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ScienceWeek
SCIENCE-WEEK
A Weekly Email Digest of the News of Science
A journal devoted to the improvement of communication
between the scientific disciplines, and between scientists,
science educators, and science policy-makers.
March 2, 2001 -- Vol. 5 Number 9
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A new scientific truth does not triumph by
convincing its opponents and making them see
the light, but rather because its opponents
eventually die, and a new generation grows up
that is familiar with it.
-- Max Planck (1858-1947)
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Section 1
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Contents of this Issue (Full reports in Section 2):
1. PLANETARY SCIENCE:
GEOCHEMICAL EVIDENCE FOR WATER WITHIN MARS
Observations of Martian surface morphology have suggested that an
ancient ocean once existed on Mars. It has been proposed that
significant quantities of such water could have been supplied to
the Martian surface via outgassing, but this suggestion is
apparently contradicted by the low magmatic water content that is
generally inferred from chemical analysis of igneous Martian
meteorites. Researchers now report the distributions of trace
elements within pyroxenes of the Shergotty meteorite -- a basalt
body apparently ejected 175 million years ago from Mars -- as
well as hydrous and anhydrous crystallization experiments that
together imply that water contents of pre-eruptive magma on Mars
could have been up to 1.8 percent. The authors suggest that water
was present in pyroxenes at depth, but was largely lost as
pyroxenes were carried to the surface during magma ascent. The
authors conclude that ascending magmas possibly delivered
significant quantities of water to the Martian surface in recent
times. (H.V. McSween Jr et al: Nature 25 Jan 01 409:487)
2. GEOPHYSICS: ON CRUSTAL DEFORMATIONS AND MANTLE DYNAMICS
Recent discoveries have produced a vivid and dynamic picture of
the motions of the mantle, and researchers are beginning to
understand that these motions shape the surface of the Earth in
many ways. Such motions help to drive the horizontal movement of
tectonic plates, but they also lift and lower the continents.
Enigmatic dips and swells have occurred over continent-size
swaths of the Earth's surface several times in the past. Southern
Africa, for example, has been lifted approximately 1000 feet over
the past 20 million years, and the highest peaks of a sunken
continent today form the islands of Indonesia. The causes of
these vertical motions apparently lie deep within the interior of
the planet and involve mantle dynamics. Perhaps the most
intriguing discovery is that motion in the deep mantle lags
behind the horizontal movement of tectonic plates.
(Michael Gurnis: Scientific American March 2001)
3. MATERIALS SCIENCE: ON THERMOELECTRICS
Semiconductor material can be made to conduct electricity well by
supplying mobile electrons or holes derived from the doping of
the material with "impurities" that slightly disorder the crystal
lattice. In these same semiconductors, heat is carried mostly by
vibrational waves (phonons) moving through the atomic lattice.
Most semiconductors have high thermal conductivity. But if the
propagation of phonons in a semiconductor can be impeded by
scattering due to disorder in the crystal structure, the normally
high value of thermal conductivity will drop: the more disorderly
the internal atomic arrangement, the more poorly the material
will conduct heat. An ideal thermoelectric material, therefore,
would be a well-ordered atomic structure allowing electric charge
to move freely, but an atomic structure that at the same time
scatters phonons. An ideal thermoelectric material should be a
"phonon glass" and an "electron crystal".
(G.S. Nolas and G.A. Slack: Amer. Scientist Mar/Apr 2001 89:136)
4. EVOLUTIONARY BIOLOGY:
A CRITIQUE OF THE COEVOLUTION THEORY OF THE GENETIC CODE
The "code coevolution" hypothesis proposes that the genetic code
coevolved with the evolution of biosynthetic pathways for new
amino acids. A re-examination of the biochemical basis of the
genetic code coevolution theory to test the validity of its
statistical support demonstrates that the theory's definition of
"precursor-product" amino acid pairs is unjustified biochemically
because it requires the energetically unfavorable reversal of
steps in extant metabolic pathways to achieve desired
relationships. In addition, it is suggested that coevolution
theory neglects important biochemical constraints when
calculating the probability that chance could assign precursor-
product amino acids to contiguous codons. The authors report a
conservative correction for these errors reveals a surprisingly
high 23 percent probability that apparent patterns within the
code are caused purely by chance.
