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ScienceWeek - August 9, 2002 Vol. 6 Number 32
An Online Research Digest Published Weekly Since 1997
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Section 1
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1. Spotlight: Origin of Supposed Biogenic Magnetite in a
Meteorite
2. Spotlight: On Scientific Terminology
3. Clusters and Coulomb Explosion
4. Mammalian Sperm-Egg Interactions
5. On Axon Growth Cones
6. On the E. Coli Genome
7. Spontaneous Assembly of Ordered Nanocluster Arrays
8. On the Spin-Galvanic Effect
9. Edwin Land (1909-1991) and Color Vision
10. On Biofilms
11. On Cat-Scratch Disease
12. Children and Aids in Africa
13. In Focus: On the Emergence of Modern Human Behavior
14. ScienceWeek Notices and Subscription Information
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Section 2
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1. SPOTLIGHT: ON THE ORIGIN OF SUPPOSED BIOGENIC MAGNETITE IN A
METEORITE
D.J. Barber and E.R. Scott (Cranfield University, US) discuss
the Martian meteorite AH84001, the authors making the following
points:
1) Nanocrystals of magnetite [Fe(sub3)O(sub4)] inside
3.9-Gyr-old(1) carbonate in Martian meteorite Allan Hills (ALH)
84001 contain a record of the ancient Martian magnetic field (2)
and possible evidence for fossil Martian life(3). Two recent
studies have advanced the claim that the magnetites were made by
microorganisms. Thomas-Keprta et al.(4) used transmission
electron microscopy (TEM) to study magnetites obtained by
dissolving carbonates in acid. They found that about 25% were
shaped like elongated prisms (later called "truncated
hexa-octahedrons"(5), and had sizes (4-100 nm), shapes,
structures, and compositions that appeared to be
indistinguishable from those produced by one strain of
magnetotactic bacteria. Because terrestrial inorganic magnetites
have very different properties, they inferred that the Martian
magnetites were biological in origin and constitute evidence of
the oldest life yet found. In the second study, Friedmann et al.
(2001) examined broken rock surfaces and identified what they
interpreted as chains of magnetite crystals like those in modern
magnetotactic bacteria.
2) Both these claims for fossil life have been disputed. Buseck
et al. (2001) questioned the identification of magnetite by
Friedmann et al. (2001) and noted that intact magnetite chains
from magnetotactic bacteria were unlikely to survive in rocks.
Buseck et al. (2001) found that magnetites from three different
strains of bacteria had different shapes and criticized the
techniques used by Thomas-Keprta et al. (4, 5) to select
magnetite crystals and to study their shapes. They also argued
that the crystal size distributions of bacterial and meteoritic
magnetites appeared to be different. Although Buseck et al.
(2001) recognized that the similarities between the Martian and
bacterial magnetites are intriguing, they, like Treiman (2001)
inferred that strong evidence for biogenic Martian magnetites
was lacking. Buseck et al. (2001) called for more detailed
studies of the shapes of the Martian magnetite crystals to
resolve this issue.
3) Arguments against a biogenic origin for magnetites in ALH
84001 were previously advanced by Bradley et al. (1996,1998).
Their TEM studies showed that whisker-like magnetites had grown
epitaxially on a carbonate fracture surface so that two crystal
lattices were closely aligned at their interface. They concluded
that these magnetites had condensed from a vapor above 120°C.
However, Thomas-Keprta et al(4) pointed out that the
whisker-like magnetites, which comprised 7% of their sample,
were found only in the interiors of carbonate grains. They
inferred that the elongated prismatic magnetites, which they
suggested were biogenic, are largely located in the optically
opaque rims where epitaxial relationships are not observed. Four
other origins have been suggested for some or all of the
magnetite in ALH 84001: deposition of abiogenic grains that
formed elsewhere (4, 5), thermal oxidation of sulfides,
precipitation from aqueous solutions, and in situ formation
because of shock-induced, thermal decomposition of carbonate.
4) The authors report transmission electron microscope studies
of the ALH 84001 magnetites and their associated carbonate,
silicate, oxide, and sulfide minerals in situ. In magnesian
carbonate, periclase occurs as aggregates of crystals (grain
size 3 nm) that are preferentially oriented with respect to the
carbonate lattice. Larger periclase crystals 50 nm in size are
commonly associated with voids of similar size. Periclase
clearly formed by precipitation from carbonate as a result of
partial decomposition and loss of CO(sub2). Magnetite occurs in
more ferroan carbonate, and, like periclase, it is associated
with voids and microfractures and the two oxides may be
intermixed. Magnetite nanocrystals that are commonly euhedral
and entirely embedded in carbonate are topotactically oriented
with respect to the carbonate lattice, showing that they formed
as solid-state precipitates. Magnetites in Fe-rich carbonate
rims are not well oriented. These magnetites are generally more
irregular in shape and diverse in size than the euhedral
variety. The authors conclude: "All occurrences of magnetite and
periclase are entirely consistent with in situ growth by
solid-state diffusion as a result of carbonate decomposition
during impact heating. Biogenic sources should not be invoked
for any magnetites."
References (abridged):
1. Borg, L. E. et al (1999) Science 286, 90-94
2. Weiss, B. P. et al (2001) Science 290, 791-95
3. McKay, D. S. et al (1996) Science 273, 924-930
4. Thomas-Keprta, K. L. et al (2000) Geochim. Cosmochim. Acta
64, 4049-4081
5. Thomas-Keprta, K. L. et al (2001) Proc. Natl. Acad. Sci. USA
98, 2164-2169
Proc. Nat. Acad. Sci. 2002 99:6556
Web Links: meteorite ALH84001 Martian meteorites
Related Background:
ON BIOSIGNATURES IN A MARTIAN METEORITE ALH84001
On August 6, 1996, a team of scientists, a group headed by David
Mckay at the US National Aeronautics and Space Administration
(NASA), held a press conference to announce they had found
evidence of former life on Mars in a Martian meteorite. The
meteorite was part of a collection made in 1984 at Allan Hills
in Antarctica and has the name ALH84001 [*Note #1]. About the
size of a potato, this meteorite weighs 1.9 kilograms. Its
Martian origin was first recognized in 1993, and the material of
the meteorite has been dated as crystallized 4.5 billion years
ago, with the meteorite ejected from Mars 14.4 million years
ago, and the meteorite resident in Antarctic ice for 13,000
years. The meteorite consists largely of orthopyroxenite, which
is an *igneous rock comparable to *basalt.
The 1996 claim of evidence of former life in this meteorite was
based on apparent fossilized "nannobacteria" [see related
background material below], and was not widely accepted by
either the paleobiology community or by other scientific groups
at NASA. From the beginning, it has been known that carbonate
globules found in this meteorite have dark margins of abundant
tiny crystals of the form of iron oxide known as "magnetite".
Comparable magnetite crystals are produced by some bacteria
(e.g., Magnetospirillum magnetotacticum), which apparently use
them for orientation within Earth's magnetic field. The crystals
of these bacteria are similar to the magnetic crystals of the
ALH84001 meteorite, and the McKay group has been claiming for
several years that these magnetite crystals are further evidence
of former life in the meteorite.
Now two contiguous reports of analyses of these magnetite
crystals have appeared, one report from the McKay group and the
other report from another group, both reports concluding that
the crystals are of biological origin.
K.L. Thomas-Keprta et al (10 authors at 6 installations, US CA)
present an analysis of a subpopulation of magnetite
[Fe(sub3)O(sub4)] present in abundance within the iron-rich rims
of the carbonate globules considered previously to be the fossil
remains of Martian microbes. The authors make the following
points:
1) The authors report these Martian magnetites to be both
chemically and physically identical to terrestrial, biogenically
precipitated, intracellular magnetites produced by magnetotactic
bacteria (strain MV-1). Specifically, both magnetite populations
are single [magnetic] domain and chemically pure crystals, and
exhibit a unique crystal form the authors call "truncated
hexa-octahedral".
2) The authors point out there are no known reports of inorganic
processes to explain the observation of truncated
hexa-octahedral magnetites in a terrestrial sample, and that in
bacteria of strain MV-1 their presence is likely a product of
natural selection. Unless there is an unknown and unexplained
inorganic process on Mars that is conspicuously absent on the
Earth and forms truncated hexa-octahedral magnetites, the
authors suggest that these magnetite crystals in the Martian
meteorite ALH84001 were likely produced by a biogenic process,
and that these crystals are Martian magnetofossils and
constitute evidence of the oldest life yet found.
E.I. Friedmann et al (4 authors at 4 installations, US ES DE)
present an analysis of chains of magnetite crystals in the
meteorite ALH84001, the authors making the following points:
1) The authors report the presence of magnetite crystal chains
in the meteorite is demonstrated by high-power stereo
*backscattered scanning electron microscopy. Five
characteristics of such chains (uniform crystal size and shape
within chains, gaps between crystals, orientation of elongated
crystals along the chain axis, flexibility of chains, and a halo
that is a possible remnant of a membrane around chains),
observed or inferred to be present in magnetotactic bacteria but
incompatible with a nonbiological origin, are shown to be
present.
2) The authors conclude: "We suggest the following scenario:
First, decomposed remains of dead magnetobacteria suspended in a
carbonate-rich fluid penetrated fissures of ALH84001, already
crushed by previous asteroid impact... Second, evaporation of
the liquid led to the deposition of pancake-shaped carbonate
globules, and magnetite crystals and chain fragments were
deposited in the periphery of the carbonate discs, perhaps
through the mechanism for ring deposition of particles dispersed
in liquid drops."
