|
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.
December 24, 1999 -- Vol. 3 Number 52
-----------------------------------------------
We measure things. We spend countless dull hours measuring
the swing of a pendulum, the heat of an acid, the twitch
of a muscle. But only with these measurements in hand can
we begin our dialogue with the Cosmos.
-- Anonymous
-----------------------------------------------
The Editors extend to everyone their best wishes
for the holiday season and the New Year.
-----------------------------------------------
The following reports from back issues have been posted at the
ScienceWeek website. Links are on the main page (URL:
http://www.scienceweek.com) in the panel to the right of the
current issue contents (you many need to scroll down on the web
page). Main page links to these reports will be up for 1 week.
After that the reports go into the archive, where they can be
accessed via the SW website search engine.
Science & Society: Ammonia and the Population Explosion
Earth Science: On the Natural Occurrences of Diamond
Materials Science: On Rotating Superfluid Helium-3
Astrobiology: Polarized Starlight and Amino Acid Homochirality
Cell Biology: Protein Sorting and Golgi Components
Science Policy: Human Embryonic Stem Cell Research
-----------------------------------------------
Contents of This Issue:
1. Science Policy: On the Regulation of Human Genetic Tests
2. Applied Mathematics: Conformal Mapping
3. Astrophysics: On the Complexity of the Death of Stars
4. Neurobiology: Modification of Dendritic Spines by Calcium
5. Paleoanthropology: Dating of Late Pleistocene Hominid Remains
6. Evolutionary Biology: Two Species of Living Coelacanths
In Focus: On the Foundations of Quantum Mechanics
-----------------------------------------------
1. SCIENCE POLICY: ON THE REGULATION OF HUMAN GENETIC TESTS
As more and more human genes related to diseases are identified,
the commercialization of tests designed to detect the presence of
such genes in individuals gathers momentum and introduces a
spectrum of problems that will most likely be of considerable
importance in the coming decades. Neil A. Holtzman (Johns Hopkins
University, US) comments on the situation in and editorial, the
author making the following points:
1) The author suggests that the Human Genome Project has
engendered "genohype", from early pronouncements that our destiny
is in our genes to recent declarations that new discoveries will
minimize or prevent the appearance of diseases in individuals
altogether. As a result of these claims, commercial enterprises
have sprung up to identify the presence of susceptibility-
conferring genes in individuals. As early as 1995, over 50
biotechnology companies were developing or providing tests to
diagnose genetic disorders or to predict the risk of their future
occurrence. Common complex disorders, usually disorders of adult
onset such as Alzheimer's disease and breast and colon cancer,
make up the single largest category for which tests are under
commercial development.
2) The author suggests that the "educational" materials
prepared by companies for physicians and patients considering
genetic tests frequently make exaggerated claims for predictive
tests for common complex disorders. In particular are exaggerated
claims for a) clinical validity (i.e., the probability of a
detectable susceptibility-conferring gene occurring in those who
would get the disease, and the probability that those with a
susceptibility-conferring gene would actually get the disease);
and b) claims for utility (i.e., how a positive test result could
help people cope with future disease).
3) The author suggests that this situation has arisen
because of the double standard which the US Food and Drug
Administration (FDA) uses to regulate in vitro clinical
diagnostic devices: If a genetic test is to be marketed as a kit,
the manufacturer of the test kit must first demonstrate its
clinical validity to the satisfaction of the FDA, and scrutiny by
the FDA of the labeling of the test kit can ensure the utility of
the test is not exaggerated. But if, on the other hand, a test is
marketed as a clinical laboratory service, the laboratory
providing the service is not even required to notify the FDA. The
author states that the FDA admits it has the authority to
regulate clinical laboratory tests marketed as services, but
(according to the author) the FDA says it does not have the
resources to carry out such regulation.
4) The author suggests that, with respect to statements of
clinical validity and utility, the FDA regulation of genetic
tests marketed as services should be as stringent as the
regulation of tests marketed as kits.
-----------
Neil A. Holzman: Are genetic tests adequately regulated?
(Science 15 Oct 99 286:409)
QY: Neil A. Holtzman, Johns Hopkins University, Baltimore, MD
21205 US.
-------------------
Summary by SCIENCE-WEEK [http://scienceweek.com] 24Dec99]
-------------------
Related Background:
CONTROVERSY IN US: GENETIC TESTING FOR ALZHEIMER'S DISEASE
During the past 2 decades in the US, one relatively new
feature of the scientific enterprise has been a mushrooming of
the number of prominent academic researchers in molecular biology
who have become high-level corporate research managers. Often
these high-level research managers maintain ties to the
universities that originally hosted their research, ties, for
example, that may involve patent partnerships. Where patents are
involved, in many cases, the research underlying the patents was
financed by US federal funds, while the patents are now the basis
for extensive private commercial ventures. This situation was
made possible by explicit US Congressional legislation in the
1980s.
Last May, in a long article, the journal *Science* presented
a detailed profile of Allen Roses, a neurologist at Duke
University (US), who in 1997 became head of genetics research at
Glaxo Wellcome, Roses overseeing a US$50 million genetics
research budget that is part of the Glaxo Wellcome US$2 billion
annual research and development effort.
Prior to his move to Glaxo Wellcome, Roses achieved
prominence as the head of a research group at Duke University
that discovered a gene variant that apparently increases a
carrier's risk of developing the common late-onset form of
Alzheimer's disease (the most common form) -- a discovery that
was initially ignored by many researchers in the field but is now
considered to be of some importance.
After assuming his new position at Glaxo Wellcome, Roses
apparently set about creating an international "network of
clinicians" to provide data and clinical material to Glaxo in its
"hunt for disease-related genes", with the evident interest of
Glaxo Wellcome that of patenting key discoveries (including
genes) and manufacturing drugs based on the new discoveries. The
focus is on pathologies such as asthma, cardiovascular disease,
mental depression, schizophrenia, inflammatory bowel disease,
dermatitis, and susceptibility to infectious agents -- in other
words, a wide array of human diseases with possible genetic
involvements. The essential idea is apparently to build up
detailed indexes of variations in human genes and use these
indexes to scan the genomes of patients or volunteers.
Concerning Alzheimer's disease, the approach of Roses and
his group has been criticized because their marker for
Alzheimer's disease, a gene variant (called _APOE4_) of an
*apolipoprotein gene called _APOE_, does not appear to cause the
disease directly but appears to only increase the risk. Many
researchers believe that other genes and other proteins,
particularly so-called *beta-amyloid proteins, are involved in
Alzheimer's disease.
The Allen Roses profile in *Science* appeared 15 May 1998.
On 28 August 1998, J.F. Merz et al, in a letter to the
journal *Science*, pointed out that the article about Roses and
his advocacy of wide genetic testing for Alzheimer's disease did
not mention that Roses is named as an inventor on a patent
claiming exclusive rights to the detection of the _APOE_ *allele,
a patent now held in exclusive license from Duke University and
Roses by a company called AthenaDiagnostics, and that
AthenaDiagnostics has attempted to stop anyone anywhere from
performing _APOE_ genotyping for the purpose of diagnosing
Alzheimer's disease. In other words, AthenaDiagnostics
effectively owns the gene that may be one of the causes of
Alzheimer's disease, and no one can use that gene (which when
present appears as part of human chromosome 19) for diagnostic
purposes without paying a royalty fee. Considering the advocacy
by Roses of genetic testing for Alzheimer's disease, J.F Merz et
al stated: "This situation raises ethical concerns, not the least
of which is that those who benefit financially from the
performance of genetic testing and screening could be said to
have a conflict of interest that might lead to aggressive
promotion of those tests."
On 18 September 1998, Allen Roses responded to the J.F. Merz
et al letter in *Science*, and also to other related commentary
by J.F. Merz et al in *Nature Medicine*. In summary, Roses
criticized his critics for "incorrect notions and opinions",
stated that it is not true that he receives 50% of the licensing
fees for the _APOE_ gene, stated that he was being attacked
personally without relevant facts, and that he had not been able
to respond to the criticisms in *Nature Medicine* because that
journal does not entertain responses.