(T.A. Ronneberg et al:
Proc. Natl. Acad. Sci. US 5 Dec 00 97:13690)
5. ZOOLOGY: A UNIQUE EXAMPLE OF CHEMICAL DEFENSE IN INSECTS
Researchers report that males of the moth C. myrodora acquire
pyrrolizidine alkaloid by feeding on the excrescent fluids of
certain plants (e.g., Eupatorium capillifolium). The male moths
incorporate the alkaloid systemically and as a result are
protected against spiders. The males have a pair of abdominal
pouches, densely packed with fine cuticular filaments, which in
alkaloid-fed males are alkaloid laden. The male discharges the
filaments on the female in bursts during courtship, embellishing
the female with alkaloid as a result. This topical investiture
protects the female against spiders. Alkaloid-free filaments,
from alkaloid-deprived males, convey no such protection. The
males also transmit alkaloid to the females by seminal infusion.
The systemic alkaloid thus received by the female, which itself
may contribute to the female's defense against spiders, is
bestowed in part by the female on the eggs.
(W.E. Conner et al: Proc. Natl. Acad. Sci. US 19 Dec 00 97:14406)
6. ANTHROPOLOGY:
DIET AND THE EVOLUTION OF THE EARLIEST HUMAN ANCESTORS
Recent research with the scanning electron microscope has
revealed tiny pits and scratches on the surfaces of tooth enamel
("microwear patterns") that provide evidence of dietary
conditions. An analysis indicates that the dietary capabilities
of the early hominids changed dramatically in the time period
between 4.4 to 2.3 millions years ago. Most of the evidence has
come from five sources: analyses of tooth size, tooth shape,
enamel structure, dental microwear, and jaw biomechanics. Taken
together, the evidence suggests a dietary shift in the early
australopithecines, a shift to increased dietary flexibility in
conjunction with apparent climate variability. Moreover, changes
in diet-related adaptations from A. anamensis to A. afarensis to
A. africanus suggest that hard abrasive foods became increasingly
important through the Pliocene, perhaps as critical items in the
diet.
(M.F. Teaford and P.S. Ungar:
Proc. Natl. Acad. Sci. US 5 Dec 00 97:13506)
7. IN FOCUS: ON THE CURIES
8. FROM THE SCIENCEWEEK ARCHIVE:
ON THE DUAL-CAREER-COUPLE PROBLEM IN PHYSICS
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Section 2
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1. PLANETARY SCIENCE:
GEOCHEMICAL EVIDENCE FOR WATER WITHIN MARS
Mars is now a desert world on which liquid water, because of
ambient conditions, is not likely to be found at the surface:
average temperatures are below 273 degrees kelvin and atmospheric
pressures are at or below water's triple-point vapor pressure of
6.1 millibars. However, in 1972 the Mariner 9 orbiter mission
photographed evidence -- in the form of apparent giant flood
channels and arborized networks of small valleys -- that liquid
water might have been stable in the surface environment at some
time in the past. Analysis of Mars 4 and Mars 5 data, Viking
orbiter images (1976-1980), and observations of flood terrain by
Mars Pathfinder in 1997 supported this conclusion.
In general, the term "metamorphism" refers to the process of
changing the characteristics of a rock in response to changes in
temperature, pressure, or volatile content. Most metamorphic
changes of rocks do not include bulk chemical changes, but merely
the crystallization of new mineral phases. Examples of the
transformation of sediments through metamorphism are sand to
sandstone and peat to coal. The term "regional metamorphism"
refers to the recrystallization of pre-existing rocks in response
to simultaneous changes of ambient conditions that include
temperature and the weight of overlying rocks (lithostatic
pressure). The term "contact metamorphism" (thermal metamorphism)
refers to the recrystallization of rocks surrounding an igneous
intrusion in response to the heat supplied by that intrusion.
In general, the term "magma" refers to molten rock generated
by partial melting of a planet's crust. Igneous rocks are rocks
that have congealed from a molten mass, particularly from magma.
The term "pyroxene" refers to any of a group of rock-forming
minerals of variable composition, among which calcium-,
magnesium-, and iron-rich varieties predominate. Pyroxenes are
found in almost every variety of igneous rock, and they also
occur in rocks of widely different compositions formed under
conditions of regional and contact metamorphism.