[ScienceWeek Editor's note: The response of the astrobiology and
paleobiology communities to these reports has for the most part
apparently been skepticism. Indeed, it is difficult to support
an interpretation of a biological origin of the Martian
magnetite crystals on the primary grounds that no contrary
evidence for such inorganic crystal formation yet exists. Future
research will demonstrate whether or not such crystals can be
formed by nonbiological processes. The same is true of the
apparent magnetite "chains" in the meteorite. Commenting several
years ago on the controversy concerning claims of fossils of
former life in the Martian meteorite ALH84001, Malcolm Walter
said: "What we are observing is the messy business of science in
action at the frontiers of knowledge. In many ways, there is
nothing unusual about it, except that it is happening in a very
public way." In addition, there is the old adage that has been
stated and restated in many forms: "Extraordinary claims require
extraordinary evidence." As one paleobiologist recently
remarked, perhaps no one will be satisfied until a Martian
meteorite is found to contain a clamshell.]
Proc. Natl. Acad. Sci. US 2001 98:2164,2176
Malcolm Walter: _The Search for Life on Mars_. Perseus Books,
New York 1999, p.91
Text Notes:
... ... *Note #1: The number of meteorites found in Antarctica
is as great as in the rest of the world combined: a)
dark-colored rocks are readily visible in the Antarctic ice-cap;
b) the ice sheet concentrates moveable rocks in small areas.
... ... *igneous rock: Igneous rocks are rocks that have
congealed from a molten mass.
... ... *basalt: Basalt is a dark gray to black igneous rock of
volcanic origin that cools rapidly. It is found as basement rock
on land, and on sea floor spreading from mid-ocean ridges.
... ... *backscattered: In general, in this context,
"backscattering" is the deflection of radiation by scattering
processes through angles greater than 90 degrees with respect to
the original angle of travel.
Related Background:
EVIDENCE SUPPORTING POSSIBILITY OF LOW-TEMPERATURE LIFE-BEARING
METEORITES
In 1984, a 1.9 kilogram meteorite the size of a potato
(designated ALH84001) was found in Antarctica, and because of
its chemical composition the consensus is that this meteorite
(and a dozen similar meteorites) originated from the planet
Mars. The basis for the consensus is the detailed quantitative
correspondence of the trapped gases in the meteorites to Martian
atmospheric gases, and the specific distributions of oxygen
isotopes. In 1996 a group of researchers, D. McKay et al
(National Aeronautics and Space Administration Johnson Space
Center, US; Stanford University, US) reported they had concluded
that unusual characteristics of the meteorite ALH84001 can be
most reasonably interpreted as vestiges of ancient Martian
bacterial life. In particular, the authors noted the presence of
tubules 20 to 40 nanometers in diameter (called by some
"nannobacteria" or "nanobacteria"), and they proposed these
structures were fossilized bacteria or parts of microorganisms.
The report was first delivered at a press conference in August
1996 (published as a paper 9 days later) and provoked
considerable media attention and controversy when it appeared.
The controversy has continued, with many biologists objecting to
the interpretation of the rock data, and in particular objecting
to the idea of "bacteria" 20 to 40 nanometers in diameter.
B.P. Weiss et al (7 authors at 3 installations, US CA) present
the results of analysis of images of the magnetic field of the
Martian meteorite ALH84001, the authors making the following
points:
1) The authors point out that large-body impacts are the only
known natural processes capable of ejecting a rock from Mars. It
has been suggested that some rocks could be ejected without
being shocked and heated, and laboratory experiments have
produced chipped fragments of only lightly shocked material
moving at approximately 20 percent of Martian escape velocity,
and thermal conductivity calculations demonstrate that passage
through Earth's atmosphere will not heat the interior of
meteorites larger than 0.3 centimeters above 100 degrees
centigrade.
2) The authors report that ultra-high resolution magnetometer
images of the magnetic field of meteorite ALH84001 reveal a
spatially heterogeneous pattern of magnetization associated with
fractures and rock fragments. Heating the meteorite to 40
degrees centigrade reduces the intensity of some magnetic
features, indicating that the interior of the rock has not been
above this temperature since before its ejection from the
surface of Mars.
3) The authors report their results thus indicate that major
impact events are capable of moving rocks from the surface of
Mars to the surface of Earth without subjecting them to
temperatures high enough to cause thermal sterilization of
eukarya or bacteria. Dynamic simulations of Martian impact
events indicate that materials can be launched into a wide
variety of orbits. The authors suggest that although most of the
approximately 1 ton of Martian rocks that are believed to land
on Earth each year have spent several million years in space,
one in 10^(7) of the arriving rocks will have made the journey
in less than a year, and that every million years, approximately
10 rocks larger than 100 grams are estimated to be transferred
in just 2 to 3 years of travel time. It has been estimated that
bacterial spores, as well as microorganisms within rocks, can
survive in deep space for more than 5 years. Thus, the authors
suggest, their results indicate that it may not be necessary to
protect Earth's present biosphere by quarantining rocks
retrieved by a Mars sample return mission, since conditions have
been appropriate to allow low-temperature rocks -- and, if
present, microorganisms -- from Mars to be transported to Earth
throughout most of geological time.
4) In summary, the authors state: "[Our] data support the
hypothesis that meteorites could transfer life between planets
in the Solar System."
Science 2000 290:791
Related Background:
THE MARTIAN METEORITE MICROBES CONTROVERSY: AN UPDATE
Allan Treiman (National Aeronautics and Space Administration,
US) presents a review and update of the ALH84001 meteorite
controversy, the author making the following points:
1) Early hopes for a fast resolution of the controversy
concerning meteorite ALH84001 have evaporated: no agreement has
emerged on whether or not the meteorite ever contained Martian
life.
2) There is no disagreement that ALH84001 formed on Mars
approximately 4.5 billion years ago, that the meteorite was
probably ejected into space approximately 16 million years ago
by an asteroid impact, that the meteorite fell in Antarctica
13,000 years ago, and that the meteorite remained in Antarctica
until found on the ice in 1984. 3) ALH84001 is an igneous rock
(i.e., a rock congealed from a molten mass) that apparently
crystallized slowly from molten lava and which contains globules
of carbonate minerals scattered along fractures. All the
evidence for life is in the carbonate globules or their rims. 4)
The 4 lines of evidence originally proposed by the McKay group
were a) the presence in the meteorite of carbon compounds
(polycyclic aromatic hydrocarbons) suggestive of decayed organic
matter; b) the presence in the meteorite of unusual small
crystals of magnetite (an iron oxide) matching identical
crystals believed to be produced only by Earth bacteria; c) the
presence in the meteorite of apparently incompatible minerals
(e.g., iron-sulfide and iron-oxide) close together whose
proximity would suggest organic action if the rock were from
Earth; and d) the presence in the meteorite of bacteria-shaped
formations. 5) The author [Treiman] suggests that a) The
polycyclic aromatic hydrocarbons may or may not be Martian, and
if they are, they may or may not be related to life. b) The
magnetite crystals are indeed Martian, but there is evidence
that some of these crystals formed without life and the origin
of the others remains unclear. c) The mineral associations in
the carbonate globules do not prove life, but also do not
exclude it. d) The bacteria-shaped objects in ALH84001 are not
fossil bacteria but could be fossils of bacteria fragments.
(McKay's group now agrees that the objects are too small to be
fossil microbes.) 6) The author concludes: "McKay's original
hypothesis (as expressed in the 1996 paper) depended on all four
lines of evidence working together... The evidence has not been
verified, so the hypothesis has not succeeded... Despite world
attention, significant spending, and the work of the best
laboratories on Earth, the question [of life on Mars] is
unresolved."
Sky & Telescope April 1999
Related Background:
EVIDENCE THAT MARTIAN METEORITE AMINO ACIDS ARE CONTAMINANTS
As the subunits that compose protein polymers in living systems,
the detection of certain amino acids in a material is often
interpreted as indicating a possible biological origin. The
meteorite ALH84001, along with a number of other discovered
meteorites, has a composition that suggests it was apparently
ejected from the surface of Mars, and during the past year it
has been proposed that microanalysis of this meteorite indicates
the possible presence of bio-organics and biogenic fossils. This
proposal, however, has met with considerable controversy, and
the controversy is still in full force.
Bada et al (4 authors at 3 installations, US) now report that
the amino acids present in a sample of the ALH84001 meteorite
appear to be terrestrial in origin and similar to those found in
the ice where the meteorite was discovered, although the
possibility remains that minute amounts of endogenous amino
acids are preserved in the meteorite. The authors suggest that
radiocarbon studies (cf. contiguous report: Jull et al, Science
279:366 1998), coupled with their own amino acid results,
indicate that major and minor organic constituents in the
Martian meteorites are contaminants.
Science 1998 279:362
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2. SPOTLIGHT: ON SCIENTIFIC TERMINOLOGY
J.L. Heilbron Worcester College, UK discusses terminology in
science and make the following points:
1) The history of scientific terminology opens a royal road to
the history of scientific culture. The eighteenth century, which
spent much of its intellectual energy classifying and
summarizing its burgeoning knowledge, devised terminology that
transformed botany and chemistry. The binomial designation of
natural species introduced by Carolus Linnaeus (1707-1778) and
the systematic names of chemical compounds invented by Antoine
Lavoisier (1743-1794) and his collaborators remain in use,
although they are not free from damaging idolatry. Linnaeus
embedded his binomials in a system of arithmetically defined
taxa that sometimes put species in the wrong families. The
French chemists admitted the substance caloric, which does not
exist, among their elements, and coined "oxygen'' on the
mistaken idea that the gas so designated gave acids their
acidity. But the terminology, erected on the enlightened
principles of rationality, order and universality, proved
flexible enough to drop erroneous reifications (like caloric)
and ignore misnomers (like oxygen).