On 9 October 1998, A.J. Ivinson, the editor of *Nature
Medicine*, published a letter in *Science* in response to the
Roses letter, Ivinson stating that *Nature Medicine* does
sometimes invite responses, and that during a face-to-face
discussion Roses was specifically invited to respond to the
*Nature Medicine* text and he failed to do so, "making his
comments regarding our policy on responses all the more
surprising."
Finally, we return to 19 February 1998, to a paper published
in the *New England Journal of Medicine* by a large research
group that included the Roses research team, in which study the
authors reviewed clinical and autopsy _APOE_ data on 2188
patients at various installations referred for evaluation of
dementia, and in which paper the authors (Allen Roses among them)
conclude: "APOE genotyping does not provide sufficient
sensitivity or specificity to be used alone as a diagnostic test
for Alzheimer's disease, but when used in combination with
clinical criteria, it improves the specificity of the diagnosis."
All the backbiting and considerations of conflict of
interest aside, the last paragraph is the essence of this
brouhaha: Given that the direct and unique involvement of the
_APOE_ gene in Alzheimer's disease has not been demonstrated,
should _APOE_ genotyping (and consequent labeling of people as
"Alzheimer's prone") be widely used? The bioethicists say no,
that given the uncertainties in diagnosis, the social dangers are
too great; while Allen Roses, Glaxo Wellcome, and others say yes,
that genotyping can substantially improve clinical diagnostics.
As is seen in the attached background material below, the
problem is now not unique to the US. In fact, it is a safe guess
that in the next few decades, for many human genes, this sort of
problem will be commonplace.
[Editor's note: A collection of relevant past SCIENCE-WEEK
briefs can be found at URL http://scienceweek.com in a Focus
Report under the title "Science and Commerce in Conflict".]
-----------
A.J. Ivinson (*Nature Medicine*)
Nature Medicine's reply policy.
(Science 9 Oct 98 282:239)
QY: Adrian J. Ivinson [a.ivinson@natureny.com]
-----------
A.D. Roses (Genetics Glaxo Wellcome Research and Development, US)
Patent income.
(Science 18 Sep 98 281:1805)
QY: Allen D. Roses [adr69412@glaxowellcome.com]
-----------
J.F. Merz et al (3 authors at University of Pennsylvania, US)
Testing for Alzheimer's.
(Science 28 Aug 98 281:1288)
QY: Jon F. Merz [merz@mail.med.upenn.edu]
-----------
E. Marshall (*Science)
Allen Roses: From "street fighter" to corporate insider.
(Science 15 May 98 280:1001)
QY: Eliot Marshall [science_editors@aaas.org]
-----------
R. Mayeux et al (10 authors at 8 installations, US)
Utility of the apolipoprotein E genotype in the diagnosis of
Alzheimer's disease.
(New England J. Med. 19 Feb 98 338:505)
QY: Richard Mayeux, Columbia Univ. 212-854-1754.
-----------
Text Notes:
... ... *apolipoprotein gene: An apolipoprotein is the protein
component of a lipoprotein (lipid + protein) complex. In its non-
pathological form, the apolipoprotein gene is involved in the
metabolism of fats. Concerning the pathological form of the gene,
apparently confirmed data indicate that white persons 60 to 80
years old with two copies of the variant allele are 9 times more
likely to get Alzheimer's disease than those who do not carry the
variant. But almost everything else about the gene is in
controversy.
... ... *beta-amyloid proteins: Post-mortem tissue analysis of
Alzheimer's disease patients and Down syndrome patients reveals
anomalous protein deposits (beta-amyloid protein) in brain nerve
cells. Many researchers believe these deposits are in some way
related to the etiology of both of these disease entities.
... ... *allele: An allele is one of two or more forms of a given
gene that control a particular characteristic, with the
alternative forms occupying corresponding loci on homologous
chromosomes.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 30Oct98
-------------------
Related Background:
REPORT URGES CAUTION ON GENETIC TESTING FOR MENTAL DISORDERS
The continuing identification of specific gene mutations involved
in the pathogenesis of certain diseases will have as one
consequence an improvement in diagnostic and therapeutic
techniques that will considerably benefit patients with these
diseases. There is another consequence, however, a consequence
that is less certain to be uniformly beneficial: there are no
doubt many diseases whose pathogenesis involves one or more gene
mutations, but with the process such that the pathological
outcome is only more or less probable but not certain. The
identification of people with such probabilistic genetically
based pathologies is of great interest to commercial health
insurance and other interests, and there is thus a danger of a
type of branding of potential commercial insurance clients,
employees, children in various social contexts, and others on the
basis of genetic testing for "susceptibility genes". Britain's
main bioethics advisory panel has now issued a strong warning
against attempts to use genetic screening to predict an
individual's susceptibility to common mental health disorders
such as schizophrenia and Alzheimer's disease. There is concern
that commercial over-the-counter genetic tests for such diseases
will soon be available, and that the voluntary industry and
government constraints now in place will not be sufficient to
prevent the widespread use of such tests. The Nuffield Council on
Bioethics, Britain's national forum in this domain, has therefore
suggested in its recent report that regulations could be needed.
The report is titled: *Mental disorders and genetics: the ethical
context*. In a letter to the journal *Nature*, a group from the
Nuffield Council writes: "Even if a number of susceptibility
genes were identified for a particular disorder, the Nuffield
Council takes the view that, without an understanding of their
interaction, they would not be adequate for predicting individual
risk in a clinical setting. It has therefore recommended that
genetic testing for susceptibility genes which offer relatively
low predictive or diagnostic certainty be discouraged unless and
until there is clear medical benefit to the patient."
-----------
D. Dickson (*Nature*)
Panel urges caution on genetic testing for mental disorders.
(Nature 24 Sep 98 395:309)
QY: David Dickson [nature@nature.com]
----------
S. Thomas (Nuffield Council on Bioethics)
Restrict genetic susceptibility tests.
(Nature 24 Sep 98 395:317)
QY: Sandy Thomas, Nuffield Council on Bioethics, 28 Bedford
Square, London WC1B 3EG, UK
-------------------
Summary by SCIENCE-WEEK [http://scienceweek.com] 23Oct98
-------------------
Related Background:
NO LARGE US COMMERCIAL MARKET FOR CANCER GENE TESTING
Contrary to the expectations of financial analysts, commercial
genetic tests for breast cancer and colon cancer in the US are
not only not making money for the companies that offer them, but
they are proving to be overwhelmingly rejected by the public. The
tests are based on identification of inherited mutations in
specific genes, and have high predictive value for certain types
of family-related breast and colon cancers. Oncormed Inc.
(Gaithersburg, Md US), which offers breast and colon cancer
tests, is reports to be doing approximately 100 tests a month,
one-tenth of what was expected. Myriad Genetics Inc. (Salt Lake
City, Ut US), which offers the breast cancer gene test, has
reported only 262 tests in the entire last quarter of 1997. A
third company, Genetics IVF Institute (Fairfax, Va US) declines
to give numbers, but admits a similar low total of tests
administered. One problem is apparently that genetic testing may
result in individuals being refused health insurance coverage by
US insurers. At the present time, cancer gene tests cost
approximately $2000 each, and are often paid for by insurance
companies. Wall Street analysts had predicted a US$100 million a
year market in the US, but the market has not materialized.
Timothy Triche, CEO at Oncormed, says, "Everyone misunderstood
this field."