Meteorites are divided roughly into 3 main classes according
to their composition. "Iron meteorites" consist of an alloy of
iron and nickel; "stony meteorites" consist of silicate minerals;
and "iron-stony meteorites" are a mixture of the two previous
types. The stony meteorites are further divided into "chondrites"
and "achondrites". Chondrites contain small spherules of
high-temperature silicates ("chondrules") and constitute more
than 85 percent of recovered meteorites. The achondrites (which
contain no chondrules) range in composition from rocks made up
essentially of single minerals (e.g., olivine) to rocks
resembling basaltic lava. Each category is further subdivided on
the basis of chemical composition. (The olivines are a group of
metal-silicate minerals, with the metal as magnesium, iron,
manganese, and calcium, with minor amounts of nickel. The most
common form is a solid solution of magnesium silicate and iron
silicate.)
Basalt is a dark gray to black igneous rock of volcanic
origin that cools rapidly.
In this context, the term "outgassing" refers to the release
of gases by volcanic activity.
"Shergottite" is a very rare type of achondrite meteorite,
named after the meteorite that fell at Shergotty, India, in 1865,
the first known meteorite fall of this type. The Shergotty
meteorite consists primarily of pyroxene, and it belongs to the
class of meteorites known as SNC meteorites (shergottites,
*nakhlites, *chassignites; called "snick" meteorites") which on
the basis of their composition are considered to be from Mars.
The SNC meteorites are igneous rocks that apparently solidified
from a cooling magma near the surface of their parent body. All
but one are relatively young (less than 1.3 billion years old);
the single ancient SNC meteorite found to date was apparently
formed 4.5 billion years ago. The proportions and isotopic
abundance of noble gases trapped in one shergattite resemble the
composition of the Martian atmosphere as analyzed by the Viking
landers. The current consensus is that the SNC meteorites were
ejected from Mars by impacts, and entered orbits around the Sun
before falling to Earth.
... ... H.Y. McSween Jr et al (7 authors at 4 installations, US)
present an analysis of the pyroxenes in the Shergotty meteorite,
the authors making the following points:
1) The authors point out that observations of Martian
surface morphology have been used to argue that an ancient ocean
once existed on Mars. It has been proposed that significant
quantities of such water could have been supplied to the Martian
surface via outgassing, but this suggestion is apparently
contradicted by the low magmatic water content that is generally
inferred from chemical analysis of igneous Martian meteorites.
2) The authors now report the distributions of trace
elements within pyroxenes of the Shergotty meteorite -- a basalt
body apparently ejected 175 million years ago from Mars -- as
well as hydrous and anhydrous crystallization experiments that
together imply that water contents of pre-eruptive magma on Mars
could have been up to 1.8 percent. The authors report they have
found that in the Shergotty meteorite, the inner cores of
pyroxene minerals (which are believed to have formed at depths in
the Martian crust) are enriched in soluble trace elements when
compared to the outer rims of the minerals (which are believed to
have crystallized on or near to the Martian surface). The authors
suggest this implies that water was present in pyroxenes at
depth, but was largely lost as pyroxenes were carried to the
surface during magma ascent. The authors conclude that ascending
magmas possibly delivered significant quantities of water to the
Martian surface in recent times, this conclusion reconciling
geologic and petrologic views of the outgassing history of Mars.
-----------
H.V. McSween Jr et al: Geochemical evidence for magmatic water
within Mars from pyroxenes in the Shergotty meteorite.
(Nature 25 Jan 01 409:487)
QY: Harry V. McSween Jr: mcsween@utk.edu
-----------
Text Notes:
... ... *nakhlites: A rare type of achondrite meteorite, named
after the meteorite that fell at Nakla, Egypt, in 1911, the first
known meteorite fall of this type. The nakhlites (also called
"augite-olivine achondrites) consist of approximately 80 percent
by weight of augite (a type of pyroxene) and approximately 14
percent by weight of iron-rich olivine. The textures of nakhlites
suggest they formed within cooling magmas.
... ... *chassignites: Another rare type of achondrite meteorite,
named after the meteorite that fell at Chassigny, France, in
1815, the only known meteorite fall of this type. Chassignite
(also called "olivine achondrite) is an olivine-rich rock: the
Chassigny meteorite contains 92 percent olivine.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 2Mar01
For more information: http://scienceweek.com/swfr.htm
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2. GEOPHYSICS: ON CRUSTAL DEFORMATIONS AND MANTLE DYNAMICS
Studies of seismic wave velocities have been central to our
understanding of the structure of the deep interior of the Earth.
