pollen. These entities are very resistant to
destruction, and in many sedimentary rocks they are the only
fossils that can be used for stratigraphical correlation.
... ... *extinction debt: In this context, a delayed extinction
of a competitive species as habitat destruction increases.
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SCIENCE-WEEK 19 Oct 2001 http://scienceweek.com
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Related Background:
EVIDENCE FOR AN IMPACT EVENT RELATED TO THE PERMIAN EXTINCTION
The available evolutionary record on Earth provides evidence
of recurrent mass extinctions of biological species. Apparently,
environmental catastrophes, occurring for various reasons, have
suddenly removed many groups with a resultant collapse of
ecosystems. Eventually new forms appear and evolution continues,
but the periods of mass extinction apparently are a major factor
in the various patterns of evolution.
The geological period ranging approximately from 146 million
years ago to 65 million years ago is called the "Cretaceous
period", and the geological period comprising the approximate
time-frame 65 million to 3 million years ago is called the
"Tertiary period". The largest mass extinction of the past 200
million years apparently occurred 65 million years ago at the end
of the Cretaceous period, when approximately half of the genera
of multicellular organisms on Earth, including all of the
dinosaurs, suddenly died off. The geological record indicates
that a layer of impact-produced minerals and the element iridium
(an element rare in the crust of the Earth but more abundant in
primitive meteorites) was deposited at the time the dinosaurs
vanished -- the so-called Cretaceous/Tertiary or K/T boundary. In
addition to this, the largest known crater on Earth (Chicxulub,
Yucatan, MX) to be dated at less than 1 billion years old was
apparently formed at this time. Taken together, these data imply
that the K/T mass extinction was caused by the impact into the
Yucatan peninsula of an asteroid or comet of approximately 10
kilometers in radius...
"Stony" meteorites (aerolites) are meteorites formed solely
of rock-forming silicates, and 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. A "carbonaceous chondrite" is a chondritic
meteorite that contains a relatively large amount of carbon, with
a resultant dark appearance. The "Murchison meteorite" is a
carbonaceous chondrite that fell in 1969 near Murchison,
Australia.
"Fullerenes" are large molecules composed entirely of
carbon, with the chemical formula C(n), where n is any even
number from 20 to over 100. They apparently have the structure of
a hollow spheroidal cage with a surface network of carbon atoms
connected in hexagonal and pentagonal rings, and the cage large
enough to trap atoms and small molecules. Fullerenes have been
identified in the Murchison meteorite.
In this context, the term "planetary" refers to the
primordial aggregates (primitive planets) surrounding the Sun
from which the current planets were formed.
... ... L. Becker et al (5 authors at 4 installations, US)
present new evidence concerning an impact event at the Permian-
Triassic boundary, the authors making the following points:
1) The authors point out that the extinction event that
marks the Permian/Triassic boundary (251.4 +- 0.3 million years
ago) was the most severe in the past 540 million years, killing
off over 90 percent of all marine species, approximately 70
percent of terrestrial vertebrate genera, and most land plants.
Several new studies have demonstrated that these extinctions were
much more abrupt than previously thought, with estimates of the
extinction interval ranging from less than 500,000 to
approximately 8000 years. Proposed catastrophic hypotheses for
the Permian/Triassic boundary extinction include an exploding
meteor (bolide) (asteroidal or cometary) and or massive volcanic
lava flows (flood basalt volcanism). Other extinctions mechanisms
involving ocean anoxia, as well as changes in sea level and
climate, have also been proposed.
2) The authors report that fullerenes [C(sub60) to
C(sub200)] from sediments of the Permian/Triassic boundary
contain trapped helium and argon with isotope ratios similar to
the apparent planetary component of carbonaceous chondrites. The
authors suggest these data imply that an impact event (asteroidal
or cometary) accompanied the extinction, as was the case for the
Cretaceous/Tertiary extinction event approximately 65 million
years ago.
3) The authors conclude: "Based on the measured helium-3
content for the Permian/Triassic boundary and Murchison
fullerenes, the estimated size of the bolide is 9 +- 3 kilometers
or comparable to the K/T Chicxulub impactor. Such an event could
have caused the severe end-Permian mass extinction. Our results
are consistent with recent paleontological studies that now point
to a very rapid extinction event. The unique planetary signature
measured in fullerenes isolated from the Murchison carbonaceous
chondrite and from the Permian/Triassic boundary sediments
demonstrates that this distinctive noble gas carrier can survive
major impact events and contribute to the unique gas signature of
the terrestrial planetary atmospheres."
-----------
SCI 2001 291:1530
SW 2001 9 Mar
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SCIENCE-WEEK 19 Oct 2001 http://scienceweek.com
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Related Background:
FULLERENES AND CAGED NOBLE GASES IN METEORITES
Fullerenes are large molecules composed entirely of carbon,
with the chemical formula C(n), where n is any even number from
32 to 100+. They apparently have the structure of a hollow
spheroidal cage with a surface network of carbon atoms connected
in hexagonal and pentagonal rings. They were discovered by
Richard E. Smalley, who received the Nobel Prize in Chemistry for
the discovery in 1996.