(New York Times 27 Mar 98) (Science-Week 3 Apr 98)
-------------------
Related Background:
CONSTRAINTS IMPOSED IN USE OF BREAST CANCER GENE PATENTS
_BRCA1_ and _BRCA2_ are known breast cancer susceptibility genes,
discovered in 1994 and 1995, and it has been reported that a
protein made by BRCA2 plays a critical role in cell repair of DNA
damage. Both cancer genes are apparently somehow involved in
tumor suppression or in the production of proteins vital for DNA
repair and the suppression of mutations. Patents for both genes
have been awarded to university-industry consortiums. From the
commercial standpoint, the value of such a patent is the
potential for making profit from diagnostic tests, since if one
has a patent on an important gene, one also has exclusive rights
to market diagnostic tests for disease-producing mutations of
that gene. Since breast cancer kills many millions of women
worldwide each year, there are many millions of women who are a
potential market for a diagnostic test that will tell them if
they are at risk. A similar paradigm applies to many other
patents that have been awarded or will soon be awarded for genes
in the human genome. That is the commercialization aspect of the
story. The scientific aspect is that identifying genes in the
human genome, their expressed proteins and the function of their
expressed proteins, and coupling such genes to specific disease
entities -- all of it is enormously expensive. So for the past
decade such research projects have been deliberately opened to
venture capital from private interests with the idea that
exclusive patents would be used to recoup the initial investments
with a profit. The basic science of the human genome has thus
been commercialized. A reasonable analogue would be the use of
private venture capital in particle physics to identify a new
elementary particle with the arrangement such that the private
commercial interest would have an exclusive patent on that
elementary particle. In any case, this is the current situation
in molecular biology. Now a British patent for the _BRCA2_ gene
has been awarded to a consortium of the Cancer Research Campaign
Technology (UK) and Duke University (US), and this consortium has
in turn granted an exclusive worldwide license to the patent for
diagnostic services and products to the company Oncormed of
Gaithersburg, MD US. What is apparently a new wrinkle in this
game is that Oncormed must apparently meet certain strict
conditions in exercising its license: broad sub-licensing of
diagnostic tests to other concerns, a requirement for pre- and
post-test counseling for women tested, a ban on direct
advertising to the public for screening tests, and no charge for
use of the techniques by the UK National Health Service. Everyone
agrees that genetic testing ought to be conducted ethically and
responsibly, but apparently not everyone agrees that agreements
with private companies can effectively replace government
regulation. The story of the commercialization of molecular
biology has yet to unfold its denouement.
(Nature 27 Nov 97) (Science-Week 19 Dec 97)
-------------------
Related Background:
GENETIC TESTING FOR CANCER RISK
At the present time, the following cancers are known to be
associated with Mendelian-type single gene inheritance: pituitary
adenoma, parathyroid adenoma, islet tumors of the pancreas,
medullary thyroid carcinoma, pheochromocytoma, parathyroid
hyperplasia, mucosal neuroma, Gardner's syndrome, familial
polyposis of the colon, nevoid basal cell carcinoma, and tricho-
epithelioma. A dozen precancerous conditions are also in the same
genetics category. During the next decade, there will no doubt be
discovered multiple-gene etiologies of various other malignant
conditions. All of which poses a serious problem with many
aspects and ramifications: Should asymptomatic individuals,
particularly the young, be tested for the presence of genes
associated with specific high cancer risk? Bruce Ponder
(University of Cambridge, UK), in a review of the present status
of genetic testing for cancer, points out that although genetic
testing for cancer is now clinically routine for families
afflicted with known inheritable cancers, genetic testing in the
population at large is not common, and provoking awareness and
discussion in the public and among health professionals is still
a challenge.
-----------
QY: B. Ponder [bajp@mole.bio.cam.ac.uk]
(Science 7 Nov 97) (Science-Week 5 Dec 97)
[For more information: http://scienceweek.com/search/search.htm]
2. APPLIED MATHEMATICS: CONFORMAL MAPPING
In mathematics, a "conformal transformation" is a transformation
which preserves angles. Such a transformation must be an analytic
function and have a nonzero derivative. In general, a "conformal
map" is a map of one region upon another that is one-to-one and
continuous and such that angles are preserved; in particular,
maps that preserve the form (or shape) of small-scale features
are called "conformal".
... ... Steven G. Krantz (Washington University St. Louis, US)
presents a review of conformal mapping and certain applications,
particularly applications to neurobiology, the author making the
following points:
1) Neurobiologists would like to unfold the complicated
geometry of bulges and folds of the brain into a simple flat map
of the surface of the brain -- but there is no perfect way to do
this: any flat map of a curved surface must sacrifice something.
2) At any place in the surface of a conformal map, the map
expands or contracts distances equally in all directions, so
small circles in the surface still look circular. All angles in
the original surface are also represented faithfully in the map.
But this preservation of form on the small scale is achieved --
just as in the Mercator map of the Earth -- at the expense of
size and large-scale form, which may be greatly distorted.
3) Conformal maps have been a part of mathematics for
centuries. The first conformal map of the globe was known to the
ancient Greek astronomer Hipparchus of Rhodes (c. 190-120 B.C.)
in the second century B.C. and may have been discovered even
earlier. In the 19th century, great strides were made in
conformal mapping theory by using the idea of complex numbers.
The culmination of this work was the Riemann Mapping Theorem,
considered by many to be the greatest accomplishment of 19th
century mathematics. In 1851, Georg Friedrich Bernhard Riemann
(1826-1866) proved that any planar region with no holes in it can
be conformally mapped to a disk. Unfortunately, the theorem
provided no way to actually construct the map whose existence was
proved: the theorem was a "nonconstructive theorem", i.e., it
proved the existence of conformal mapping of a planar region with
no holes in it by demonstrating that nonexistence would lead to a
logical contradiction.
4) Between 1868 and 1870, mathematicians Herman Schwarz
(1843-1921) and Elwin Christofel (1829-1900) independently found
formulas for mapping regions with piecewise straight boundaries
(polygonal regions) to a disk. In theory, any region without a
hole can be approximated by a polygon, and suitable mapping
found. Before the age of computers, however, the Schwarz-
Christofel transformation formulas were not of much use, since
they involved calculations intractable by hand. Currently,
however, conformal mappings and their applications are
flourishing as a result of computer solutions of Schwarz-
Christofel formulas. But new mapping techniques have also been
developed: a recent breakthrough involves the idea of "circle
packing". This is a purely geometric method that requires none of
the intricate formulas of Schwarz-Christofel transformations.
5) Concerning applications to neurobiology, the surface of
the human *cerebellum, for example, can be reconstructed from
*magnetic-resonance images and displayed in three dimensions with
colors showing the anatomic regions. But encoding spatial
information such as location or distance, information important
for localizing functional data from brain scans, is difficult on
such a complex surface and requires a mapping technique that
correctly represents small-scale features and local geometry.
Conformal mapping using circle packing flattens the surface of
the cerebellum onto a plane and allows visualization of the
entire surface at once, and this technique is now being used by
neurobiologists to analyze activations of the human cerebellum
during performance of particular tasks.
6) The author concludes: "Mathematicians like to point out
that mathematical research is justified by the frequently
unpredictable applications that come years after the development
of the theory. The study of conformal mapping is an example of a
field in which the pendulum has swung both ways: The theory was a
goad to the applications and then the applications turned around
and goosed the theory. This is scientific research at its best,
and the development continues."
-----------
Steven G. Krantz: Conformal mappings.
(American Scientist Sep/Oct 1999 87:436)
QY: Steven G. Krantz [sk@math.wustl.edu]
-----------
Text Notes:
... ... *cerebellum: The cerebellum is a large neural structure
at the base of the brain involved in motor coordination, posture,
and balance.