The center of the Earth, located below 2900 kilometers, comprises
the "core", which is divided into two parts. The "outer core" is
believed to be composed of molten metallic iron alloyed with
nickel and some light elements such as oxygen, hydrogen, carbon,
or sulfur. In contrast, the "inner core", which extends from 5155
kilometers to the Earth's center, is probably composed of solid
iron-nickel metal with significantly lesser amounts of light
components. Overlying the core is the "mantle", divided into
"upper mantle" and "lower mantle" by a zone of rapid increase in
density and seismic wave velocity at approximately 700 kilometers
depth. The so-called "crust", the outermost solid layer of the
Earth, represents less than 1 percent of the Earth's volume and
varies in thickness from approximately 5 kilometers beneath the
oceans to approximately 60 kilometers beneath continental
mountain chains. The term "lithosphere" refers to the upper
(oceanic and continental) layer of the solid Earth, the
lithosphere comprising all crustal rocks and the brittle part of
the uppermost mantle. At the high temperatures that exist beneath
the lithosphere, the mantle is apparently slowly deforming, its
flow associated with "plate tectonics".
In general, the modern theory of plate tectonics provides a
framework for understanding the origin of continents, ocean
basins, and mountain ranges. In its presently accepted form, the
theory of plate tectonics divides the surface area of the Earth
into several "plates" that move relatively independently over the
surface of the planet. These plates consist of the crust and the
uppermost part of the mantle to a depth of approximately 100
kilometers: a plate is thus a segment of the lithosphere. Each
plate essentially behaves as a rigid solid shell, while the
material at greater depths in the Earth slowly flows under it.
Plates usually contain both oceanic and continental crust, with
the familiar continents passive passengers on the topmost parts
of their respective plates. In general, a plate may be as broad
as 10,000 kilometers (e.g., the Pacific plate) or as small as a
few thousand kilometers (e.g., the Philippines plate). There are
12 major plates (Antarctica, Africa, Eurasia, India, Australia,
Arabia, Philippines, North America, South America, Pacific,
Nazea, and Cocos) and several minor plates (e.g., Scotia,
Caribbean, Juan de Fuca).
Although the lateral movements of plates and plate-to-plate
contacts have received much attention, there is evidence that
vertical deformations of plates by underlying mantle dynamics are
of significance.
... ... Michael Gurnis (California Institute of Technology, US)
presents a review of current research on mantle dynamics and
crustal deformations, the author making the following points:
1) Recent discoveries have produced a vivid and dynamic
picture of the motions of the mantle, and researchers are
beginning to understand that these motions shape the surface of
the Earth in many ways. Such motions help to drive the horizontal
movement of tectonic plates, but they also lift and lower the
continents. Enigmatic dips and swells have occurred over
continent-size swaths of the Earth's surface several times in the
past. Southern Africa, for example, has been lifted approximately
1000 feet over the past 20 million years, and the highest peaks
of a sunken continent today form the islands of Indonesia. The
causes of these vertical motions apparently lie deep within the
interior of the planet and involve mantle dynamics. Perhaps the
most intriguing discovery is that motion in the deep mantle lags
behind the horizontal movement of tectonic plates, so that
positions of ancient plate boundaries can be related to the way
the surface of the Earth is shaped many millions of years later.
2) The author suggests that our ability to view the dynamics
of mantle convection and plate tectonics will rapidly expand as
new ways of observing the mantle and techniques for simulation
its motions are introduced. When mantle convection changes, the
gravitational field of the Earth changes, so that tracking
variations in Earth's gravitational field is an important
research objective. The author concludes: "Plans to make
unprecedented images and measurements of the mantle in the coming
decade, together with the use of ever more powerful
supercomputers, foretell an exceptionally bright future for
deciphering the dynamics of the Earth's interior."
-----------
Michael Gurnis: Sculpting the Earth from inside out.
(Scientific American March 2001)
QY: Michael Gurnis: Calif. Institute of Technology 818-395-6811
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 2Mar01
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
GEOPHYSICS: ON SEISMIC TOMOGRAPHY AND MANTLE DYNAMICS
Seismic studies indicate the interior of the Earth consists
of three parts: a metallic core, a dense rocky mantle, and a thin
low-density crust. The central part of the core is solid, but the
outer part of the core is evidently liquid. The mantle, the layer
of dense rock and metal oxides between the molten part of the
core and the surface, has plastic properties (i.e., it is a solid
capable of flow under pressure).