2) After the Second World War, Americans gained by priority of
discovery the right to name the elementary particles. Their
terminology tended to be facetious and jocular. Thus, quarks in
their various flavors and colors; gluons to paste quarks
together; quantum chromodynamics, which does not study color;
and GUTs and TOEs, not body parts but Grand Unified Theories and
Theories of Everything. Did the jocularity indicate the easy
confidence of people who felt close to finishing physics? It
certainly demonstrated that the sober conservatism of European
scientists of the nineteenth and early twentieth centuries had
given way to the flippant equality of Americans during their
time of world dominance. The playful names coined by high-energy
physicists have been criticized as inelegant, non-ancient,
capricious and misleading. No doubt it is unlucky that quark
means garbage in German, but gluon is an inspired put-on: it
looks Greek, means nothing in German, puns in English and
satisfies Bacon's requirement that a word express a clear and
distinct idea.
3) Genetics and molecular biology have a taxing and awkward
terminology. Students of fruit flies favour bouncy names in the
style of particle physicists: armadillo, hedgehog,
lost-in-space. Mouse geneticists like dull ones, such as
beta-catenin, which happens to be the same gene as armadillo. A
single gene (selectin L) has 15 different aliases, whereas MT1
refers to at least 11 different genes.The cure for this genetic
disorder is a computer, which identifies a gene not by its name
but by systematic descriptors.
Nature 2002 415:585
Web Links: scientific nomenclature scientific jargon
Related Background:
HISTORY OF SCIENCE: ON LANGUAGE REFORM IN CHEMISTRY
An argument can be made that nomenclature in science is as
important as data, since nomenclature represents the prevailing
conceptual organization of observations. Certainly, researchers
in most sciences are constrained to adhere to the nomenclature
rules of their field. Molecular biology is currently in a phase
of general nomenclature chaos with respect to the naming of
genes, but hopefully that phase will soon pass. Meanwhile,
nomenclatures in other areas of biology are more organized, and
18th century plant taxonomy, in fact, served as a model for the
nomenclature revolution in chemistry that occurred in
conjunction with the "new chemistry" proposed by Antoine
Lavoisier (1743-1794). Lavoisier is often cited as the
instigator of chemical nomenclature reform at the end of the
18th century, but four chemists were the prime movers of this
reform: Lavoisier, Louis Guyton de Morveau (1737-1816), Claude
Berthollet (1748-1822), Antoine Fourcroy (1755-1809). Of the
four, Guyton de Morveau, probably deserves more credit than the
others, his efforts culminating in the publication of his
_Method of Chemical Nomenclature_ in 1787 [*Note #1]. All the
above chemists, however, collaborated in the nomenclature
revision program, which quickly became accepted after the
publication of Lavoisier's influential textbook _Elementary
Treatise on Chemistry_ in 1789 [*Note #2]. Perhaps the most
important general nomenclature revision was the adoption of a
binomial scheme for naming compounds (influenced by the scheme
then current in botany), but of specific importance was the
renaming of "*dephlogisticated air" ("empyreal air; vital air)
as "oxygen", and the renaming of "inflammable air" as
"hydrogen", both new names based on prevailing knowledge of
chemistry rather than on ambiguous attributes.
Bernadette Bensaude-Vincent (University of Paris, FR) presents a
commentary on language reform in chemistry, the author making
the following points:
1) Guyton de Morveau initiated the French 18th century chemical
nomenclature reform project and established a set of basic
principles: a) nomenclature should reveal "the nature of
things"; b) simple substances should have simple names evoking
their most characteristic property; c) compound names should
express the composition of chemical compounds; d) Greek
etymologies should be used in preference to Latin.
2) Guyton de Morveau began his attempt to reform chemical
nomenclature in 1782 and submitted his project to the Paris
Academy of Sciences in January 1787. At the Academy, Guyton
encountered a fierce debate concerning the existence of
"phlogiston", the principle that was believed to explain
combustion and reduction. Although most chemists at that time
believed in phlogiston, Lavoisier's explanation of combustion
was quite different. Guyton allied himself with Lavoisier, and
with the help of Lavoisier, Berthollet, and Fourcroy, Guyton
published a revised project in the spring of 1787, the revision
making no mention of "phlogiston", but instead containing new
words such as "oxygen", from Greek words meaning "acidifying
principle", the new term stemming from Lavoisier's idea that all
acids contained oxygen.
3) The author points out that the language reform of 1787-1789
was an integral part of the formation of the autonomous
discipline of chemistry, contributed to the subordination of
pharmacy to chemistry, and contributed to the redefinition of
the chemical arts as applied chemistry. The new language forged
by academic chemists separated many users of chemical substances
from their own traditions. The new language ignored the
physiological senses of chemists, banished all reference to
geographical origins or the discovery of the substances, and
imposed an analytical quantitative logic on chemical
nomenclature. Although the use of this logic proved to be a
valuable method over time, the principles of the system were
never strictly applied. Oxygen, for example, should have been
renamed when Humphrey Davy (1778-1829) established that many
acids do not contain oxygen. Colors and odors were restored
after the discovery of chlorine and iodine, named from the Greek
for "yellowish-green" and "violet", respectively. Bromine was
named from the Greek word for "stink". Morphine was named after
Morpheus, the god of dreams. Benzene was named after Styrax
benzoin, a tree native to Sumatra and Java. Scandium, germanium,
and polonium were named after political entities, and in the
20th century various new elements were named after historical
scientific figures. In general, the systematization imposed by
the four 18th century reformer chemists in the name of
rationality remained an ideal often contradicted by practice. At
present, nomenclature rules in chemistry are under the control
of a permanent commission, the International Union of Pure and
Applied Chemistry (IUPAC).
Nature 2001 410:415
Text Notes:
... ... *Note #1: Louis Bernard Guyton de Morveau (1737-1816)
was an interesting personage. His first profession was that of
an attorney. In 1776, while still an attorney, he published the
_Elements of Theoretical and Practical Chemistry_, a major
attempt to quantify chemical affinities. In 1782, he gave up the
law and devoted himself full-time to chemistry. In 1795, he
founded the Ecole Polytechnique and taught there until 1805.
Guyton was one of the first to conclude that iron and steel
differ solely in their carbon content. He made improvements in
the manufacture of gunpowder. He was the first to use chlorine
and hydrochloric acid gas as disinfectants. He was one of the
first balloonists, making two flights in 1784 and helping in the
organization of the world's first air force, the Compagnie
d'Aerostiers, whose reconnaissance balloonists assisted the
French army in several battles during the Napoleonic wars.
... ... *Note #2: Concerning nomenclature in chemistry, the
following passage appears in Lavoisier's _A General Introduction
to Chemistry_ (1789):
"It is impossible to dissociate language from science or science
from language, because every natural science always involves
three things: the sequence of phenomena on which the science is
based; the abstract concepts which call these phenomena to mind;
and the words in which the concepts are expressed. To call forth
a concept, a word is needed; to portray a phenomenon, a concept
is needed. All three mirror one and same reality. Words are thus
required to preserve and transmit ideas, so that it is clear
that the advancement of a science and the improvement of its
technical vocabulary go hand in hand. No matter how certain we
are of the phenomena, no matter how adequately our concepts
reflect them, we cannot help perpetuating wrong ideas unless we
have a precise terminology in which to express ourselves."
Lavoisier, considered the father of modern chemistry, was no
doubt the most eminent scientist to ever suffer death by the
guillotine. In 1780, as a member of the French Academy of
Sciences, Lavoisier was active in rejecting the application to
the Academy of a certain physician Jean-Paul Marat (1743-1793).
Marat apparently did not forget. During the French Revolution
(1787-1799), Marat became a powerful revolutionary leader, and
Marat was instrumental in bringing Lavoisier to trial for his
investments in a much-hated company that collected taxes for the
French government. Lavoisier was guillotined May 8, 1794 and
buried in an unmarked grave. (Marat did not live to see this:
Marat himself was assassinated in July 1793.)
... ... *dephlogisticated air: In this context, the term
"phlogiston" refers to a 17th and 18th century chemical theory
involving a hypothetical principle of fire. The idea was that
every combustible substance is in part composed of phlogiston,
with the phenomenon of burning caused by the liberation of
phlogiston and the "dephlogistonated" substance remaining as ash
or residue. The phlogiston theory was experimentally discredited
by Lavoisier beginning in 1770, who showed that the newly
discovered element oxygen was always involved in combustion.
Related Background:
ON LINGUISTIC CHAOS IN MOLECULAR BIOLOGY
Nomenclature anarchy in molecular biology is apparently once
again the focus of attention, although no remedies are evident.
In a recent article, Paul Smaglik writes, "Gene and protein
names often are based on the flamboyant, the descriptive, and
the intentionally obscure. For many researchers, naming their
discovery may be a rare opportunity to imbue their science with
creativity." But Lawrence Puente (University of Alberta, CA)
points out that creativity plus competition can equal confusion.
Julia A. White (University College London, UK), a member of the
Nomenclature Committee of the Human Genome Organization, says
that although the committee strives to sort out linguistic
chaos, the committee remains behind as a result of the speed and
scope of the Human Genome Project. With hundreds of thousands of
genes and proteins still to be named, molecular biology is in
dire need of nomenclature regulation.