... ... *magnetic resonance images: Magnetic resonance imaging is
a technique involving images produced by mobile protons of a
tissue excited by the application of a magnetic field, and when
used in functional cerebral imaging, the basis of the technique
is that it images very small metabolic, blood-flow, and
perfusion-diffusion changes in vivo, in real time, and with no
risk to the subject.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 24Dec99
[For more information: http://scienceweek.com/search/search.htm]
3. ASTROPHYSICS: ON THE COMPLEXITY OF THE DEATH OF STARS
In astronomy, the term "nebula" was originally applied to any
astronomical object that appeared fuzzy and extended in a
telescope, and over 100 such objects had already been catalogued
in the 18th century. The majority of these objects were later
identified as galaxies and star clusters. At the present time,
the term "nebula" (i.e., cloud) refers to a region of
interstellar gas and dust. "Emission nebulae" are bright diffuse
nebulae that emit light and other radiation as a result of
ionization and excitation of gas atoms by ultraviolet radiation,
the source of the UV usually one or more hot stars. A "planetary"
nebula is a nebula formed when a *red giant or *supergiant star
sheds its outer layers in the last stage of its evolution,
leaving a hot core that ionizes the expunged gas. The size of
planetary nebulae range from approximately the diameter of our
solar system to a light year across. Their lifetime is only about
10,000 years, and they are all expanding with speeds of
approximately 20 kilometers/second. In this context, the term
"planetary" has nothing to do with planets: the term is
historical, the first planetary nebulae discovered so named
because they gave the impression of planetary disks around stars
when viewed in small telescopes. The term "protoplanetary nebula"
has two separate meanings. In the context of this report, the
term refers to an early mass of gas and dust that will become a
planetary nebula. In the context of studies of planetary
formation, the term refers to the mass of gas and dust
surrounding young stars, the material from which planets will be
formed.
... ... Yervant Terzian (Cornell University) presents a review of
current research concerning planetary nebulae produced by the
death of stars, the author making the following points:
1) The images of planetary and protoplanetary nebulae
provided by the *Hubble Space Telescope have revealed previously
unsuspected morphological complexities of nebulae, and these new
complexities represent a challenge for researchers who study the
mechanisms of star death.
2) When a star similar to the Sun, with a mass of up to a
few solar masses, reaches the last stages of its evolution, the
star expands and becomes a relatively cool (e.g., 2500 degrees
kelvin) red giant, with a size so large that its outer perimeter
would include the orbit of Mars. Such a star loses mass in the
form of *stellar wind, followed by a more intense mass loss that
results from a "superwind". The star loses a substantial fraction
of its mass, and the ejected material forms a planetary nebula.
Following this, the stellar core contracts and becomes a "white
dwarf star", a star approximately the size of the Earth, with a
density of approximately 10^(7) times the density of the Earth,
and with a surface temperature of approximately 10^(5) degrees.
3) The material ejected from such a dying star initially
consists for the most part of atomic and molecular gas, which
partly coalesces shortly after ejection to form warm dust
particles. Eventually, the ultraviolet radiation of the hot white
dwarf ionizes the nebula, and the nebula becomes an emission
nebula. Within a few tens of thousands of years, the expanding
planetary nebula diffuses into the interstellar medium with a
velocity of approximately 20 kilometers per second.
4) The general stages of late stellar evolution are
reasonably well understood, but the mechanisms by which these old
stars eject their envelopes remain unknown, and researchers have
been unable to understand what factors contribute to creating any
particular nebular morphology. Not one of the images provided by
the Hubble Space Telescope shows a simple expanding bubble: most
objects have complex bipolar structures with central torii,
multipolar bubbles, jet-like filaments, globules, and in some
cases sets of circular rings. Many nebula show a detailed point
and mirror symmetry that can extend as much as 100,000
*astronomical units from the central star.
5) Hydrodynamical and magnetohydrodynamical simulations of
such dying stars show a multitude of morphologies, but the models
require magnetic field strengths and stellar rotations that are
not well understood in the context of present theory.
6) The author concludes: "The Hubble Space Telescope
observations have greatly enriched our knowledge about the death
of sun-like stars. The new discoveries illustrate the complexity
of stellar explosions near their deaths and pose a multitude of
new questions for observers and theorists alike."
-----------
Yervant Terzian: The complexity of stellar death
(Science 15 Oct 99 286:425)
QY: Yervant Terzian [terzian@astrosun.tn.cornell.edu]
-----------
Text Notes:
... ... *red giant: A "red giant star" is a star in a late
stage of evolution, the star having exhausted the hydrogen fuel
in its core. It has a surface temperature of less than 4700
degrees Kelvin and a diameter 10 to 100 times that of the Sun.
... ... *supergiant star: A supergiant star is an extremely
luminous star of large diameter and low density. The diameter can
be as large as 1000 times that of our Sun.
... ... *Hubble Space Telescope: The Hubble Space Telescope was
launched from a space shuttle in 1990 into a 600-kilometer
low-Earth orbit and has been providing extensive imaging and
spectroscopic observations critical for the development of
astronomy and astrophysics. The new information has concerned hot
stars, stellar chromospheres and coronas, the interstellar
medium, galaxies and galactic clusters, quasars, etc. -- all of
it information uncorrupted by the Earth's atmosphere, which is
the problem for ground based telescopes.
... ... *stellar wind: In general, the term "stellar wind" refers
to the outflow of gas from the surface of a star. The Sun, for
example, loses approximately 10(-14) of its mass each year via
such a wind (solar wind).
... ... *astronomical units: (AU) 1 AU = the mean distance from
the Sun to the Earth = approximately 93 million miles, and
exactly 149,597,870 kilometers.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 24Dec99
[For more information: http://scienceweek.com/search/search.htm]
-------------------
Related Background:
ON PLANETARY NEBULA AND THE DEATH OF STARS
... In a review of planetary nebulae, Sun Kwok (University of
Calgary, CA) makes the following points: 1) During the final
10,000 years of their life, stars with masses up to 8 times the
mass of the Sun pass through a stage in which they produce
planetary nebulae. Such nebulae are not only bright in visible
light, but they are also radio, infrared, and x-ray sources.
Immediately preceding the first planetary nebula formation, there
is a progenitor phase called the "protoplanetary nebula", and
this entity has recently come into its own as a focus of
research. 2) A full-grown planetary nebula is completely ionized
by the UV light from its central hot star. The central star of a
protoplanetary nebula, however, is relatively cool and does not
emit UV radiation, so the nebula is not ionized and shines by
reflected light only (i.e., the protoplanetary nebula is not yet
an emission nebula). 3) The first protoplanetary nebulae were
observed in the 1970s, when it became apparent that many terminal
stage stars are obscured by dust and can be found only by
searching for emissions at mid-infrared wavelengths. More than
2000 mid-infrared stars in our Galaxy were thus identified. 4)
The classification of planetary nebula is based not on appearance
by on their emission spectra. Because planetary nebula expand
with time, their radio surface brightness decreases as the nebula
ages and becomes more diffuse. The youngest planetary nebula are
thus small and radio bright. 5) While all protoplanetary nebula
have similar infrared characteristics, they differ greatly from
one another in their visual brightness. But since protoplanetary
nebula are not ionized, whatever brightness they possess arises
from starlight reflected off the surrounding dust. A bright
central star typically outshines the small faint protoplanetary
nebula, so that protoplanetary nebulae are best identified when
the system is observed edge-on. 6) Planetary nebula often have
bipolar shapes, and the origin of this form has been a focus of
research. In 1978, Kwon et al proposed a stellar fast wind
hypothesis which has been successful in simulations and has had
some observational support. The author concludes: "While
planetary nebulae have been well-known objects for more than 200
years and have fascinated generations of astronomers, the nature
of their immediate progenitors [protoplanetary nebulae] was not
known until recently. At last, we are now filling in this
missing-link in our understanding of stellar evolution."
QY: Sun Kwon, University of Calgary, CA.
(Sky & Telescope October 1998) (Science-Week 4 Sep 98)
-----------
Text Notes:
... ... *red giant: See notes to previous report.
... ... *supergiant star: See notes to previous report.