Since the synthesis, isolation, and characterization of
fullerenes, there has been considerable interest in determining
whether or not this family of carbon molecules occurs naturally
on the Earth and elsewhere in the universe. But despite an
intensive effort by researchers, terrestrial discoveries of
fullerenes have been limited to the detection of mostly C(sub60)
and C(sub70) fullerenes, while extraterrestrial evidence is based
on trace findings of C(sub60) and a single mass spectrum of a
high mass carbon envelope from the *Allende meteorite.
... ... L. Becker et al (3 authors at 3 installations, US) now
report the discovery of naturally occurring fullerenes [C(sub60)
to C(sub400)] in the Allende and *Murchison meteorites and in
some sediment samples from the 65 million-year-old
*Cretaceous/Tertiary boundary layer. The authors point out that
unlike the other pure forms of carbon (diamond and graphite),
fullerenes are extractable in an organic solvent. By exploiting
this property, and also the unique ability of the fullerene cage
structure to encapsulate and retain noble gas atoms, the authors
report they have determined that both the Allende and Murchison
fullerenes and the Cretaceous/Tertiary boundary fullerenes
contain trapped noble gases with isotope ratios that can only be
described as extraterrestrial in origin. The authors suggest they
have thus identified a carrier phase for noble gases associated
with impact (asteroidal or cometary) and meteorite carbon, and
that the recognition of the fullerene carrier phase with an
extraterrestrial noble gas signature in *carbonaceous chondrites
provides pathways for understanding the origin and evolution of
planetary atmospheres and/or presolar environments. The authors
conclude: "Future searches for fullerene and (sup3)He as a tracer
of the flux of extraterrestrial material in the sediments
throughout the geologic record, and its possible association with
changes observed in climate and the biostratospheric record,
could have broad implications for the [study of the] evolution of
life on the Earth."
-----------
PNAS 2000 97:2979
-----------
Notes:
... ... *Allende meteorite: One of the more interesting
carbonaceous chondrites (see note below) is the meteorite called
Allende, which fell on 8 February 1969 at Pueblito de Allende in
northern Mexico, and which scattered 5 metric tons of material
over an area 48 kilometers long by 7 kilometers wide.
... ... *Murchison meteorite: This meteorite, which fell near
Murchison AU in 1969, contains 8 amino acids and the nucleotide
bases adenine, guanine, and uracil.
... ... *Cretaceous/Tertiary boundary layer: See main report.
... ... *carbonaceous chondrites: Chondrite meteorites are
characterized by the presence of chondrules, small spherical
inclusions of glassy rock that can be seen easily with the naked
eye if the meteorite is cut and one of the cut faces polished.
Carbonaceous chondrites contain both chondrules and volatiles,
and since the presence of the volatiles is believed to indicate
an origin which did not involve heat, the carbonaceous chondrites
are considered the least altered remains of the solar nebula from
which the planets are believed to have formed.
-----------
SW 2000 26 May
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SCIENCE-WEEK 19 Oct 2001 http://scienceweek.com
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Related Background:
ON THE PERMIAN EXTINCTION AND THE GREENHOUSE EFFECT
In geology and paleontology, a "period" is a major subdivision of
an era of geological time distinguished by a particular system of
rocks and associated fossils. The Permian Period is the most
recent period of the Paleozoic Era, and occurred approximately
225 to 270 million years ago. The Permian was characterized by an
abundance of vertebrate and invertebrate forms, a proliferation
of reptilian forms, and a variety of vegetation forms. The
arrangement of the continents was apparently quite different than
at present, with a closer packing of land masses, and the
southern end of Africa well below the Antarctic circle. One of
the more interesting features of the Permian Period is that it
was apparently terminated by a mass biotic extinction of enormous
proportions. ... ... Peter D. Ward (University of Washington
Seattle, US), reviewing the Permian extinction and a current
model to explain it, makes the following points: 1) The Permian
extinction was the greatest mass extinction in Earth's history,
involving the extinction of 90 percent of the species in the
ocean and 70 percent of the species on land. Despite its scale,
the Permian extinction remains a deep mystery. 2) In the fossils
of the past 530 million years, there is evidence of many mass
extinctions, but evidence only of 5 extinctions that killed more
than half the extant species. The best known extinction is the
Cretaceous-Tertiary (also called K/T) event of 65 million years
ago, apparently caused by the impact of a comet or an asteroid,
the event characterized by the extinction of the dinosaurs. But
the K/T event destroyed only about 50 percent of the species on
Earth, which means it was a much less extensive extinction than
the Permian. 3) Knoll et al, in 1995, proposed that the Permian
extinction in the oceans was essentially caused by the release in
the ocean, due to movements of land masses, of carbon dioxide
trapped in sediments, and that it was ocean carbon dioxide, known
to be highly toxic to marine life, that was responsible for the
ocean mass biotic extinction. 4) In the current article, Ward
suggests that ocean carbon dioxide and volcanic gases, both
emerging into the atmosphere, resulted in a heating of the
atmosphere to critical levels. Ward proposes that the surge in
temperature is reflected in the common redness of rock strata
associated with the end of the Permian, the red color a result of
the rusting of iron compounds, and suggesting a climate change of
massive proportions. 5) The author suggests that the Permian
extinction now appears to be a new type of mass extinction,
unrelated to extraterrestrial causes, and occurring much faster
than typical extinctions triggered by internal changes to the
climate and chemistry of the Earth. The author concludes: "Are we
walking down the same path that killed off so much life 250
million years ago -- not from carbon dioxide liberated from the
oceans but from carbon dioxide liberated from our cars and
industry?"