The Scientist 30 Mar 1998
Related Background:
MORE DISCUSSION OF ACRONYM ANARCHY IN MOLECULAR BIOLOGY
There are approximately 100,000 genes in the human genome, and
approximately 100,000 expressed proteins, the total certainly
enough to require a dictionary of names. Add to this total the
total of acronyms used to identify cell-lines, cell receptors,
metabolic pathways, carbohydrates, etc., and the dictionary
would require a second volume. In the early days of biochemistry
and molecular biology, when few genes and their expressed
proteins had been identified, everyone could more or less
remember the names of the macromolecular entities being studied
by the people in the laboratory down the hall. These days that
is unlikely, and made more unlikely by the tendency of many
molecular biologists to choose ad hoc names that are often more
cute than technically pertinent, and to obfuscate their research
papers with acronyms by the dozen in a single paper. We know of
at least one instance where an acronym for a cell-line in a
paper from a group at the US National Institutes of Health was
not defined anywhere in the paper, where telephone calls to
molecular biologists produced no one who knew what cell-line was
involved, and where a query to the authors of the paper did not
produce a response for nearly three weeks. As one scientist
recently put it: "If you make your paper difficult to read, at
least no one can call you stupid." A recent exchange of letters
in the journal Nature revisits this recurrent problem of
nomenclature in molecular biology. It seems there are indeed
existing committees concerned with regulating the nomenclature
of molecular biology, but it also seems no one pays any
attention to them. Puente et al (Univ. of Alberta, CA) refer to
the present situation as "acronym anarchy". We agree. We would
add that if the in-house editors of the leading general journals
such as Science and Nature would refuse to publish these unduly
obfuscated papers, they would be doing a service to the
scientific community.
Nature 1997 390:329
Related Background:
A CRITICISM OF NOMENCLATURE IN MOLECULAR BIOLOGY
Nomenclature is a serious problem in all the sciences, since as
new discoveries are made, new entities identified, new concepts
formulated, new names for these things must be found so that
scientists can communicate with each other with some degree of
precision. Most sciences have nomenclature committees that meet
regularly to standardize current terminology and make decisions
about new terminology. Molecular biology, one of the most active
scientific disciplines these days, has no such constraints, and
apparently there is growing concern that the arbitrary and
sometimes whimsical naming of new entities ("miranda",
"prospero", "numb", "inscrutable") in molecular biology, with
the same entity often sporting a number of names, has reached
the stage of promoting confusion and the inability of scientists
to deal efficiently with the literature. In a recent editorial
criticizing nomenclature practices in molecular biology, the
journal Nature says, "Regrettably, molecular biologists have
followed the particle physicists' whimsy with obscurantist
enthusiasm." In particle physics, of course, we already have
"quark", "strangeness", "charm", "color", "top", "bottom", etc.,
which the editorial calls a "descent into whimsy" started by
Murray Gell-Mann in the 1960s, who evidently took the term
"quark" from a phrase in James Joyce's FINNEGAN'S WAKE. What is
interesting is that the same journal which is criticizing
whimsical scientific nomenclature is apparently quite fond of
headlines involving whimsical wordplay, puns, and metaphors when
describing scientific research results. If a consequence of this
attention to nomenclature will be a more rational use of
language in science, many people will no doubt be appreciative
of it.
Nature 1997 389:1
Related Background:
PHYSICISTS ORGANIZE AGAINST IMPENETRABLE JARGON IN PHYSICS
A group of working physicists and journal editors, under the
leadership of Mitio Inokuti (Argonne National Laboratory, US)
and Ugo Fano (University of Chicago, US) has come into existence
with the objective of reforming the publication standards for
papers in physics. The problem is that physicists no longer
understand each other, their communication warped by
"unexplained acronyms, cryptic symbols, endless sentences, and
monstrous graphs". Analyzing the psychology of why this exists,
Phillip Schewe (American Institute of Physics, US) says, "You
lose all your readers, but at least you can't be accused of
being an idiot. Instead, the readers are made to feel like
they're idiots." The problem, of course, is just as severe in
chemistry and biology.
Science 1997 277:895
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3. ON CLUSTERS AND COULOMB EXPLOSION
In general, a "Coulomb explosion" is any intense dispersion of
like-charged particles due to Coulomb repulsion. In physics, one
method of producing the phenomenon is to accelerate a molecule
to a high velocity, and then have it strike a solid, which
results in stripping of bonding-electrons in the molecule due to
repulsive collisions with electrons in the solid, and subsequent
explosive repulsive dispersal of like-charged atomic fragments.
L. Poth et al (Degussa Metals Catalysts Cerdec AG, DE) discuss
clusters and Coulomb explosion, the authors making the following
points:
1) In this context, the term "cluster" refers to a collection of
weakly bonded atoms or molecules whose size distribution can
readily be altered by changing the techniques used to make them.
These small aggregates have properties that often differ
significantly from those of bulk material, and their behavior is
not strictly that of a solid, liquid or gas. Indeed, clusters
are often referred to as a new state of matter. Because clusters
are so small, their constituents lie largely on the surface. The
small size also constrains energy levels within clusters so that
quantum effects often arise. The excitement in cluster research
lies in investigating new fundamental phenomena through
clusters, and also in the possibility of connecting what is
known about the gaseous and condensed states by exploring states
lying between them. On the more pragmatic side, clusters may
someday also serve as building blocks for assembling new
materials engineered at nanometer scales.
2) Among the fastest processes of chemical interest are those
involving charged species. In general, processes involving the
transfer of electrons take place on the shortest timescales, but
those involving other ionic constituents take only a little
longer. An example of particular interest is an interaction
involving ionized atoms or molecules. Because of their
electrical charge, ionized particles sense one another over
comparatively long distances. Their interactions are generally
strong and often take place very quickly. Just as the like poles
of a child's magnets repel, so do particles of like charge repel
each other -- Coulomb repulsion, named for Charles Augustin de
Coulomb (1736-1806), the French engineer and scientist who first
proposed laws of electrical attraction and repulsion. And if a
collection of like-charged particles is produced suddenly in
close proximity, the repulsion between them can be so large as
to be explosive -- hence the name "Coulomb explosion". Clusters
have a particular propensity to undergo such processes when
exposed to the high-intensity light fields available from
femtosecond pulsed lasers, and their behavior during Coulomb
explosion offers information on the time evolution of
intermediates -- the ephemeral species that exist temporarily
during a chemical reaction.
3) The consequences of like-charge repulsion in clusters have
been known for years, but the fingerprints left by the process
were not very detailed. Until recently, most observations of
Coulomb phenomena in clusters were confined to intact and
multiply-charged clusters that were large enough to retain more
than one charge center, making them detectable with a mass
spectrometer. In these cases, the charges were sufficiently
separated to prevent them from overcoming the cohesive energy of
the systern, keeping the cluster from completely disassembling.
Unfortunately, species that did undergo explosive separation had
only a fleeting existence and were undetectable by the
spectrometer. The implementation of ionization sometimes enabled
the observation of a stability change induced by laser heating
of stable clusters, which caused them to evaporate. The ensuing
reduction in size could lead to clusters suffidently small that
the embedded charges became too close for the system to remain
stable, leading to fission-like disassembly. Thus, some of the
results of Coulomb effects could be observed, but the actual
explosion could not.(1-5)
References (abridged):
1. Card, D. A., D. E. Folmer, S. Sato, S. A. Buzza and A. W.
Castleman, Jr. 1997. Covariance mapping of ammonia clusters:
evidence of the connectiveness of clusters with Coulombic
explosion. Journal of Physical Chemistry A 101:3417-3426.
2. Card, D. A., E. S. Wisniewski, D. E. Folmer and A.W.
Castleman, Jr. 2002. Dynamics of Coulomb explosion and kinetic
energy release in clusters of heterocyclic compounds. Journal of
Chemical Physics 116:3554-3567
3. Ditmire, T., J. Zweiback, V. P. Yanovsky, T. E. Cowan, G.
Hays and K. B. Wharton. 1999. Nuclear fusion from explosions of
femtosecond laser-heated deuterium clusters. Nature 398:489-492.
4. Douhal, A., S. K. Km and A. H. Zewail. 1995. Dynamics of
automerization in Model Base Pairs. Nature 378: 260.
5. Douhal, A., and J. Santamaria, eds. 2002. Femtochemistry and
Femtobiology: Ultrafast Dynamics in Molecular Science.
Singapore: World Scientific Publishing Company.
American Scientist 2002 90:342
Web Links: Coulomb explosion
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4. ON MAMMALIAN SPERM-EGG INTERACTIONS
P. Primakoff and D.G. Myles (University of California Davis, US)
discuss mammalian sperm-egg interactions and make the following
points:
1) In mammals, fertilization is completed by the direct
interaction of sperm and egg, a process mediated primarily by
gamete surface proteins. Therefore, an essential task in the
study of sperm-egg interaction is an exploration of the
capabilities of a distinct set of surface proteins, some gamete
specific and others more widely expressed. On gametes, these
proteins act in a sequential pattern to orchestrate the close
approach and ultimate fusion of the two cells.
2) To penetrate the substantial cumulus cell barrier surrounding
ovulated eggs of most mammalian species, sperm use
hyperactivated motility (1) and a
glycosylphosphatidylinositol-anchored surface hyaluronidase,
named PH-20 (2). The motility and surface hyaluronidase are
necessary, and perhaps sufficient, to digest a path through the
extracellular matrix of the cumulus cells; no proteases have yet
been implicated in this process.
3) The egg's zona pellucida is a cell type-specific
extracellular matrix or coat composed of three glycoproteins
termed ZP1, ZP2, and ZP3. Sperm that reach and bind to the zona
pellucida receive a signal to acrosome react, i.e., release by
exocytosis the contents of their large secretory granule, the
acrosome. The currently favored model is that sperm bind to
O-linked carbohydrate on ZP3.