-------------------
Related Background:
BIRTH AND EARLY EVOLUTION OF A PLANETARY NEBULA
White dwarf stars are extremely dense and compact stars that have
undergone gravitational collapse. White dwarfs are of great
interest to cosmologists, because it is believed their masses and
luminosities have little variance and they can thus be used as
"standard candles" to estimate distances. The final expulsion of
a gas by a star as it forms a planetary nebula (the ionized shell
of gas often observed surrounding a young white dwarf star) is
one of the most poorly understood stages of stellar evolution.
Particularly puzzling is how a spherical star can produce a
highly asymmetric nebula with collimated outflows (outflows
aligned parallel to a particular axis). ... ... Bobrowsky et al
(4 authors at 4 installations, US IN ES) now report optical
observations of the nebula surrounding the star He3-1357 (called
by the authors the "Stingray nebula"), a nebula that has
evidently become an ionized planetary nebula within the past few
decades. The authors find that the collimated outflows are
already evident, and they have identified the nebular structure
that focuses the outflows, and have also found a companion star,
which reinforces previous suspicions that binary companions play
an important role in shaping planetary nebulas and in changing
the direction of successive outflows. The authors suggest the
Stingray nebula demonstrates how far the nebular structure can
develop by the time the nebula becomes ionized, and that no other
planetary nebula in this phase of its evolution has been
previously identified.
QY: Matthew Bobrowsky (mattb@cta.com)
(Nature 2 Apr 98) (Science-Week 24 Apr 98)
[For more information: http://scienceweek.com/search/search.htm]
4. NEUROBIOLOGY: MODIFICATION OF DENDRITIC SPINES BY CALCIUM
Nerve cells (neurons), the cellular units of all nervous
systems, have diverse morphologies within an individual, and a
variety of unique morphologies across species. In general,
however, all neurons have two fundamental anatomic components: a
cell body (soma) and a single long filamentary extension from the
cell body, the so-called "axon" (which may branch along its
length), that propagates electrical activity from the cell body
(or from the vicinity of the cell body) to other locations (e.g.,
to muscle cells or to other nerve cells).
Most neurons also possess a third anatomic component:
extensions of the cell body (few or numerous) that provide
junction points for the axons of other neurons (i.e., provide
surface area for synapses), and thus serve as loci for receiving
inputs. In some neurons, dendrites are extensively branched
(arborized), the single neuron as a whole receiving inputs from
as many as 100,000 other neurons, while at the other extreme
there are neurons with only one dendrite receiving input from
only one or a few other neurons.
During the past several decades, the detailed anatomy of
dendrites has been a focus of much research, in particular the
often-present parts of dendrites called "dendritic spines". These
spines are small (1 to 2 microns) thorn-like protuberances along
the length of a dendrite, and there is evidence that such spines
may be important components in many kinds of neural
microcircuits. In the human nervous system, dendritic spines are
especially prominent in the *cerebellar cortex, *basal ganglia,
and *cerebral cortex, and in the cerebral cortex approximately 80
percent of all *excitatory synapses are evidently made onto
dendritic spines, whereas only approximately 30 percent of
*inhibitory synapses are made onto dendritic spines. Currently,
neurobiologists have ascribed literally dozens of different
functions to dendritic spines, and investigations of these
structures are underway in many laboratories.
... ... Kristen M. Harris (Boston University, US) presents a
commentary on current research concerning the involvement of
calcium ions in dendritic spine morphology, the author making the
following points:
1) Since the discovery of dendritic spines at the turn of
the century by *Ramon y Cajal (1852-1934), these structures have
been of interest to neurobiologists because of their special
morphology and their possible involvement in cognitive functions
such as learning and memory.
2) Dendritic spines have diverse shapes that range more than
100-fold in size. Larger spines have proportionately larger
synapses and more diversity in subcellular organelles and
molecular composition, and such differences may be involved in
functional differences of the synapses located upon spines.
3) Existing data suggest that spines are maintained by
optimal activation: more spines form when neurons have less
excitatory activation, and spines are lost when activation is too
high or when presynaptic axons degenerate. This pattern suggests
that neurons may *homeostatically regulate input by means of the
number of axon-dendritic spine synapses.
4) New evidence by Korkotian and Segal (1999) now points to
the release of calcium ions from intracellular stores as a
possible modulator of dendritic spine structure. When neurons in
culture are exposed to caffeine, calcium is released from the
intracellular stores into dendrites and dendritic spines, and
under these conditions most of the dendritic spines elongate
approximately 33 per cent during the following several hours.
5) The finding that release of calcium from intracellular
stores might have a direct effect on dendritic spine structure is
especially interesting because changes in spine structure have
long been thought to be an important mechanism of memory. The
increase in intraspine calcium caused by release of calcium from
intracellular stores is much less than the increase in intraspine
calcium resulting from high synaptic activity (which decreases
the number of spines), and this suggests that a small increase in
intraspine calcium causes a spine to elongate, whereas high
intraspine calcium from excessive activity causes a spine to
collapse. Thus, spine morphology may be closely regulated by
local intracellular calcium ion concentration.
-----------
Kristen M. Harris: Calcium from internal stores modifies
dendritic spine shape.
(Proc. Natl. Acad. Sci. US 26 Oct 99 96: 12213)
QY: Kristen M. Harris [kmharris@bio.bu.edu]
-----------
Text Notes:
... ... *cerebellar cortex: The cerebellum is a large neural
structure at the base of the brain involved in motor
coordination, posture, and balance. The cortex of the cerebellum
is a thin corrugated outer layer containing cell bodies of
cerebellar neurons.
... ... *basal ganglia: The term "basal ganglia" refers to a
group of nuclei lying deep in the subcortical white matter of the
frontal lobes, these nuclei involved in the organization of motor
behavior. In this context, a "nucleus" is a cluster of nerve
cells.
... ... *cerebral cortex: The cerebral cortex is a thin surface
layering of nerve cells of the brain, the region only several
millimeters thick but covering all of the brain surface. This is
the part of the central nervous system most intimately involved
with the so-called "higher faculties", although the cortex
operates in concert with other parts of the brain. The structure
is primitive in lower mammals, and is found progressively more
pronounced and with greater surface area in primates and man.
... ... *excitatory synapses: A synapse which when activated
produces excitation of the postsynaptic nerve cell.
... ... *inhibitory synapses: A synapse which when activated
produces inhibition of the postsynaptic nerve cell.
... ... *Ramon y Cajal (1852-1934): Santiago Ramon y Cajal is one
of the more important historical figures in neurobiology. His
specialty was the histology of the nervous system as revealed by
cellular staining. By 1889 he worked out the connections of the
cells of the gray matter of the brain and spinal cord with a
demonstration of the extreme complexity of the system. He also
worked out the structure of the retina of the eye. He established
the "neuron theory", which postulated the nervous system to
consist entirely of individual nerve cells and their processes.
In 1906, Ramon y Cajal shared the Nobel Prize in Physiology and
Medicine with Camillo Golgi (1843-1926), whose stains for the
nervous system were used by Ramon y Cajal in his work.
... ... *homeostatically regulate: The term "homeostasis" refers
to a physiological equilibrium necessary in general for the
viability of an organism, and in particular for the operation of
many cellular functions. Homeostatic mechanisms in biological
systems usually involve an element of negative feedback
signaling. In vertebrates, for example, when blood temperature is
too high, temperature receptors provoke a sequence of events
involving many pathways that ultimately results in a lowering of
body temperature. Similar homeostatic mechanisms operate at
cellular levels.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 24Dec99
[For more information: http://scienceweek.com/search/search.htm]
-------------------
Related Background:
NEUROBIOLOGY: DENDRITE MORPHOLOGY INDUCED BY SYNAPTIC ACTIVITY
Viewed as a computing machine analog, the developing brain
consists of a large number of elements (neurons) engaged in
various mechanisms of self-wiring. To the degree that brain
programs are hard-wired, this self-wiring during development (and
apparently after gross development is completed) constitutes
programming of brain functions. "Self-wiring" in the brain is the
biological production of new or increased connections between
neurons, either as a result of genetically programmed biochemical
events, or as a result of functional activity of the neurons
already connected or about to be connected ab initio. These two
aspects constitute two major investigative areas in developmental
neurobiology, each with a long history of intensive research.