-----------
Discover 1998 August
SW 1998 28 Aug
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SCIENCE-WEEK 19 Oct 2001 http://scienceweek.com
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6. ON THE MULTIFUNCTIONAL HORMONE DHEA
The hormone dehydroepiandrosterone (DHEA) is an intermediate in
*androgen and *estrogen biosynthesis, and is itself
biosynthesized in testis, ovary, and adrenal cortex. In plasma,
the sulfate salt DHEAS is approximately 1000 more concentrated
than DHEA. The secretion of this hormone begins during fetal
life, reaches a peak at about age 30, and declines steadily
thereafter, the level at age 80 only 10 to 20 percent of the peak
level. The decline has been speculatively associated with the
changes of aging, and commercial formulations of DHEA are
marketed as dietary supplements that are promoted for the
prevention of various aging-associated degenerative diseases,
including atherosclerosis, Alzheimer dementia, and parkinsonism
-- all without any benefits demonstrated in large randomized
clinical trials. Analyses of commercial dietary supplement
preparations has revealed large differences between label
concentrations and actual concentrations.
... ... S.S. Yen (University of California San Diego, US)
discusses DHEA. This hormone was isolated in urine in 1934, and
DHEA 3-beta-sulfate (DHEAS) identified in 1944. Another decade
passed before both DHEA and DHEAS were identified in peripheral
blood. More recently, fatty acid esters of DHEA, *testosterone,
and estradiol have been isolated from a variety of tissues,
particularly fat. In 1981, DHEA and DHEAS were reported in
mammalian brain, but isolation of the enzyme responsible for the
synthesis of DHEA was unsuccessful. Recently, expression of the
this enzyme and biosynthesis of DHEA were found to interact by
way of neuroglial cells and neurons. The adrenals of humans and a
few higher primates synthesize and secrete large amounts of DHEA
and DHEAS that are biotransformed into biologically active
androgens and estrogens in peripheral tissues. It is estimated
that more than 30 percent of total androgen in men and over 90
percent of estrogen in postmenopausal women are derived from
peripheral conversion of DHEA/DHEAS. Thus, intracellular
biotransformation of DHEA to active sex steroids may result in
the binding of active sex steroids locally to their specific
intracellular receptors in the cell nucleus with minimal loss of
concentration or time, an economical system to exert maximal
functional activities. During the past 50 years, many animal
experiments have suggested that DHEA is a multifunctional hormone
with beneficial effects, including anti-aging properties.
-----------
PNAS 2001 98:8167
-----------
Notes:
... ... *androgen: A generic term for the male sex hormones.
... ... *estrogen: Estrogen is a collective term for the female
hormones, the most powerful of which is estradiol. They control
female secondary sexual characteristics, and prepare and maintain
the uterine lining.
... ... *testosterone: the principal and most potent androgen
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SCIENCE-WEEK 19 Oct 2001 http://scienceweek.com
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7. CONDITIONS AT EARTH'S CORE
A. Jephcoat and K. Refson (University of Oxford, UK) discuss
Earth's core. Although in the beginning, shortly after the Earth
was formed, there was apparently no inner core, this has since
grown to a spherical region of dense and nearly pure solid iron
approximately 2240 kilometers in diameter. The size and some of
the characteristics of the internal structure of the core are
known because of their effect on seismic waves produced by
earthquakes, which pass through the core on their way to detector
stations at the surface. A longstanding puzzle is that the speed
of a seismic compressional wave (an ultra-low frequency sound
wave) depends on its direction across the core. Such waves travel
faster along the axis of Earth's rotation (north-south) than in
the equatorial plane (east-west), and this difference (called
"seismic anisotropy") appears to increase with depth within the
inner core and is patchy on a variety of length scales. Other
vital evidence for seismic anisotropy comes from free
oscillations, studies of the natural vibrational frequencies of
the Earth. Steinle-Neumann et al (2001) have now presented
calculations of the behavior of iron at high temperatures and
pressures, and they predict that the crystal structure of iron
will distort unexpectedly when subjected to the conditions at the
center of the Earth. Together with a model of the orientation of
iron crystals in the inner core by Buffet and Wenk (2001), these
results apparently explain the observed seismic anisotropy, and
it seems the fundamentals for understanding the phenomenon are
now in place.