4) In summary: Fertilization is the sum of the cellular
mechanisms that pass the genome from one generation to the next
and initiate development of a new organism. A typical, ovulated
mammalian egg is enclosed by two layers: an outer layer of ~5000
cumulus cells and an inner, thick extracellular matrix, the zona
pellucida. To reach the egg plasma membrane, sperm must
penetrate both layers in steps requiring sperm motility, sperm
surface enzymes, and probably sperm-secreted enzymes. Sperm also
bind transiently to the egg zona pellucida and the egg plasma
membrane and then fuse. Signaling in the sperm is induced by
sperm adhesion to the zona pellucida, and signaling in the egg
by gamete fusion. The gamete molecules and molecular
interactions with essential roles in these events are gradually
being discovered.(3-5)
References (abridged):
1. R. Yanagimachi, in The Physiology of Reproduction, E. Knobil,
J. D. Neill, Eds. (Raven, New York, 1994), pp. 152-162.
2. Y. Lin, et al., J. Cell Biol. 125, 1157 (1994)
3. J. D. Bleil, et al., Cell 20, 873 (1980)
4. J. Chen, et al., Proc. Natl. Acad. Sci. U.S.A. 95, 6193 (1998)
5. R. A. Kinloch, et al., Proc. Natl. Acad. Sci. U.S.A. 92, 263
(1995)
Science 2002 296:2183
Web Links: sperm egg interaction ovum fertilization
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5. ON AXON GROWTH CONES
B.J. Dickson and K-A. Senti discuss axon growth cones, the
authors making the following points:
1) Ramon y Cajal first observed neuronal growth cones a century
ago, and correctly guessed their function in guiding the axons
and dendrites of differentiating neurons towards their targets.
Since then, successive generations of neuroscientists have
sought to understand how growth cones accomplish this
extraordinary feat. In a classic series of experiments in the
1940s and 50s, Roger Sperry [1] showed that growth cones are
guided by specific chemical cues, and suggested that they might
often be distributed in concentration gradients. In the 1970s
and 80s, Friedrich Bonhoeffer and colleagues [2] developed
clever in vitro assays for growth cone guidance, and used these
assays to show that growth cones can detect gradients that
differ by as little as 1-2% across their diameter.
2) More recently, in the mid-late 1990s, a remarkable
convergence of biochemical studies in vertebrates and genetic
studies in invertebrates led to the identification of four major
families of guidance molecules -- netrins, Semaphorins, ephrins
and Slits -- all of which can act as graded cues to guide axonal
growth cones [3]. With these extracellular guidance cues
identified, and powerful in vitro and in vivo assays available,
attention is now increasingly turning to the inner workings of
the growth cone. What exactly happens inside the growth cone to
make it turn in response to these extracellular gradients?
3) In a surprising new development, Campell and Holt [4] have
demonstrated that these turning responses are dependent upon
localized protein synthesis and degradation within the growth
cone. Holt and colleagues, like Sperry and Bonhoeffer before
them, study the guidance of retinal ganglion cell axons to their
targets in the tectum. In earlier work, they showed that Xenopus
retinal axons are responsive to both the netrin-1 and
Semaphorin-SA (Sema3A) guidance cues [5]. Netrin-1 appears to
guide these axons at an early stage (stage 24), attract ing them
to the head of the optic nerve on the first leg of their journey
towards the tectum [5]. Only later (after about stage 32) do
retinal axons respond to Sema3A, which provides a repulsive
signal to help keep them on course as they approach the tectum.
The netrin-1 and Sema3A responses can both be reproduced in
vitro, using a growth cone turning assay initially developed for
Xenopus spinal axons by Poo and colleagues [1992] and adapted
for retinal axons in the Holt lab [5]. In this assay, an
isolated Xenopus axon growing on a coverslip is confronted with
a gradient of netrin-1 or Sema3A, delivered from a micropipette
positioned just ahead and to the side of the advancing growth
cone. Retinal axons turn towards the pipette if it delivers
netrin-1, and away from it if it provides Sema3A.
References (abridged):
1. Sperry R.W. (1963). Chemoaffinity in the orderly growth of
nerve fiber patterns and connections. Proc. Natl. Acad. Sci.
U.S.A. 50,703-710.
2. Baier H. and Bonhoeffer, F. (1992). Axon guidance by
gradients of a target-derived component. Science 255, 472-475.
3. Yu, T.W. and Bargmann, C.I. (2001). Dynamic regulation of
axon guidance. Nat. Neurosci. 4Suppl, 1169-1176.
4. Campbell, D.S. and Holt, C.E. (2001). Chemotropic responses
of retinal growth cones mediated by rapid local protein
synthesis and degradation. Neuron 32,1013-1026.
5. de la Torre, J.R., Hopker, V.H., Ming, G.L., Poo, M-m.,
Tessier-Lavigne, M., Hemmati-Brivanlou, A., and Holt, C.E.
(1997). Turning of retinal growth cones in a netrin-1 gradient
mediated by the netrin receptor DCC. Neuron 19,1211-1224.
Current Biology 2002 12:R218
Web Links: axon growth cones nerve axons
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6. ON THE E. COLI GENOME
V. Souza et al (National Autonomous University of Mexico, MX)
discuss the E. Coli genome, the authors making the following
points:
1) Although it has been subjected to scientific scrutiny for
more than a century and has occupied center stage in the
development of genetic-engineering technologies in the
laboratory, the bacterium Escherichia coli continues to confound
our ideas of how bacteria reproduce, adapt and colonize new
niches. In 1994, for example, Stephen Jay Gould (1941-2002)
wrote that "the most salient feature of life has been the
stability of its bacterial mode from the beginning of the fossil
record until today." Just eight years later, new insights into
the nature of £. coli and its close relatives have made such a
view of the bacterium as a stable organism seem superficial.
Having now sequenced certain E. coli genomes and studied the
population genetics of numerous bacterial species, we know that
although the bacteria have undergone little change in
morphology, their genome is a small but dynamic and changing
entity that has not stopped evolving.
2) Recent advances in our understanding of the genetics and
physiology of £. coli have in fact been spectacular. We know the
entire genome sequences of three strains of this species, and
the E. coli genome is undoubtedly the best understood of any
genome (approximately 70 percent of its genes being "annotated,"
in the terminology of genomics). It has also been used as a
model organism in evolutionary studies, both in natural
populations and in the laboratory in so-called "experimental
evolution" studies. These investigations have allowed us to
understand better the action of two evolutionary forces,
selection and mutation, over a very long time. These studies
were based on the prevailing notion that these bacteria are
clonal, passing genes from generation to generation with little
scrambling or swapping -- a notion that was entirely upset as
the 20th century came to a close.
3) However, we have just begun to investigate bacterial ecology
and evolutionary biology in natural populations. Such studies
have gained urgency in connection with recent outbreaks of some
pathogenic, foodborne strains of E. coli. These strains have
virulence factors and genetic "pathogenicity islands" that have
made E. coli, long a killer of infants in poor countries, a
growing threat everywhere in the world.(1-5)
References (abridged):
1. Elliott, S. J. et al. 1999. The complete sequence of the
locus of enterocyte effacement (LEE) from enteropathogenic
Escherichia coli E2348/69. Molecular Microbiology 28:1-4
2. Guttman, D. S. 1997. Recombination and clonality in natural
populations of Escherichia coli. Trends in Ecology & Evolution
12(127):15-22.
3. Levin, B. R., and C. T. Bergstrom. 2000. Bacteria are
different: Observations, interpretations, speculations and
opinions about the mechanisms of adaptive evolution in bacteria.
Proc. Natl. Acad. Sci. 97:6981-6985.
4. Maynard-Smith, J. 1991. The population genetics of bacteria.
Proc. Roy. Soc. London 8245:37-41.
5. Nataro, J. P., and J. B. Kaper. 1998. Diarrheagenic
Escherichia coli. Clinical Microbiology Reviews 11:142-201.
American Scientist 2002 90:332
Web Links: Escherichia coli genome
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7. SPONTANEOUS ASSEMBLY OF ORDERED NANOCLUSTER ARRAYS
J-L. Li et al (Chinese Academy of Sciences, CN) discuss
nanocluster arrays, th authors making the following points:
1) Ordered arrays of metal nanoclusters are promising materials
for next generation microelectronics [1,2], ultra-high-density
recording [3], and nanocatalysis [4,5]. Self-organization in
heterogeneous strained thin-film growth and self-assembly in
chemical synthesis are two of the most commonly used methods to
obtain such nanostructures spontaneously. No method has
succeeded in producing reproducibly identical nanoclusters/dots
with periodic spatial distribution, which is highly desirable
for practical device applications and is an open question in
molecular and solid-state physics of these "artificial atoms".
Fabrication of uniform-size cluster arrays at the ultrasmall 1-2
nm size regime is even more challenging because fluctuation at a
level of only a few atoms could substantially alter their
electronic properties.
2) On the other hand, ultrasmall cluster arrays of such
dimension have potential for quantum application because the
Fermi wavelength for most metals is around 1 nm. At such a
length scale, one could also maximize quantum confinement
effects and test the fundamental limitations that such effects
could impose on the electronic properties. Self-assembly of
nanoclusters on periodic solid surface has been shown to be a
promising approach to the problem, but growth of ordered arrays
of nanoclusters with identical size and tunable composition is
still a daunting challenge.
3) Certain clusters with a specific ("magic") number of atoms
exhibit electronic and/or atomic closed-shell structures and
hence remarkable stability. For substrate-supported clusters,
while the "closed-shell structure" is still under debate,
several recent studies suggest that supported clusters of
specific or "magic" sizes indeed exist with remarkable relative
stability. As the substrate can interact with the clusters, a
substrate modification of the magic sizes may be unavoidable. On
the other hand, such an interaction could play a pivotal role by
automatically selecting identically sized clusters. The authors
report they explore this concept and exploit the magic
clustering process to assemble ordered cluster arrays. The
authors report a demonstration that substrate-induced
spontaneous clustering can be realized by delicate control of
growth kinetics, and that periodic identical-size metal
nanocluster arrays can be fabricated on Si(lll)-(7 X 7) surfaces.