Concerning new connections as a result of neuron activity, one
important question is what changes in morphology, if any, occur
in the various parts of nerve cells in response to input from
other nerve cells. Again, viewing the brain as a computing
machine analog, at this level the question concerns how the input
history of a computing element (e.g., the input history of a
transistor) changes the physical architecture (and thus the
input-output behavior) of that element. Neurons, the biological
elements, contain 3 unique structural features that differentiate
them from other biological cells: the "axon", which is
specialized for intracellular information transfer over a
relatively long distance (e.g., impulse propagation); one or more
"dendrites", which are often the sites at which information is
received from other neurons; and the "synapse", which is the
point of information transfer between neurons. In general,
dendrites are neuronal processes that tend to be thicker and much
shorter than axons. Dendrites are often highly branched, giving
rise to a dense network of processes. In certain types of
neurons, close optical examination of dendrites reveals the
presence of numerous finger-like projections or thickenings
(dendritic spines) arising from the main shaft of the dendrite,
and each spine is apparently a synaptic input site at which the
neuron receives information from another nerve cell. Relatively
little is known about the role of activity in the development of
dendritic morphology, and the studies that have been done have
produced inconsistent results. In the mammalian brain, dendrites
develop in a stereotyped sequence: Soon after birth the
relatively smooth dendrites of neonates sprout numerous thin
filamentous extensions of the cell surface ("filopodia-like
protrusions") which are later replaced by dendritic spines as the
brain matures. ... ... M. Maletic-Savatic et al (3 authors at
Cold Spring Harbor Laboratory, US) now report a study using
*vital imaging of dendrites in *living rat brain slices to
directly observe dendritic morphogenesis evoked by synaptic
activity. The authors report their results indicate that synaptic
activity can produce rapid growth in *postsynaptic dendrites.
Growth was input specific, occurring only close to activated
parts of the dendrite, and required synaptic *NMDA receptor
activation. This activity-induced growth was most prominently
expressed as an increase in long thin filopodia-like protrusions.
The authors suggest that if such structures develop into
dendritic spines and synapses, they will have greater likelihood
of connecting with presynaptic axons that were active during the
synaptic stimulus, providing a mechanism for *synaptic plasticity
satisfying *Hebbian rules. The authors suggest such a mechanism
could play a role in the establishment of functional neural
circuits during development and memory storage.
-----------
M. Maletic-Savatic et al: Rapid dendritic morphogenesis in CA1
hippocampal dendrites induced by synaptic activity.
(Science 19 Mar 99 283:1923)
QY: M. Maletic-Savatic, Cold Spring Harbor Laboratory, Cold
Spring Harbor, NY 11724 US.
-----------
Text Notes:
... ... *vital imaging: The visualization technique in this
report involved the infection of neurons in cultured rat brain
slices (CA1 hippocampus pyramidal neurons) by a neurotropic virus
genetically engineered to express a green fluorescent protein.
Living infected neurons were visualized with a custom-built laser
scanning microscope, the infected neurons exhibiting bright
homogeneous fluorescence throughout their dendritic and axonal
branches, with detailed morphology revealed.
... ... *living rat brain slices: "Rat brain slices" are exactly
that, the rat brain removed from the animal and a thin slice of a
particular region prepared in an appropriate solution for
electrophysiological recording of nerve cell activity.
... ... *postsynaptic dendrites: The neuron attachments
associated with a synapse are denoted as follows: the input
attachment is called "presynaptic"; the output attachment is
called "postsynaptic".
... ... *NMDA receptor: (N-methyl-D-aspartate receptor) This is a
type of glutamate receptor. Glutamate is a major excitatory amino
acid neurotransmitter, accounting for an estimated 40 percent of
all nerve signals in the mammalian brain, and involved in
phenomena such as neural development, learning, and memory
formation. A glutamate receptor is a molecular site that mediates
the actions of glutamate neurotransmitters. In this report, the
significance of the requirement of NMDA activation is that it
suggests that the dendrite morphological changes were due to
actual synaptic activity rather than to the electrical fields
produced by electrical stimulation of the tissue. Thus, chemical
blockade of NMDA receptors prevented the morphological changes in
response to electrical stimulation.
... ... *synaptic plasticity: In general, the term "synaptic
plasticity" refers to the changeability of neuronal synaptic
connections, usually as a result of ongoing neural activity.
... ... *Hebbian rules: In 1949, psychologist D.O. Hebb
hypothesized that coordinated activity of a presynaptic terminal
and a postsynaptic neuron would strengthen the synaptic
connections between them. In general, Hebb's postulate implies
that synaptic terminals strengthened by correlated activity will
be retained or sprout new branches, whereas those terminal
connections that are persistently weakened by uncorrelated
activity will eventually lose their hold on the postsynaptic
cell. Another form of the Hebb postulate is as follows: When a
postsynaptic neuron becomes depolarized, it generates a
biochemical reaction or a trophic factor that stabilizes the
excitatory synapses that are firing at that time. Hebb's
postulate has been found useful in explaining various
neurophysiological experimental observations related to learning
and memory.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 11Jun99
[For more information: http://scienceweek.com/search/search.htm]
5. PALEOANTHROPOLOGY: DATING OF LATE PLEISTOCENE HOMINID REMAINS
The term "hominid", in general, refers to any primate in the
human family. The Pleistocene period is the time frame 2.5
million years ago to 11,000 years ago, an epoch of rapid hominid
evolution, and the appearance of cattle and the modern horse. One
continually intriguing aspect of human evolution during the
Pleistocene period concerns the hominid group known as the
"Neandertals" (Neanderthals) (see related background reports
below). The nature of the biological relationship between the
Neandertals and early modern humans has for some time been a
highly contentious issue in paleoanthropology, with fundamental
questions having changed little since the initial Neandertal
discoveries during the second half of the 19th century. These
questions concern the taxonomy, phylogenetic position, and
"humanness" of the Neandertals. In other words, where do the
Neandertals fit into the scheme of human evolution, and how
closely related in culture were they to so-called modern humans?
... ... F.H. Smith et al (5 authors at 5 installations, US FR UK
HR) now report new *accelerator mass spectrometry radiocarbon
dates on human remains from the Late Pleistocene sites of Vindija
and Velika Pecina in the Hrvatsko Zagorje region of Croatia. The
authors state that hominid specimens from both sites have played
critical roles in the development of current ideas concerning the
emergence of modern humans in Europe. The three fossils that were
successfully dated, two Neandertal specimens and one modern human
specimen, have previously been believed to be approximately
contemporaneous at approximately 30,000 years before the present,
providing support for a temporal overlap of Neandertals and
modern humans in a single geographic region. The authors now
report the dating of the two Neandertal specimens (from Vindija)
at approximately 28,000 and 29,000 years before the present, and
the dating of the human specimen (from Velika) at approximately
5000 years before the present. The authors suggest their results
render the human specimen "no longer pertinent to discussions of
modern human origins." The authors conclude: "Apart from
invalidating the only radiometrically based example of temporal
overlap between late Neandertal and early modern human fossil
remains from within any region of Europe, these dates raise the
question of when early modern humans first dispersed into Europe
and have implications for the nature and geographic patterning of
biological and cultural interactions between these populations
and Neandertals."
-----------
F.H. Smith et al: Direct radiocarbon dates for Vindija G(sub1)
and Velika Pecina Late Pleistocene and hominid remains.