-----------
NAT 2001 413:27,57,60
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SCIENCE-WEEK 19 Oct 2001 http://scienceweek.com
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Related Background:
ON EARTH'S CORE AND THE GEODYNAMO
Seismic wave propagations are the propagated shock waves
produced by earthquakes, and quantitative analysis of these waves
can tell us much about the structure of the Earth and its
interior physical discontinuities. 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. The mantle, the layer of dense rock and metal
oxides between the molten part of the core and the surface, has
plastic properties (i.e., it is a solid capable of flow under
pressure). Apparently, the Earth's magnetic field is a direct
result of the combination of its rapid rotation and its molten
core, and the theoretical account of this is called the "dynamo
effect" (the source of the effect is called Earth's "geodynamo").
The essential idea is that the liquid metallic core is stirred by
convection, the rotation of the Earth couples this motion into a
circulation that generates electric currents, and the electric
currents in turn generate a magnetic field according to classical
electromagnetic theory.
... ... Bruce A. Buffett (University of British Columbia, CA)
presents a review of current research on the Earth's core and the
geodynamo, the author making the following points:
1) Earth apparently evolved into a layered body early in its
history. Molten metal (mainly iron) descended to form the
present-day core, while silicates and oxides were confined to the
thick shell of the mantle. The innermost part of the core is now
a solid, whereas the outer portion of the core is liquid. The
apparent viscosity of the liquid outer core is comparable to that
of water, which permits vigorous convection as the core cools.
Fluid velocities of the order of 10 kilometers per year are
evidently sufficiently rapid to sustain Earth's magnetic field
via the geodynamo.
2) Planetary rotation promotes the types of flows that are
needed to generate the magnetic field, but the resulting magnetic
field exerts a strong feedback on convection, and this
complicates quantitative predictions of the field generation. An
important advance in recent years is the development of numerical
simulations that produce self-sustaining dynamo action.
Computational limitations prevent these simulations from reaching
known Earth-like conditions, but the models obtained so far have
external magnetic fields that are similar to Earth's magnetic
field.
3) The operation of the geodynamo depends on the internal
evolution of the planet, since convection in the core is linked
to the rate of cooling. (Cooling of the core causes growth of the
inner core by solidification; the current rate of growth is
approximately 1 millimeter per year.) The transport of heat
through the mantle is crucial for powering the geodynamo, and
even the existence of *plate tectonics at the surface is an
important factor. Interactions between the core and the mantle
are expected from theory, but it is not clear how these
interactions are expressed in the magnetic field. The apparent
persistence of the magnetic field over most of the history of
Earth implies continual cooling and convection in the core. In
contrast, the absence of magnetic fields in our nearest planetary
neighbors indicates that other planetary thermal histories are
possible. As we gain a better understanding of the geodynamo and
the dynamics of the core, new perspectives about the processes
that drive the internal evolution of Earth are expected to
emerge.
-----------
SCI 2000 288:2007
-----------
Notes:
... ... *plate tectonics: The term "lithosphere" refers to
the outer layer of the Earth, comprising the crust and upper
mantle, and extending to a depth of 50 to 70 kilometers. The
traditional view of tectonics (changes in the structure of the
Earth's crust) is that the lithosphere consists of a strong
brittle layer overlying a weak ductile layer. "Plate tectonics"
is the current consensus theory that the Earth's lithosphere is
broken into fairly rigid plates, seven or eight major plates and
many smaller plates, and that convection within the underlying
less rigid "asthenosphere" causes the plates (and the associated
continents and crust) to move relative to each other.
-----------
SW 2000 14 Jul
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SCIENCE-WEEK 19 Oct 2001 http://scienceweek.com
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Related Background:
SIMULATIONS OF LIQUID IRON VISCOSITY AT EARTH'S CORE
The current idea is that the Earth's outer core consists mainly
of liquid iron, and that the convection of this metallic liquid
is responsible for the Earth's magnetic field. However, a full
understanding of the dynamics is hampered by uncertainty
regarding the viscosity of the outer core, with viscosity
estimates by various researchers ranging over 12 orders of
magnitude. ... ... Wijs et al (7 authors at 3 installations, UK
AT) present dynamical first principles simulations of liquid iron
which indicate that the viscosity of iron at Earth core
temperatures and pressures is at the low range of previous
estimates -- roughly 10 times that of typical liquid metals at
ambient pressure. The authors suggest this estimate supports the
approximation commonly made in magnetohydrodynamic models that
the outer core is an inviscid fluid undergoing small-scale
circulation and turbulent convection, rather than large-scale
global circulation.
-----------
NAT 1998 23 Apr
SW 1998 15 May
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SCIENCE-WEEK 19 Oct 2001 http://scienceweek.com
-------------------
Related Background:
EXPERIMENTAL APPROACHES TO THE ANALYSIS OF EARTH'S CORE
Computer models of the Earth's core, combined with seismic
information, have suggested that the iron crystals in the inner
core are aligned, that the inner core has an intrinsic rotation,
and that the rotation velocity is slightly faster than that of
the rest of the planet. Seismologists have also suggested, on the
basis of seismic studies, that the core is not isotropic. Gallium
is a metallic element similar to mercury, with a low melting
point (29.78 degrees Centigrade) and a high boiling point (2403
degrees Centigrade). In a review of experimental work in
geophysics, A. Frank (Univ. of Rochester, US) emphasizes the
advantages of experimental methods over computer simulations.