References (abridged):
1. A.O. Orlov et al., Science 277, 928 (1997).
2. R.P. Andres et al, Science 272, 1323 (1996).
3. S. Sun et al, Science 287, 1989 (2000).
4. M.Valden et al, Science 281, 1647 (1998).
5. M. Haruta, Catal. Today 36, 153 (1997).
Phys. Rev. Lett. 2002 88:066101
Web Links: nanoclusters
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8. ON THE SPIN-GALVANIC EFFECT
S.D. Ganichev et al (University of Regensburg, DE) discuss the
spin-glavanic effect, the authors making the following points:
1) There has been much recent interest in exploiting the spin of
conduction electrons in semiconductor heterostructures together
with their charge to realize new device concepts(1). Electrical
currents are usually generated by electric or magnetic fields,
or by gradients of, for example, carrier concentration or
temperature. Electron spin in a spin-polarized electron gas can,
in principle, also drive an electrical current, even at room
temperature, if some general symmetry requirements are met.
2) The authors demonstrate such a "spin-galvanic" effect in
semiconductor heterostructures, the effect induced by a
non-equilibrium but uniform population of electron spins. The
microscopic origin for this effect is that the two electronic
sub-bands for spin-up and spin-down electrons are shifted in
momentum space and, although the electron distribution in each
sub-band is symmetric, there is an inherent asymmetry in the
spin-flip scattering events between the two sub-bands. The
resulting current flow has been detected by applying a magnetic
field to rotate an optically oriented non-equilibrium spin
polarization in the direction of the sample plane. In contrast
to previous experiments, where spin-polarized currents were
driven by electric fields in semiconductor(2,3), the authors
report the complementary situation where electron spins drive a
current without the need of an external electric field.
3) The authors conclude: The uniformity of the spin polarization
in space is preserved during the scattering processes.
Therefore, the spin-galvanic effect differs from surface
currents induced by inhomogeneous spin orientation. It also
differs from other experiments where the spin current is caused
by gradients of potentials, concentrations and so on, like the
spin-voltaic effect and the photo-voltaic effect, which occur in
inhomogeneous samples, and like the "paramagnetic
metal-ferromagnet" junction or p-n junctions. When considering
spintronic devices involving heterojunctions, spin-galvanic
current must be taken into account.(4,5)
References (abridged):
1. Wolf, S. A. et al. Spintronics: a spin-based electronics
vision for the future. Science 294, 1488-1495 (2001)
2. Hägele, D. et al. Spin transport in GaAs. Appl. Phys. Lett.
73, 1580-1582 (1998)
3. Kikkawa, J. M. & Awschalom, D. D. Lateral drag of spin
coherence in gallium arsenide. Nature 397, 139-141 (1999)
4. Bychkov, Y. A. & Rashba, E. I. Properties of a 2D electron
gas with lifted spectral degeneracy. Sov. JETP Lett. 39, 78-81
(1984)
5. D'yakonov, M. I. & Kocharovskii, V. Yu. Spin relaxation of
two-dimensional electrons in noncentrosymmetric semiconductors.
Sov. Phys. Semicond. 20, 110-111 (1986)
Nature 2002 417:153
Web Links: spin-galvanic effect
Related Background Brief:
ELECTRICAL SPIN INJECTION AND ACCUMULATION AT ROOM TEMPERATURE
IN AN ALL-METAL MESOSCOPIC SPIN VALVE. Finding a means to
generate, control and use spin-polarized currents represents an
important challenge for spin-based electronics, or
"spintronics". Spin currents and the associated phenomenon of
spin accumulation can be realized by driving a current from a
ferromagnetic electrode into a non-magnetic metal or
semiconductor. This was first demonstrated over 15 years ago in
a spin injection experiment on a single crystal aluminium bar at
temperatures below 77 K. Recent experiments have demonstrated
successful optical detection of spin injection in
semiconductors, using either optical injection by circularly
polarized light or electrical injection from a magnetic
semiconductor. However, it has not been possible to achieve
fully electrical spin injection and detection at room
temperature. The authors report room-temperature electrical
injection and detection of spin currents and observe spin
accumulation in an all-metal lateral mesoscopic spin valve,
where ferromagnetic electrodes are used to drive a
spin-polarized current into crossed copper strips. The authors
suggest that larger signals should be obtainable by optimizing
the choice of materials and device geometry. F.J. Jedema et al:
Nature 2001 410:345
Related Background Brief:
PERSISTENT SOURCING OF COHERENT SPINS FOR MULTIFUNCTIONAL
SEMICONDUCTOR SPINTRONICS. Recent studies of n-type
semiconductors have demonstrated spin-coherent transport over
macroscopic distances, with spin-coherence times exceeding 100
ns; such materials are therefore potentially useful building
blocks for spin-polarized electronics ("spintronics"). Spin
injection into a semiconductor (a necessary step for spin
electronics) has proved difficult; the only successful approach
involves classical injection of spins from magnetic
semiconductors. Other work has shown that optical excitation can
provide a short (<500 ps) non-equilibrium burst of coherent spin
transfer across a GaAs/ZnSe interface, but less than 10% of the
total spin crosses into the ZnSe layer, leaving long-lived spins
trapped in the GaAs layer. The authors report a "persistent"
spin-conduction mode in biased semiconductor heterostructures,
in which the sourcing of coherent spin transfer lasts at least
1-2 orders of magnitude longer than in unbiased structures. The
authors use time-resolved Kerr spectroscopy to distinguish
several parallel channels of interlayer spin-coherent injection.
The relative increase in spin-coherent injection is up to 500%
in the biased structures, and up to 4,000% when p-n junctions
are used to impose a built-in bias. The authors suggest these
experiments reveal promising opportunities for multifunctional
spin electronic devices (such as spin transistors that combine
memory and logic functions), in which the amplitude and phase of
the net spin current are controlled by either electrical or
magnetic fields. I. Malajovich et al: Nature 2001 411:770
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9. ON EDWIN LAND (1909-1991) AND COLOR VISION
N. Ribe and F. Steinle (Paris Geophysical Institute, FR) discuss
Edwon Land, the authors making the following points:
1) The now-classic experiments on color vision begun in the
1950s by Land are not only a fine example of exploratory
experimentation at the frontier between physics and biology,
they also have a direct bearing on the theoretical content of
Goethe's Theory of Colors. Land's research began with a simple
experiment using two black-and-white transparencies of the same
colored scene. The first transparency, the "long record," was
taken through a filter that passed only long-wavelength light.
The second, the "short record," was taken through a filter that
passed only short wavelengths. The two records differed only in
the lightness or darkness of corresponding points; neither had
any color. The transparencies were then projected onto a screen,
directly on top of one another, using a beam of light from the
red part of the spectrum for the long record and a beam of
incandescent light for the short record. According to the
classical color theory based on the work of Newton, Thomas
Young, James Clerk Maxwell, and Hermann von Helmholtz, the image
on the screen could only be some shade of pink. What the
observer saw, however, was an image brilliantly and diversely
colored, almost like the original scene.
2) Although Land was not the first to observe such two-color
projection effects, his observation initiated a program of
exploratory experimentation lasting more than two decades. He
began with a series of 22 variations on the two-projector
experiment. Those experiments demonstrated that the unexpected
or "nonclassical" colors appeared essentially instantaneously,
and could not be explained by time-dependent adaptations in the
eye. The experiments also showed that the colors were not
substantially affected by such factors as the intensities of the
ambient illumination or of the projecting beams, the angle
subtended by the image, or the filters used to produce the short
and long records. Land then performed a more precise series of
experiments using a dual monochromator that allowed the
experimenter to vary at will the wavelengths of the projecting
beams and to study the range of colors observed as a function of
those wavelengths.(11)
3) From the experiments, Land concluded that classical color
theory was valid only for spots of light observed in totally
dark surroundings and that it had only limited relevance to
color perception in natural situations involving multiple
objects and variable illumination. In particular, he concluded
that the stimulus for the color seen at a point in an image was
not, as usually supposed, the wavelength composition of the
radiant energy reaching the eye from that point. His subsequent
experiments were aimed at uncovering the nature of the stimulus.
Most of these experiments used "Mondrians," collages of paper
rectangles with different shapes and colors. Land began with
experiments in which colorless Mondrians in white, gray, and
black were viewed through dark goggles that allowed only the
eye's rod (night-vision) system to operate. By adjusting the
illumination of the Mondrians, Land showed that the patches
maintained a constant rank order of perceived lightness, even
though a patch that appeared dark might be sending much more
light to the eye than one that appeared light. This suggested to
Land that the eye was able to discover lightness values
independent of the flux of energy it received; the reflectance,
the physical correlate of lightness, might be the color stimulus
he was seeking. This idea led Land to a series of experiments in
which he illuminated colored Mondrians with long-, middle-, and
short-wavelength light that could be mixed in any proportion. In
one set of experiments, the illumination was adjusted so that,
for example, a white area of one Mondrian sent to the eye
exactly the same triplet of radiant energies as a green area of
another Mondrian. The two areas continued to appear white and
green, a dramatic demonstration that their perceived colors were
independent of the flux of energy they emitted as a function of
wavelength. In another set of experiments, observers were asked
to choose from a standard set of 1150 color chips the one that
best matched the color of a given area on an illuminated
Mondrian. Land found that when a match was made, it was the
reflectances of the two areas that corresponded, and not the
triplets of radiant energy being sent to the eye in the three
illuminating wave-bands.(12)
4) The "retinex" theory of color vision that Land developed on
the basis of his experiments has two essential elements: It
recognizes lightness (that is, reflectance) as the fundamental
stimulus of color, and it emphasizes the importance of
boundaries, which allow the eye to estimate lightness by seeking
out singularities in the ratio of energy flux from closely
spaced points.