(Proc. Natl. Acad. Sci. US 26 Oct 99 96:12281)
QY: Fred H. Smith [fsmith@niu.edu]
-----------
Text Notes:
... ... *accelerator mass spectrometry radiocarbon dates: The
accelerator mass spectrometry dating method makes use of smaller
quantities of material than required for conventional radiocarbon
dating and extends the radiocarbon dating range to beyond 50,000
years from the approximately 0 to 25,000 years for conventional
radiocarbon dating. Carbon-14 decays by beta particle emission to
nitrogen-14 with a half-life of 5730 years. In a living system
the ratio of carbon-14 to carbon-12 is maintained constant by
continual incorporation of new carbon-14 lost by decay. When the
organism dies, the renewal stops and the residual fraction of
carbon-14 in the organism decays radioactively. The technique of
accelerator mass spectroscopy involves the combination of a mass
spectrometer and an accelerator to measure the concentration of
rare isotopes such as carbon-14 at levels lower than parts per
trillion. An accelerator mass spectrometer can be used to count
the carbon-14 atoms from only a milligram sample of carbon,
whereas the conventional radiocarbon dating method would require
as much as a gram of the same material to achieve the same
resolution by counting current beta particle emission.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 24Dec99
[For more information: http://scienceweek.com/search/search.htm]
-------------------
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."
-----------
I. Tattersall and J.H. Schwartz: Hominids and hybrids: The place
of Neanderthals in human evolution.
(Proc. Natl. Acad. Sci. US 22 Jun 99 96:7117)
QY: Ian Tattersall [iant@amnh.org]
-----------
Text 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: 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, France.
Mousterian flakes could be used for many purposes, including
cutting flesh, scraping hides, and working wood.
... ... *Levallois technique: 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.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 3Sep99
-------------------
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. 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. 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."
-----------
G.A. Clark: Highly visible, curiously intangible.
(Science 26 Mar 99 283:2029)
QY: G.A. Clark [gaclark@asu.edu]
-------------------
Summary by SCIENCE-WEEK [http://scienceweek.com] 28May99
-------------------
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."
-----------
Paul Mellars (University of Cambridge, UK)
The fate of the Neanderthals.
(Nature 8 Oct 98 395:539)
-------------------
Summary by SCIENCE-WEEK 6Nov98
-------------------
Related Background:
FIRST ANALYSIS OF DNA FROM A NEANDERTHAL BONE
... For more than a century, one of the central questions in
paleoanthropology has been whether modern man evolved from this
race [the Neanderthals] -- or was the Neanderthal a separate
branch that became extinct? Until recently, the primary
laboratory method of investigation of such a question was
analysis of the morphology of bone fragments. This week, the
field of paleoanthropology has apparently crossed an important
watershed, as M. Krings et al (University of Munich, DE;
Pennsylvania State University, US) report the first analysis of
DNA from an extinct human, in this case DNA extracted from the
actual Neanderthal skeleton found near Dusseldorf in 1856. The
key to the investigation was the analysis of mitochondrial rather
than nuclear DNA. Mitochondrial DNA is usually present in
concentrations two or three orders of magnitude greater than
nuclear DNA, and they were able to find enough of it still intact
to amplify with the PCR technique and piece together a total DNA
sequence of 379 base pairs. Comparison of this sequence with
contemporary human sequences leads to the conclusion that
Neanderthal and modern man are separate evolutionary lines, and
that the latter did not evolve from the former. The work will
have to be replicated with other Neanderthal fossils, but most
paleoanthropologists are excited by the results and expect them
to be confirmed. The technology of evolutionary paleoanthropology
has evidently now progressed from caliper measurements of bones
to measurements of bone DNA fragments. (Cell 11 July)
(Science-Week 18 Jul 97)
-------------------
Related Background:
A POSSIBLE COMMON ANCESTOR OF NEANDERTHAL AND HOMO SAPIENS
The reconstruction of human evolutionary history is a slow and
halting process, the evidence scattered and fragmentary. But
paleoanthropologists persist. They gather the evidence, when they
can find it, and at intervals they conclude it necessary to
redraw the theoretical lines of evolution leading to modern man.
This week, from Spain, there is a suggestion that once again the
lines need to be redrawn. Antonio Rosas et al [National Museum of
Natural Sciences, Madrid ES; Complutense University of Madrid
(ES); Rovira i Virgili University, Tarragona (ES)] have presented
evidence from a find of 80 human fossil remains recovered between
1994 and 1996 in the Pleistocene cave site of Gran Dolina, Sierra
de Atapuerca, Burgos (ES). Included among the fossils is that of
an adolescent male, the bones approximately 800,000 years old,
with facial features a mix of both Neanderthal and modern
aspects. The Spanish team has named this a new species, Homo
antecessor, and they suggest the simplest explanation for the
various skeletal attributes is that H. antecessor is the common
ancestor of both H. neanderthalensis and H. sapiens, and that the
evolutionary lines need to be redrawn to show these two as
separate branches from the H. antecessor stem. Not all
paleoanthropologists agree, primarily because the basis for the
introduction of a new species into the evolutionary gallery is
the partial fossil of a single immature individual, in this case
a boy 10 or 11 years old. Despite the disagreement, what is
certain is that the evidence will be the focus of attention among
paleoanthropologists for some time.
(Science 30 May) (Science-Week 5 Jun 97)
6. EVOLUTIONARY BIOLOGY: TWO SPECIES OF LIVING COELACANTHS
The discovery of a single living coelacanth off the coast of
South Africa in 1938 came as a surprise to the scientific
community because the fish is a member of a lineage that was
thought to have become extinct approximately 80 million years
ago. In 1952, it was determined that the home of this single
coelacanth (species: Latimeria chalumnae) was in the Comores
Islands in the western Indian Ocean, and since that time more
than 200 coelacanths have been captured in the vicinity of these
islands. By 1994, the population of coelacanths in the Comores
was thought to have dwindled to a few hundred animals. Then, in
1998, a coelacanth was discovered off the coast of Manado Tua
Island, Sulawesi, Indonesia -- the first coelacanth recorded from
a location outside the western Indian Ocean. Interviews with
Indonesian fishermen have revealed a history of catches of
coelacanths in the north Sulawesi area, and these facts and the
general pattern of ocean current flow from north Sulawesi toward
the Comores have implied that the Indonesian specimens cannot be
strays from the Comoran population.
... ... M.T. Holder et al (5 authors at 2 installations, US) now
report a molecular biological analysis of *mitochondrial DNA from
the Indonesian specimen. The authors report they have obtained
the sequence of 4823 nucleotide base pairs of mitochondrial DNA
from this fish, and that the Comoran coelacanth (L. chalumnae)
and the Indonesian coelacanth (recently described as a new
species, L. menadoensis) indeed appear to be separate species
based on divergence of mitochondrial DNA. The authors suggest
that given the apparent biological and geological restrictions to
dispersal of coelacanths, it is perhaps not surprising that the
Comoran and Indonesian populations represent distinct
evolutionary lineages of Latimeria. The authors conclude: "This
finding clearly does not preclude the existence of further living
species of coelacanths in the area between Sulawesi and the
Comores or elsewhere, and genetic studies of any such population
will help unravel the mysteries of coelacanth distribution and
evolution."
-----------
M.T. Holder: Two living species of coelacanth?
(Proc. Natl. Acad. Sci. US 26 Oct 99 96:12616)
QY: Mark T. Holder [mtholder@mail.utexas.edu]
-----------
Text Notes:
... ... *mitochondrial DNA: Mitochondria are double-membrane
enclosed organelles of cells that are involved with several
important biochemical pathways, including electron transport and
oxidative metabolism. Various types of eukaryotic cells may
contain from a few to several thousand mitochondria in each cell
type. The mitochondria are relatively large cylindrical
structures up to 10 microns long and up to 2 microns in diameter,
and they are believed to have originated as organisms that became
symbiotic with eukaryotic cells (i.e., cells containing
membrane-bound organelles such as a nucleus). In biology,
"symbiosis" is an intimate and protracted association of
individuals of different species. Mitochondrial DNA (sometimes
denoted as mtDNA), found in the mitochondria of all eukaryotes,
is believed to evolve in parallel with nuclear DNA, but since
sperm lose their mitochondria, it is inherited only in the
maternal lineage in animals.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 24Dec99
[For more information: http://scienceweek.com/search/search.htm]
-------------------
Related Background:
EVOLUTION: A NEW COELACANTH DISCOVERY
The coelacanth is a primitive *teleost fish that apparently first
appeared in the *Devonian Period, with the most recent fossil
specimens dating from the *Cretaceous Period. The group was
assumed to be extinct, but a living specimen was caught in 1938
in the mouth of the Chalumna River in South Africa and named
Latimeria chalumnae. The 1938 specimen was 2 meters in length and
weighed 40 kilograms. More specimens have since been caught, all
off the coast of South Africa and near the island of Madagascar
(Comoros archipelago) in the Indian Ocean.