Recent experiments involve using molten gallium as a model for
the Earth's core, with measurements of the effects of local
conditions such as the flow of heat, magnetic fields, and
rotation on the grain of the liquid metal. Among other
observations, the Olson group (Johns Hopkins Univ., US) has
apparently already provided evidence that temperature changes in
Earth's core did not create the crystalline arrangements, and
that it is rotation that may be the cause of the apparent core
asymmetries.
-----------
Earth 1998 February
SW 1998 2 Jan
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SCIENCE-WEEK 19 Oct 2001 http://scienceweek.com
-------------------
Related Background:
ROTATION DIFFERENCES OF EARTH'S INNER CORE AND MANTLE
Seismic wave propagations are the propagated shock waves produced
by earthquakes, and quantitative analysis of these waves can tell
us much about the structure of the Earth. 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. The mantle, the layer of dense
rock and metal oxides between the molten part of the core and the
surface, has plastic properties (i.e., it is a solid capable of
flow under pressure). Apparently, the Earth's magnetic field is a
direct result of its rapid rotation and its molten core, and the
theoretical account of this is called the "dynamo effect". The
essential idea is that the liquid metallic core is stirred by
convection, the rotation of the Earth couples this motion into a
circulation that generates electric currents, and the electric
currents in turn generate a magnetic field according to classical
electromagnetic theory. K. Creager (Univ. Washington Seattle, US)
reports a model that uses observations of particular seismic wave
propagations and proposes that the inner core of the Earth is
rotating 0.2 degrees to 0.3 degrees per year faster than the
mantle. The author suggests this low difference raises the upper
limit for inner core viscosity and thus constrains parameters for
future dynamo models.
-----------
SCI 1997 14 Nov
SW 1997 5 Dec
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SCIENCE-WEEK 19 Oct 2001 http://scienceweek.com
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8. OBSERVATION OF THE KAPITZA-DIRAC EFFECT
D.L. Freimund et al (University of Nebraska Lincoln, US) discuss
the Kapitza-Dirac effect. In their famous 1927 experiment, C.L.
Davisson (1881-1958) and L. Germer (1896-1971) observed the
diffraction of electrons by a periodic material structure, thus
demonstrating that electrons can behave like waves. Shortly
afterwards, P.L. Kapitza (1894-1984) and P.A.M. Dirac (1982-1984)
predicted that electrons should also be diffracted by a standing
light wave. This "Kapitza-Dirac effect" is analogous to the
diffraction of light by a grating, but with the roles of the wave
and matter reversed. The electron and the light grating interact
extremely weakly, via the "*ponderomotive potential", so attempts
to measure the Kapitza-Dirac effect had to wait for the
development of the laser. The idea that the underlying
interaction with light is enhanced by resonance in the case of
electrons in an atom led to the observation that atoms could be
diffracted by a standing wave of light. Deflection of electrons
by high-intensity laser light, which is also a consequence of the
Kapitza-Dirac effect, has also been demonstrated, but the
coherent interference that characterizes wave diffraction has not
so far been observed. The authors report the diffraction of free
electrons by a standing light wave -- a realization of the
Kapitza-Dirac effect as originally proposed. In their experiment,
an electron beam crosses two counter-propagating laser beams that
form the standing wave light grating. The authors suggest the
study of the interaction of free electrons with laser light can
probably be extended from quantum mechanics to include particle
spin, chaotic behavior, and relativistic mechanics.
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NAT 2001 413:142
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Notes:
... ... *ponderomotive potential: A charged particle bouncing in
the field of a constant-amplitude wave will oscillate in a
perfectly periodic manner. If the wave is modulated, however, so
that its amplitude varies with position, the particle will
experience a net force working on a time scale long compared to
the wave period. This is the "ponderomotive force", which can be
derived by taking the time-average of the force on the particle
caused by the wave field.
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SCIENCE-WEEK 19 Oct 2001 http://scienceweek.com
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9. SUPERCONDUCTIVITY IN A FERROMAGNETIC STATE
Charles Day (PT) discusses *superconductivity in a *ferromagnetic
state. Electrons require help to form *superconducting pairs. In
conventional superconductors, lattice vibrations push the
electrons together into the superconducting state. In exotic
superconductors and those with a high superconducting transition
temperature, magnetic fluctuations are believed to play the same
role. Magnetic fluctuations are strongest when magnetic order is
about to form or disappear, a point known as the quantum critical
point. At this point, electron spins wobble most and spin
fluctuations have the lowest frequencies: the lower the
frequency, the more favored the electron pairing. In high-
temperature superconductors, superconductivity is most likely in
the vicinity of the quantum critical point. However,
superconductivity is not expected in the magnetic state itself,
especially in a ferromagnetic material. Internal magnetic fields
tend to twist apart electron pairs whose spins are antiparallel.