The term "lithosphere" refers to the outer layer of the
Earth, comprising the crust and upper mantle, and extending to a
depth of 50 to 70 kilometers. The traditional view of tectonics
(changes in the structure of the Earth's crust) is that the
lithosphere consists of a strong brittle layer overlying a weak
ductile layer.
The term "subduction" refers to the process of
underthrusting of the edge of an oceanic plate into the mantle
underlying an adjacent plate. In this context, the term "plate"
derives from "plate tectonics", the current consensus theory that
the Earth's lithosphere is broken into fairly rigid plates, seven
major plates and many smaller plates, and that convection within
the underlying less rigid "asthenosphere" causes the plates (and
the associated continents and crust) to move relative to each
other.
In general, the term "tomography" refers to a representation
in cross-section in which neighboring 2-dimensional cross-
sections are combined to provide a 3-dimensional model. The use
of computer-aided tomography (CAT) in medical diagnosis is well-
known as a non-invasive method of examining internal organs for
abnormal regions. X-rays or ultrasonic waves are absorbed
unequally be different materials, and computer-aided tomography
consists of studying the attenuation of x-rays or ultrasonic
waves that pass through the body in distinctly controlled planar
sections. The technique of "seismic tomography" uses the same
principles, with the difference that the travel-times of the
signals, rather than their attenuation, are observed. Thus, the
technique of seismic tomography may be described as the 3-
dimensional modeling of the velocity distribution of seismic
waves in the Earth. In general, the technique requires powerful
computational facilities and sophisticated programming.
... ... T. Tanimoto and T. Ley (2 installations, US) present a
review of current research on mantle dynamics and seismic
tomography, the authors making the following points:
1) The authors point out that the advent of the theory of
plate tectonics approximately 30 years ago established that most
near-surface geological phenomena such as earthquakes, volcanoes,
and mountain belts can be understood in the context of a unifying
model of interacting surface plates. However, our understanding
of this system has largely been limited to detailed kinematics of
plate motions, leaving the nature of the driving motions in the
interior as a puzzle. Questions such as what is the configuration
of convection, and how are surface tectonics controlled by
internal processes, have long been raised, but a lack of tools
and a lack of evidence prevented evaluation of various
hypotheses. Thus, most views regarding mantle dynamics remained
highly speculative until recently. Seismic tomography, which
emerged in the early 1980s, has provided a major probe of the
dynamical system of which plates are just the surface veneer.
2) The primary question concerning mantle dynamics is
whether mantle convection occurs in mantle-wide convective cells
or whether it involves a layered system, with separate flow
regimes in the upper mantle (i.e., above 650 kilometers) and
lower mantle. One of the most exciting results from work during
the last 5 years is the verification of deep penetration of
former oceanic lithosphere into the lower mantle. Tomography
shows thickened tabular extensions of subducted material to
depths as great as 2000 kilometers directly below deep subduction
zones where earthquakes occur in oceanic slabs down to
approximately 650-kilometer depth. Thus, strictly layered mantle
convection can now be ruled out with good confidence.
3) In summary, seismic tomography has resulted in
breakthrough advances in the last two decades, revealing
fundamental geodynamical processes throughout the Earth's mantle
and core. Convective circulation of the entire mantle is taking
place, with subducted oceanic lithosphere sinking into the lower
mantle, overcoming the resistance to penetration provided by the
phase boundary near 650-kilometer depth that separates the upper
and lower mantle. The boundary layer at the base of the mantle
has been revealed to have complex structure, involving local
stratification, extensive structural anisotropy, and massive
regions of partial melt. The Earth's high *Rayleigh number
convective regime is now recognized to be much more interesting
and complex than suggested by textbook cartoons, and continued
advances in seismic tomography, geodynamic modeling, and high-
pressure-high-temperature mineral physics will be needed to fully
quantify the complex dynamics of our planet's interior.
-----------
T. Tanimoto and T. Ley: Mantle dynamics and seismic tomography.
(Proc. Natl. Acad. Sci. US 7 Nov 00 97:12409)
QY: Toshiro Tanimoto, Univ. of Calif. Santa Barbara 805-893-8000.