... ... M.V. Erdmann et al now report the capture and observation
of a live coelacanth specimen near the island of Manado Tua
(north Sulawesi) in Indonesia on 30 July 1998. The Indonesian
specimen was caught in a net at a depth of 100 to 150 meters. The
specimen is 1.24 meters in length and weighs 29.2 kilograms, and
the authors report it was observed by them live for more than 3
hours before the carcass was deep frozen and tissue samples
collected for molecular analysis. The authors suggest that
interviews with the local fishermen, and the vast distance from
the Comoros archipelago to Indonesia (approximately 10,000
kilometers), strongly support the idea that the Indonesian
coelacanths (which are evidently well known in the region and
called "raja laut" or King of the Sea) are part of an established
north Sulawesi coelacanth population and not simply strays. Like
its Indian Ocean counterpart, the Indonesian coelacanth was found
in the vicinity of oceanic volcanic caves, its presumed habitat.
... ... In a companion article concerning the M.V. Erdmann et al
paper, P. Forey makes the following points: 1) Before the 1938
discovery, the date of the youngest coelacanth fossil recovered
was approximately 80 million years ago. This raises the question
of how the lineage survived for that time without leaving any
trace in the fossil record. 2) The theory of relationships
prevalent in the late 1930s held that the coelacanth was a direct
descendant of Devonian fish-like ancestors of land-living
vertebrates (tetrapods), but from modern evolutionary analysis it
seems that coelacanths are more distantly related to land-living
vertebrates, and that lungfishes are the closest living relatives
to the tetrapods. 3) Significant in the coelacanth are paired
fins that move in a manner unlike that seen in most fishes but in
a manner identical to limb movement in vertebrates. There are
also sensory structures in the coelacanth that are apparently
precursors of structures responsible for hearing in air. The
author concludes: "The new discovery underlines how little we
really know about the coelacanth in particular and oceanic life
in general."
-----------
M.V. Erdmann et al (2 installations, US ID)
Indonesian "king of the sea" discovered.
(Nature 24 Sep 98 395:335)
QY: Mark V. Erdmann, Univ. of Calif. Berkeley 510-642-6000.
-----------
P. Forey (Natural History Museum London, UK)
A home from home for coelacanths.
(Nature 24 Sep 98 395:319)
QY: Peter Forey
-----------
Text Notes:
... ... *teleost: In general, this refers to any of the bony
fish, the most advanced in terms of evolution and the largest
group of fish. Besides the calcified internal skeleton, the most
obvious uniform characteristic of the teleost fish is their tail,
with upper and lower halves of about equal size, whereas in
cartilaginous fish the tail has two lobes of unequal size. Almost
all sport, commercial, and ornamental fish are teleosts.
... ... *Devonian Period: From approximately 400 million to 345
million years ago. Sometimes called the Age of the Sea, since
more of the Earth was underwater than is now.
... ... *Cretaceous Period: The geological period ranging
approximately from 146 million years ago to 65 million years ago.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 16Oct98
[For more information: http://scienceweek.com/search/search.htm]
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
IN FOCUS: ON THE FOUNDATIONS OF QUANTUM MECHANICS
"The foundational aspects of physics during the first half of the
20th century have been principally concerned with the
characterization of the "elementary" constituents of matter and
the elucidation of the nature of the space-time framework in
which their interactions take place. The discovery of the
electron by Thomson, the precise characterization of its charge
by Millikan, the demonstration of the nuclear atom by Rutherford,
the photon hypothesis of Planck and Einstein, and Bohr's
explanation of the spectrum of hydrogen were some of the
landmarks of that history. These early efforts culminated in the
mid-twenties with the formulation of quantum mechanics by
Heisenberg, Dirac, and Schroedinger. The revolutionary
achievements in the period 1925 to 1927 stemmed from the
confluence of a theoretical understanding (the description of the
dynamics of microscopic particles by quantum mechanics) and the
apperception of an approximately stable ontology (electron and
nuclei). Approximately stable meant that these particles
(electrons, nuclei), the building blocks of the entities (atoms,
molecules, simple solids) that populated the domain that was
being carved out, could be treated as ahistoric objects (whose
physical characteristics were seemingly independent of their mode
of production and whose lifetimes could be considered as
essentially infinite). These entities could be assumed to be
"elementary" pointlike objects that were specified by their mass,
spin, and statistics (whether bosons or fermions), and by
electromagnetic properties such as their charge and magnetic
moment. Quantum mechanics came to be seen as correctly describing
that domain of nature delineated by Planck's constant (h): any
system whose characteristic length (l), mass (m), and time (t)
were such that the product ml^(2)/t was of the order h, and such
that l/t was much smaller than c, the velocity of light, was
quantum mechanical and was to be described by the new
nonrelativistic quantum mechanics. Quantum mechanics reasserted
that the physical world presented itself hierarchically. The
world was not carved up into terrestrial, planetary, and
celestial spheres, but was layered by virtue of certain constants
of nature... Planck's constant allows the world to be parsed into
microscopic and macroscopic realms."
-----------
Silvan S. Schweber: _QED and the Men Who Made it: Dyson, Feynman,
Schwinger, and Tomonaga_
(Princeton University Press, Princeton 1994, p.xxi)
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
NOTICES
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
CHANGE OF EMAIL ADDRESS: If at any time you need to change the
Email address at which you receive SW, please send the
information to [request@scienceweek.com], and the change will be
made and confirmed the same day.
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
SCIENCE-WEEK SUBSCRIPTIONS: The subscription rate for ScienceWeek
(52 issues per year delivered via Email only) is US$20 for one
year. Subscriptions can be obtained with a credit card
(Visa, MC, Amex) at a secure website form accessed at:
http://scienceweek.com/subinfo.htm
Information concerning other methods of payment is available at
the above URL, or via Email at swsub@scienceweek.com
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
The first issue of SCIENCE-WEEK appeared May 1, 1997, and it has
been published regularly each week since that date. SW is
designed to cross existing conceptual and linguistic barriers
between the scientific disciplines. In general, the biology is
written for physicists and chemists, and the physics and
chemistry are written for biologists, with an attempt to retain
some exactitude in the particular science involved in the news.
These are the aims. Undoubtedly, we are not always successful,
and for that we apologize. In any case, what we hope is that our
readers are reading out of their fields more than in their
fields, since that is the essence of this publication.
We welcome comments, suggestions, and criticisms from our
subscribers. Public letters relevant to any report are also
welcome. Editorial contact: [editors@scienceweek.com].
Editor/Publisher: Dan Agin
Managing Editor: Claire Haller
Associate Editor: Joan Oliner
Copyright (c) 1997-1999 SCIENCE-WEEK/Spectrum Press Inc.
All Rights Reserved
---------------------------------------------
This publication is protected by U.S. and International Copyright
Laws, and no display, transmission, or duplication in any medium,
including BBS, Internet Email, website duplication, fax, or print
is permitted without the explicit consent of the holder of the
copyright. SCIENCE-WEEK is published by Spectrum Press Inc.,
3023 N. Clark Street #109, Chicago, 60657-5205 IL, USA.
---------------------------------------------
|