References (abridged):
11. E. H. Land, Proc. Natl. Acad. Sci. USA 45, 115 (1959); 45,
636 (1959). E. H. Land, Sci. Am., December 1977, p. 108. A
recent account of Land's work and its historical context is
given by S. Zeki, A Vision of the Brain, Blackwell Scientific,
Boston (1993).
12. J. L. Benton, J. Opt. Soc. Am. 59, 103 (1969). E. H. Land,
Sci. Am., May 1959, p. 84.
Physics Today 2002 July
Web Links: Edwin Land color vision
ScienceWeek http://www.scienceweek.com
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10. ON BIOFILMS
Craig Stephens (Santa Clara University, US) discusses biofilms,
the author making the following points:
1) Adherence to surfaces by microorganisms can lead to the
development of "biofilms": durable microbial communities
embedded in polysaccharide matrices[1]. Biofilms are being
increasingly scrutinized at the cellular and molecular level,
partly in recognition of their relevance to human disease.
Pathogenic microbes that create biofilms can cause persistent
infections that defy the immune system and resist elimination by
antibiotics[2]. Recent work is revealing programs of gene
expression in biofilms that may provide ideas for new
therapeutic approaches to deal with this challenge.
2) Biofilm development begins when "planktonic" bacteria —
unattached individual cells — adhere to a surface [1]. Diverse
sites can be colonized, including mineral surfaces, living and
dead plant or animal tissues, and synthetic polymers, ceramics
and metal alloys. Adherence mechanisms vary depending on the
microbe and surface. As adherent cells grow and divide,
proximity to the surface induces physiological adaptations,
including secretion of exopolysaccharides to create a protective
matrix surrounding the cells[3]. Hydrated exopolysaccharides
contributes the bulk of the volume of a biofilm, and are
primarily responsible for its slimy macroscopic properties.
Fully developed biofilms are surprisingly elaborate structures,
with pillars rising up from mats of jumbled cells, permeated by
fluid-filled microchannels[1]. These dynamic communities can
spread across surfaces, incorporate particulates and other
microbes from the surrounding environment, and continually shed
new planktonic cells.
3) Bacterial biofilms that form in the human body can have dire
consequences[2]. Acid-producing streptococci in dental plaque
are responsible for cavity formation in teeth, and some plaque
inhabitants are associated with periodontal disease. Certain
oral streptococci can, if provided access to the bloodstream,
colonize cardiac tissue or valves, causing potentially fatal
endocarditis. Biofilms are especially problematic on in-dwelling
medical devices, including artificial heart valves, pacemakers,
synthetic joints and catheters. Biofilms are foci for persistent
inflammation, resulting in collateral damage to adjacent tissues
in addition to any direct damage by the pathogen. The
exopolysaccharide matrix shields most of the bacteria from
effective opsonization and phagocytosis, making them very
difficult to dislodge. Antibiotic treatment is frequently
unsuccessful in resolving biofilm-based infections, so surgical
removal of infected tissue or replacement of a colonized device
may be necessary.[4,5]
References (abridged):
1. Costerton J.W., Lewandowski Z., Caldwell D.E., Korber D.R.
and Lappin-Scott H.M. (1995) Microbial biofilms. Annu. Rev.
Microbiol., 49:711-745
2. Costerton J.W., Stewart P.S. and Greenberg E.P. (1999)
Bacterial biofilms: a common cause of persistent infections.
Science, 284:1318-1322
3. Kuchma S.L. and O'Toole G.A. (2000) Surface-induced and
biofilm-induced changes in gene expression. Curr. Opin.
Biotechnol., 11:429-433
4. Goldberg J.B. and Pier G.B. (2000) The role of the CFTR in
susceptibility to Pseudomonas aeruginosa infections in cystic
fibrosis. Trends Microbiol., 8:514-520
5. Withers H., Swift S. and Williams P. (2001) Quorum sensing as
an integral component of gene regulatory networks in
Gram-negative bacteria. Curr. Opin. Microbiol., 4:186-193
Current Biology 2002 12:R132
Web Links: biofilms
Related Background:
ON DNA IN BACTERIAL BIOFILMS
Among the most complex and dynamic types of common organic
coatings are "biofilms". Such films form when microbial
organisms attach to a surface and produce a highly hydrated
framework of extracellular polymers in which the microorganisms
become embedded. These biofilms may have a sorptive capacity
similar in magnitude to many reactive mineral substrates.
Cystic fibrosis is an inherited disease of the exocrine glands,
primarily affecting the gastrointestinal tract and respiratory
systems. The "exocrine" glands are glands that secret material
via excretory ducts (e.g., mucous secreting glands).
C.B. Whitchurch et al (University of Queensland, AU) discuss
bacterial biofilms, the authors making the following points:
1) Bacterial biofilms are structured communities of cells
enclosed in a self-produced hydrated polymeric matrix adherent
to an inert or living surface (1). Formation of these sessile
communities and their inherent resistance to antibiotics and
host immune attack are at the root of many persistent and
chronic bacterial infections (1), including those caused by
Pseudomonas aeruginosa, which has been intensively studied as a
model for biofilm formation (2, 3). The matrix, which holds
bacterial biofilms together, is a complex mixture of
macromolecules including exopolysaccharides, proteins, and DNA
(4). The latter has been presumed to be derived from lysed cells
and has not been thought to represent an important component of
biofilm structure. However, it has been known for many years
that some bacteria, including P. aeruginosa, produce substantial
quantities of extracellular DNA through a mechanism that is
thought to be independent of cellular lysis and that appears to
involve the release of small vesicles from the outer membrane
(5).
2) During studies of alginate biosynthesis in P. aeruginosa, the
authors report they discovered that the majority of the
extracellular material that reacted in the carbazole
colorimetric assay was not exopolysaccharide but DNA [as
determined by its peak absorbance at 260 nm, by electrophoretic
display, and by its deoxyribonuclease (DNase) but not
ribonuclease sensitivity] and therefore hypothesized that this
DNA may play a functional role in P. aeruginosa biofilms. Using
a tube ring assay (2), the authors found that addition of DNase
I to the culture medium strongly inhibited biofilm formation,
although not bacterial growth per se.
3) The authors suggest that these and other results indicate
that extracellular DNA is required for the initial establishment
of P. aeruginosa biofilms and perhaps biofilms formed by other
bacteria that specifically release DNA. The source of this DNA
is unclear, but it is presumably derived from membrane vesicles
rather than cell lysis, since the authors observed no evidence
of the latter during biofilm formation.
4) Much of the tissue damage associated with P. aeruginosa
infections of the cystic fibrosis lung epithelia is due to
inflammatory responses of the host immune system, which may
include responses to bacterial DNA. The current treatment regime
for cystic fibrosis patients includes inhalation of nebulized
recombinant human DNase I as a therapy to reduce the viscosity
of purulent sputum. Our findings suggest that DNase I treatment
might be beneficial as an early prophylactic measure to prevent
the establishment of chronic P. aeruginosa infection of the
cystic fibrosis lung by inhibiting biofilm formation. The
authors also suggest that DNase I may be useful in preventing
bacterial biofilms in other contexts.
References (abridged):
1. J. W. Costerton, P. S. Stewart, E. P. Greenberg, Science
284,1318 (1999)
2. G. A. O'Toole, R. Kolter, Mol. Microbiol. 30, 295 (1998)
3. P. K. Singh et al., Nature 407, 762 (2000)
4. I. W. Sutherland, Trends Microbiol. 9, 222 (2001)
5. Y. Muto and S. Goto, Microbiol. Immunol. 30, 621 (1986)
Science 2002 295:1487
Related Background:
ORGANIC COATINGS OF MINERAL SURFACES: BIOFILMS
In general, the term "passivation" refers to the reaction of a
solid with another substance in a way such that a protective
layer forms on the surface of the solid, the layer essentially
causing the cessation of the reaction. The solid is then said to
be "passivated". An example is the reaction of a solid metal
with gaseous oxygen to form an oxide surface coat that
subsequently prevents further oxidation.
A.S. Templeton et al (Stanford University, US) discuss mineral
surfaces, the authors making the following points:
1) The dissolved concentration of trace metal in terrestrial and
marine environments is directly linked to sorption and
precipitation reactions at mineral surfaces. However, various
types of natural organic matter form ubiquitous surface coatings
on minerals exposed to aqueous solutions.
2) Among the most complex and dynamic types of common organic
coatings are "biofilms". Such films form when microbial
organisms attach to a surface and produce a highly hydrated
framework of extracellular polymers in which the microorganisms
become embedded. These biofilms may have a sorptive capacity
similar in magnitude to many reactive mineral substrates, and
thus are potentially significant sinks for metals.
3) The presence of organic coatings may also alter the
reactivity of the underlying mineral surface through blocking of
high-energy surface sites or through modifying the electrical
properties of the mineral-water interface. In particular, it has
been observed that bacterial cells often preferentially attach
to surface features such as scratches, pits, cleavage steps, and
edges of mineral grains. Passivation of the mineral surfaces
could result if functional groups present within bacterial
surface polymers (or within the exopolysaccharide matrix) were
directly bound by a ligand-exchange mechanism to these
high-energy sites on the mineral surface. How these biofilm
coatings may alter metal ion partitioning between mineral
surfaces and the surrounding aqueous environment is largely
unresolved.