Pairs with parallel spins would not be twisted apart. But all
superconductors, regardless of pairing symmetry, expel magnetic
fields, so at first glance ferromagnetism and superconductivity
appear mutually incompatible. It was thus a surprise when S.S.
Saxena et al (2000) discovered that an alloy of uranium and
germanium exhibited superconductivity and ferromagnetism
simultaneously. Now C. Pfleiderer et al (2001) have reported a
second ferromagnetic superconductor, an alloy of zirconium and
zinc.
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PT 2001 September
NAT 2000 406:587
NAT 2001 412:58
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Notes:
... ... *superconductivity: Superconductivity is a property of
many metals, alloys, and chemical compounds at temperatures near
absolute zero, at which temperatures their electrical resistivity
vanishes and they become strongly diamagnetic. Diamagnetic
substances such as the alkalis and alkaline earth metals, the
halogens, and the noble gases are repelled by magnets and tend to
position themselves at right angles to the magnetic lines of
force.
... ... *ferromagnetic: A "ferromagnet" is a material (such as
iron) in which there may be a permanent *magnetic moment, and in
which the *spins of the atoms are aligned parallel to each other.
... ... *magnetic moments: (magnetic dipole moment) The intrinsic
spins of the electrons in an atom, together with the motion of
the electrons around the nucleus, give rise to a magnetic field
around the atom, and the magnitude of this field is related to
the magnetic dipole moment of the atom or ion.
... ... *spins: In quantum mechanics, electrons, protons, and
neutrons have an intrinsic angular momentum known as "spin", and
a magnetic moment parallel or antiparallel to that angular
momentum. When electrons are combined together to form an atom or
ion, there is a resultant angular momentum which is a combination
of the intrinsic spin of the electrons and the angular momentum
due to their motion about the nucleus, and this is the "spin" of
the atom or ion. Atoms or ions with non-zero spin are magnetic
atoms or ions. The idea of electron spin was first proposed by
Goudsmit and Uhlenbeck in 1925 to explain the splitting of atomic
spectroscopic emission lines in the presence of a magnetic field.
Elementary particle spin involves a virtual rotation about the
axis of the particle, which means only two spin states are
possible, one clockwise and one counterclockwise.
... ... *superconducting pairs: (Cooper pairs) In the late 1950s,
John Bardeen (1908-1991), Leon N. Cooper, and John R. Schrieffer
demonstrated that superconductivity involves the formation of
bound pairs of electrons, called "Cooper pairs". Their theory
(BCS theory) argued that the electron pairs were "glued together"
by small deformations (phonons) in the crystal lattice, the
deformations accompanying the motions of electrons. But although
phonons had been implicated in superconductivity many years
before BCS theory, it was not until the 1960s that it became
possible to definitively identify phonons as the "glue" (the
interaction energy quanta) in conventional superconductivity.
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10. MANY-BODY EFFECTS IN SEMICONDUCTORS
D.S. Chemla and J. Shah (University of California Berkeley, US)
discuss many-body and *correlation effects in semiconductors. One
of the most fascinating properties of quantum-mechanical systems
is that they can be in entangled states, i.e., involved in a
*coherent superposition of eigenstates unknown in classical
systems. Entangled states have been observed in atomic physics,
but they remain elusive in condensed matter. The primary
difficulty is that such states survive only as long as the
coherence is maintained, and numerous processes conspire to
produce decoherence, dephasing, and relaxation on an extremely
short timescale of approximately 10^(-12) seconds in condensed
matter, which is typically composed of 10^(22) to 10^(23)
particles per cubic centimeter that interact through the
infinite-range Coulomb force. The linear response of a solid to a
weak external perturbation is well described by the concept of
non-interacting "quasiparticles" first introduced by Lev Landau
(1908-1968). But interactions between quasiparticles can be
substantial in condensed systems. For example, studies over the
past decade have demonstrated that Coulomb correlations between
quasiparticles dominate the nonlinear optical response of
semiconductors, in marked contrast to the behavior of atomic
systems. These Coulomb correlations and other many-body
interactions are important not only for semiconductors, but also
for all condensed-matter systems.
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NAT 2001 411:549
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Notes:
... ... *correlation effects: In general, correlation effects
between entities are effects due to near-constant phase relations
between the entities.
... ... *coherent superposition of eigenstates: The term
"eigenstate" refers to any one of the wave function solutions
(probability amplitude function solutions) to the Schroedinger
equation for the given boundary conditions. The superposition
principle in quantum mechanics derives from the superposition
principle in pure mathematics, which states that for a linear
homogenous differential equation, if y(sub1)(x) and y(sub2)(x)
are solutions, then so is y(sub1)(x) + y(sub2)(x). In other
words, for such a differential equation, the sum of solutions is
itself a solution. A corollary is that any physical system which
can be described by a linear homogeneous differential equation
(or a set of such equations) will obey the superposition
principle. This principle produces various applications and
formulations in the physics of oscillating systems. In quantum
mechanics, where the time-independent Schroedinger equation is a
linear homogenous differential equation and systems are described
by oscillating probability amplitudes, the principle of
superposition results in the postulate that any state function of
a given quantum mechanical system corresponding to a given
observable (e.g., energy) can be expressed as a linear expansion
of the eigenstates of the system for the same observable.
Another way to state the quantum mechanical principle of
superposition is as follows: If a physical state of a system can
be realized in a number of different but unknown distinct ways,
then the actual state of the system is a superposition for each
distinct way, and there is a distinct probability amplitude for
each way in which the physical state can be realized. In quantum
physics, coherence involves the locking of phase differences
between wave functions: the wave functions of two or more
particles are said to be coherent if the phase difference between
their wave functions remains constant. In general, a perfectly
coherent system of particles can be described by a single
macroscopic wave function. Thus, in this context, a system with a
"coherent superposition of eigenstates" refers to a system of
phase-locked entities with highly correlated probability
distributions of one or more state variables -- in other words,
the entities are "entangled".
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11. TRANSITIONS IN MIXED-VALENCE CHEMISTRY
K.D. Demadis et al (University of North Carolina Chapel Hill, US)
discuss transitions in mixed-valence chemistry. So-called "mixed-
valence compounds" contain an element that at least in a formal
sense exists in more than one oxidation state. This is a common
phenomenon: for example, prussian blue, which has a cyanide-
bridged Fe(II)-Fe(III) structure, was one of the first chemical
materials to be described, first reported by J. Woodward in 1724.
The mixed-valence character of some minerals provides the basis
for their color, and multiple-site metalloenzymes, which undergo
multiple electron transfer, have mixed valence forms. The first
designed mixed-valence complexes were prepared in the 1970s. One
of the reasons for interest in mixed-valence molecules has been
the possibility that they could be used to measure rate constants
and activation barriers for intramolecular electron transfer,
although this approach has usually proved to have difficulties.
One of the continuing themes in mixed-valence chemistry is the
effect of electronic delocalization on mixed-valence properties.
In metal complexes, delocalization can be varied by making
changes in the metal, the bridging ligand, the ancillary ligands,
or the solvent. In organic molecules, variations can be made in
the solvent, the redox sites, or in the chemical link that joins
them. Electronic delocalization is promoted by mixing the
electronic donor and acceptor wave functions. As mixing
increases, the discrete oxidation-state character of the local
sites decreases and with it structural differences and dipole
orientational changes in the solvent also decrease.
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CR 2001 101:2655
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12. PHOTODISSOCIATION OF SIMPLE MOLECULES
Hiroyasu Sato (Mi'e University, JP) discusses photodissociation
of simple molecules. Molecules have a large number of potential
energy surfaces corresponding to excited electronic states in
addition to that of the ground electronic state (the lowest
energy state). Although we are accustomed to the ball (atom) and
stick (chemical bond) picture of a molecule, the world of
chemistry appears, if viewed in terms of potential energy, as if
many ponds (stable molecules) are separated by mountains
(transition states). In such a chemistry landscape, in addition
to the ground surface (the potential energy surface of the ground
electronic state), there are many transparent surfaces hanging in
the sky (the potential energy surfaces of excited electronic
states). Some of these potential energy surfaces cross each
other, making a "seam" of surfaces, and this is the landscape in
which electrons (quantum mechanical wave packets) play. In
general, a photodissociation reaction is a unimolecular reaction
driven by light energy. Such a reaction corresponds to the latter
half of a bimolecular reaction, where the parent molecule in the
excited state assumes the role of the transition state.
Photodissociation reactions occur under the constraint of
conservation of energy and angular momentum, and on
photodissociation, the available energy is distributed to various
degrees of freedom, both translational and internal (electronic,
vibrational, rotational). Investigations of photodissociation
dynamics have become a new form of molecular spectroscopy,
revealing information concerning the nature of chemical bonding
in both static and dynamic contexts.
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CR 2001 101:2687
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13. IN FOCUS: ON THE QUATERNARY ICE AGE
"Over millennia, the world has experienced a series of ice ages,
periods when large areas of the surface were covered with a sheet
of ice thousands of meters deep. The most widely accepted theory
for the cause of the ice ages is based on the fact that there
have been changes over millions of years in the tilt of the
Earth's axis and in the circularity of its orbit. Those changes
have been cyclical and led to long periods when some parts of the
Earth's surface received less light and heat from the Sun.
Scientists recognize five major ice ages, the first about 2
billion years ago, then three more about 600 million, 400
million, and 300 million years ago, and the last beginning 1.7
million years ago and finishing only about 10,000 years ago.
[The last], the Quaternary Ice Age, [was] a period when the
diversity of life-forms on land not covered by ice was greater
than it had been during the previous ice ages and during which
there were major effects on plants and animals as the temperature
dropped and ice sheets and glaciers formed from water that had
evaporated from the sea. As the cold took its grip, not overnight
but over thousands of years, the relationship between land and
sea changed, caused partly by climate, partly by the weight of
ice pressing down on the land. The process that froze water vapor
into ice and deposited it on the land effectively depleted the
sea of water that would otherwise condense into rain, run into
rivers, and flow on to the sea. As more water froze and water
continued to evaporate from the ocean, the sea level dropped,
revealing areas that had been covered by water and expanding the
total land area. For an area like the Hebrides, a series of
separate islands before the Ice Age, this process would have
turned them into much larger areas of land, possibly even into
one interconnecting land mass."