Proc. Nat. Acad. Sci. 2001 98:11897
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11. ON CAT-SCRATCH DISEASE
Cat-scratch disease (cat-scratch fever) is an infection caused
by the bacterium Bartonella henselae, the disease characterized
by regional lymphadenitis after the formation of a papule at the
site of a cat scratch. Small gram-negative pleomorphic bacilli,
now identified as B. henselae by polymerase chain reaction, can
be cultured from lymph nodes. The bacteria can be identified
microscopically in sections of lymph nodes and in the primary
papules. Antibodies to B. henselae have been demonstrated in 84%
of a group of patients meeting the clinical criteria for
cat-scratch disease. The domestic cat is a major reservoir for
B. henselae. The prevalence of B. henselae antibodies in cats in
the USA is 14 to 50%. In one study, 41% of pet cats had
bacteremia, although all were asymptomatic. About 99% of
patients report contact (and may remember getting scratched)
with felines, and 78% specify kittens. Most of the implicated
felines are healthy. The cat flea may be an additional vector.
The US Centers for Disease Control and Prevention presents a
report on cat-scratch disease, the report making the following
points:
1) Cat-scratch disease (CSD), a bacterial infection caused by
Bartonella henselae, has emerged as a relatively common and
occasionally serious zoonotic disease among children and adults.
CSD was first described as a clinical syndrome in 1931, but it
was not until 1983 that a bacterial etiology was determined, and
in 1992, the specific cause of CSD was identified. CSD is a
feline-associated zoonotic disease, with an estimated annual
incidence in the United States of 22,000 cases. Although CSD
occurs in persons of all ages, the highest age-specific
incidence is among children aged <10 years. Infection with B.
henselae is one of the most common causes of chronic
lymphadenopathy among children, and in some case series up to
25% of the these infections result in severe systemic illness.
Other serious manifestations of CSD are granulomatous
conjunctivitis, neuroretinitis, and atypical pneumonia. In
immunocompromised persons, B. henselae infections can cause
other potentially life-threatening disease manifestations (e.g.,
bacillary angiomatosis and peliosis).
2) Serologic testing is the standard method of diagnosis and
should be considered for patients who present with adenopathy,
fever, malaise, and history of feline contact. A single elevated
indirect immunofluorescence assay titer or enzyme immunoassay
value for IgG or IgM antibodies are generally sufficient to
confirm CSD, because initiation of a humoral immune response
generally precedes or is concurrent with symptom onset. IgG
levels rise during the first 2 months after onset of illness,
followed by a gradual decline. Other diagnostic assays,
including polymerase chain reaction and bacterial culture, are
available on a more limited basis at reference laboratories.
3) Treatment recommendations for Bartonella-associated diseases,
including CSD, depend on the specific disease presentation. For
most forms of CSD, assessing the efficacy of various antibiotics
is difficult because symptoms are generally self-limiting over
time, even in the absence of specific therapy. Recent experience
with azithromycin suggests that this antibiotic hastens
resolution of adenopathy of CSD. For patients with more severe
disease, other antibiotic regimens have been successful,
including azithromycin or doxycycline in combination with
rifampin or rifampin alone7; doxycycline or erythromycin are
considered the drugs of choice for bacilliary angiomatosis and
peliosis.
Morbidity & Mortality Weekly Report 2002 51:371
Web Links: cat scratch disease
ScienceWeek http://www.scienceweek.com
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12. ON CHILDREN AND AIDS IN AFRICA
Geoff Foster (Mutare Provincial Hospital, ZW) discusses AIDS in
Africa, the author making the following points:
1) It is estimated that in 26 African countries, the number of
children who are orphaned, for any reason, will more than double
by 2010.(1) In 1990, AIDS accounted for 16 percent of deaths
that left children orphaned in these countries; by 2010, the
proportion will be 68 percent. In southern Africa, the most
severely affected region, the number of children who have lost
both parents will increase by a staggering 1600 percent (from
0.2 million to 3.4 million). In 11 countries in Africa with a
combined population of 109 million people, 20 to 37 percent of
children under the age of 15 years will have lost one or both
parents by 2010, in most cases as a result of the AIDS
epidemic.(1)
2) Even though African children have been hardest hit by the
pandemic, the problem is not confined to Africa. Latin America,
Asia (including Cambodia, Myanmar, Vietnam, and India), the
former Soviet Union, and even the United States have large
numbers of children whose parents have died from AIDS.(1,2)
Human immunodeficiency virus (HIV) infection and AIDS are making
millions of additional children vulnerable, including those with
ill parents, those in poor households that have taken in
orphans, and those living in communities impoverished by AIDS.
3) The scale of the AIDS epidemic in Africa makes its
repercussions qualitatively different from those in other parts
of the world. Although there have been substantial gains in
improving overall survival among children, these gains are being
eroded in the African countries hardest hit by the epidemic.(3)
The economic and social effects of HIV infection and AIDS on
children include malnutrition, reduced access to education and
health care, migration, and homelessness. Psychological effects
include depression, guilt, and fear, possibly leading to
long-term mental health problems. The combination of these
effects on children increases their vulnerability to a range of
consequences, including HIV infection, illiteracy, poverty,
child labor, exploitation, and unemployment when they reach
adulthood.(4,5)
4) It has traditionally been said that there is no such thing as
an orphan in Africa, since children who have lost their parents
have been traditionally incorporated into a relative's family.
But with increased numbers of orphans, reduced numbers of
caregivers, and weakened families, the extended family is no
longer the safety net that it once was, although it remains the
predominant source of care for orphans in Africa. Relatives go
to considerable lengths to keep orphaned children in school,
including borrowing money through informal networks and selling
their own assets. For the most part, they treat these children
the same way as they treat their biologic children. Given the
scale of the AIDS epidemic in Africa, it is not surprising that
children are on the streets, in child-headed households, or
working as laborers. What is remarkable is that so few children
are slipping entirely through the safety net and ending up in
situations of extreme vulnerability.
References (abridged):
1. Hunter S, Williamson J. Children on the brink: executive
summary: updated estimates & recommendations for intervention.
Washington, D.C.: Agency for International Development, 2000.
(Also available at http://www.synergyaids.com/children.htm
2. Levine C, Stein GL. Orphans of the HIV epidemic: unmet needs
in six U.S. cities. New York: Orphan Project, 1994.
3. Foster G. Today's children -- challenges to child health
promotion in countries with severe AIDS epidemics. AIDS Care
1998;10:Suppl 1:S17-S23.
4. Foster G, Williamson J. A review of current literature on the
impact of HIV/AIDS on children in sub-Saharan Africa. AIDS
2000;14:Suppl 3:S275-S284.
5. Foster G, Germann S. The orphan crisis. In: Essex M, Mboup S,
Kanki PJ, Kalengayi MR, eds. AIDS in Africa. 2nd ed. New York:
Raven Press (in press).
New Engl. J. Med. 2002 346:1907
Web Links: AIDS and African children
ScienceWeek http://www.scienceweek.com
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13. ON THE EMERGENCE OF MODERN HUMAN BEHAVIOR
C.S. Henshilwood et al (Iziko Museum of Cape Town, ZA) discuss
the emergence of human behavior, the authors making the
following points:
1) Archaeological evidence associated with modern cognitive
abilities provides important insights into when and where modern
human behavior emerged (1). Two models for the origins of modern
human behavior are current: (a) a late and rapid appearance at
approximately 40 to 50 thousand years ago (ka) associated with
the European Upper Paleolithic and the Later Stone Age (LSA) of
sub-Saharan Africa (2,3) or (b) an earlier and more gradual
evolution rooted in the African Middle Stone Age (MSA;
approximately 250 to 40 ka)(4,5). Evidence for modern behavior
before 40 ka is relatively rare and often ambiguous (2).
However, in sub-Saharan Africa, archaeological evidence for
changes in technology, economy, and social organization and the
emergence of symbolism in the Middle Stone Age may support the
second model (4, 5). Examples of these changes include
standardized formal lithic tools (5), shaped bone implements
(5), innovative subsistence strategies such as fishing and
shellfishing, and the systematic use of red ochre.
2) Utilized ochre is found in almost all Stone Age occupations
in southern Africa that are younger than 100 ka. The ochre may
have served only utilitarian functions (e.g., skin protection or
hide tanning)(3) or may have been used symbolically as
pigment(4). Evidence for the latter is a persistent use of ochre
with saturated red hues to produce finely honed crayon or pencil
forms. However, no ochre pieces or other artifacts older than
approximately 40 ka provide evidence for abstract or depictional
images, which would indicate modern human behavior(2).
3) The authors report they have recovered two pieces of engraved
ochre from the Middle Stone Age layers at Blombos Cave, South
Africa. Situated on the southern Cape shore of the Indian Ocean,
the cave is 35 meters above sea level. A 5- to 60-cm layer of
aeolian sand containing no archaeological artifacts separates
the Later Stone Age from the Middle Stone Age occupation layers.
A mean date of 77,000 years was obtained for the layers
containing the engraved ochres by thermoluminescence dating of
burnt lithics, and the stratigraphic integrity was confirmed by
an optically stimulated luminescence age of 70,000 years on an
overlying dune. These engravings support the emergence of modern
human behavior in Africa at least 35,000 years before the start
of the Upper Paleolithic.
References (abridged):
1. The term "modern human behavior" as used here has no
chronological implication and means the thoughts and actions
underwritten by minds equivalent to those of Homo sapiens today.
Key among these is the use of symbols.
2. P. A. Mellars, K. Gibson, Eds., Modelling the Early Human
Mind (McDonald Institute Monographs, Cambridge, 1996).
3. R. G. Klein, The Human Career (Univ. of Chicago Press,
Chicago, IL, 1999).
4. H. J. Deacon, J. Deacon, Human Beginnings in South Africa:
Uncovering the Secrets of the Stone Age (David Philip, Cape
Town, South Africa, 1999).
5. S. McBrearty and A. Brooks, J. Hum. Evol. 38, 453 (2000)
Science 2002 295:1278
Web Links: evolution of human behavior
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