-----------
Karl Sabbagh: _A Rum Affair: A True Story of Botanical Fraud_
Da Capo Press, 2001, p.23
http://www.amazon.com/exec/obidos/ASIN/0306810603/scienceweek
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14. SW ARCHIVE:
SCIENCE AND SOCIETY:
ON THE TEACHING OF EVOLUTION IN US SCHOOLS
Although the recent new schoolboard election in Kansas has
resulted in a reduction of vocal objections by the scientific
community to the corruption of the teaching of biological
evolution in many US schools, the problem still exists and the
evidence is that the problem will not be easily solved.
... ... Lawrence S. Lerner (California State University Long
Beach, US) presents a commentary on the problem, the author
making the following points:
1) The author points out that nearly every science is
essentially the study of the evolution of systems in time, and
that the central organizing principle in biology is the evolution
of living things in time, just as geology centers on the
evolution of the Earth and astronomy centers on the evolution of
the Universe. That evolution of systems is the central organizing
principal of all the historical sciences is not a controversial
issue among scientists or among most educated people, and science
teaching worldwide treats evolution as routine. The US is the
exception: in much of the US, teaching evolution as scientists
see it -- particularly biological evolution -- to K-12 (age 5 to
18) students still evokes bitter controversy. A variety of
organizations and people object to the teaching of the facts and
theory of evolution in public schools at the primary and
secondary level.
2) The author suggests this controversy is not really about
science but about religion and politics. Objectors assert that
evolution has not taken place, that the picture that scientists
have of the Universe is misguided, that it is "only fair" to
present creationist views to students in tandem with evolution,
even that teaching evolution will lead children into immoral
lives. The first two assertions are part of "creation science", a
pseudoscientific rival to evolution that US courts have
repeatedly ruled to be thinly veiled religion.
3) The author points out that in contrast to the situation
in K-12 schools, no controversy exists at the university level,
where curricula are more or less fully under the control of
faculty. As science faculty members are experts in their fields,
they share a consensus as to basics. In contrast, K-12
instruction is subject to considerable intervention from people
such as schoolboard members and legislators, with no expertise
in, and often little or no knowledge of, the fields whose
curricula they govern, and hence who are vulnerable to irrelevant
lobbying and pressures.
4) The author points out that a significant number of
Americans who are not scientists feel uncomfortable about
evolution in general and biological evolution in particular.
Polls suggest that approximately one-fourth of Americans adhere
to the literal biblical account, perhaps a third are strongly
convinced of the validity of the scientific account, and the rest
are ambivalent. In a poll of 1500 Americans conducted by
telephone in November 1999, two-thirds of the respondents
believed that only evolution should be taught as science. Of
these, 20 percent held that only evolution should be taught at
all; 17 percent held that evolution should be taught in science
classes while religious creation should be taught in other
classes; and 29 percent held that evolution should be taught in
science classes as a scientific theory with creationism mentioned
as a belief. The view that evolution and creationism should be
taught together as competing views in science classes was held by
13 percent of all respondents. Finally, 16 percent believed that
only creationism should be taught.
5) The author suggests that a school district or a state
cannot argue that it is a simple matter of democracy to advocate
a scientifically unacceptable opinion because a majority or vocal
minority of citizens hold that opinion. One can understand the
desire of parents to raise their children to think as they do,
but if the parents have a poor understanding of the content and
methods of science, it is well if they hope and expect that their
children will understand science better than they do. In doing
so, parents will provide the means to expose their children to
expertise beyond their own. Indeed, that is why most parents want
to send their children to school.
6) The author points out that approximately two-thirds of US
states have science standards that are consistent with the above
educational philosophy. The author concludes: "We owe it the
students in the other third to raise the standards to the same
level in these states. Poor education, wherever it may be and in
whatever discipline, affects us all."
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NAT 2000 407:287
SW 2000 20 Oct
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15. SOURCES:
------------
AS: American Scientist; CEN: Chemical & Engineering News;
CR: Chemical Reviews; GD: Genes & Development;
GR: Genome Research; JACS: J. Amer. Chemical Society;
JAMA: J. Amer. Medical Association; JCE: J. Chem. Education;
MMWR: CDC Morbidity and Mortality Weekly Report; NAT: Nature;
NATM: Nature Medicine; NEJM: New England J. Medicine;
NS: New Scientist; NYT: New York Times; NYR: New York Review;
PNAS: Proceedings of the National Academy of Sciences;
PRL: Physical Review Letters; PT: Physics Today; PRAX: PRAXIS;
SA: Scientific American; SCI: Science; SW: ScienceWeek;
TS: The Scientist.
In the text, the affiliation following the names of authors is
the affiliation of the lead author.
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