-----------
Text Notes:
... ... *Rayleigh number: The Rayleigh number is a dimensionless
parameter used in the theory of fluid dynamics. In general, the
Rayleigh number provides a determination of when convection is
initiated in a fluid.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 24Nov00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
IN FOCUS: THE LAYERED EARTH
"During the nineteenth century, the nature of the Earth's
interior was a matter of fierce and fascinating debate. All
theories were hampered by a lack of evidence -- the nature of
rocks deep below the surface was unknown. In 1906, Richard D.
Oldham observed that compressional seismic waves (P waves) slow
abruptly deep within the Earth and can penetrate no further. This
was strong evidence in favor of a liquid core. Three years later,
Andrija Mohorovicic noticed that the velocity of seismic waves
leaps from 7.2 to 8.0 km/s at around 60 km deep. He had
discovered the 'Moho' seismic discontinuity that marks the crust-
mantle boundary. In 1926, Beno Gutenberg obtained evidence for a
seismic discontinuity at the core-mantle boundary. This, the
Gutenberg discontinuity, was confirmed during the 1950s when
world-wide records of blasts from underground nuclear detonations
were scrutinized. Subsequent studies of the Earth's seismic
properties, using seismic waves propagated by earthquakes and by
controlled explosions to 'x-ray' the planet (a technique called
seismic tomography), have revealed a series of somewhat distinct
layers or concentric shells in the solid Earth. Each shell has
different chemical and physical properties..."
-----------
Richard John Huggett: _Environmental Change_
(Routledge, London 1997, p.56)
[The author is Senior Lecturer in Geography at the University of
Manchester, UK] (Science-Week 13 Sep 99)
-------------------
Related Background:
EARTH SCIENCE: ON MANTLE PLUMES AND MOUNTAINS
... The term "hot spot" (also, hotspot) refers to a relatively
long-lasting center of surface volcanism and locally high heat
flow, and about 40 locations are now so labelled. Most hot spots
are in ocean basins, located at points where the lithosphere has
apparently upswelled, elevating the denser mantle material and
creating mass anomalies. "Mantle plumes" are thin vertical
conduits of molten rock material from the core-mantle boundary to
the crust. The term "subduction" refers to the process of
underthrusting of the edge of an oceanic plate into the mantle
underlying an adjacent plate.
... ... J.B. Murphy et al (4 authors at 3 installation, CA US)
present a review of current studies concerning the relation
between mantle plumes and mountain building (orogeny;
orogenesis), the authors making the following points:
1) Theories of mountain building were revolutionized in the
1960s by plate tectonic theory. The horizontal motions of rigid
plates of material above a pliable mantle helped explain the
creation and destruction of oceans, the generation of mountain
belts and sedimentary basins, the distribution of volcanic and
earthquake activity, and the locations of ore, oil, and gas
deposits. But plate-tectonic theorists have had difficulty
accounting for many of the geological details of southwestern
North America, including the uplifting of the Rocky Mountains,
the extent of the Basin and Range region of Nevada, Utah, and
Arizona, and the extensive volcanic deposits of the Columbia
Plateau.
2) Established methods of mountain building all depend
either directly or indirectly on subduction zones. Three methods
are currently recognized:
... ... a) A subduction zone may lead directly to mountains
formed by ascending *magma and heat, as is the case in the Andes.
... ... b) The subduction process may also transport
*microcontinental fragments to the continental margin, where they
accrete as "*terranes", a process that has added significantly to
the North American West Coast over the past 400 million years.
... ... c) When an ocean is consumed by colliding continents, as
in the example of India and Asia, spectacular mountain building
can result.
3) Mantle plumes produce island chains such as the Hawaiian
Islands. The plume remains relatively stationary while the
oceanic plate moves over it. Plumes are thought to rise all the
way from the core-mantle boundary, 2,900 kilometers below the
Earth's surface, in relatively narrow columns. On reaching the
base of the lithosphere, a plume spreads out, underplating a
large area of lithosphere, causing it to heat and be bowed
upward.
4) Hot spots, the surface manifestations of mantle plumes,
are widely distributed around the Earth, although the exact
number is controversial. Hot spots are essentially stationary
relative to the faster-moving plates. No modern ocean could be
consumed at a subduction zone without a plate margin encountering
a hot spot, so the interaction between subduction zones and hot
spots must be common throughout geological time.
5) Most of the mountain building activity on the western
margin of North America over the past 300 million years
represents episodes of *magmatism and deformation associated with
microcontinent collisions. The Sonoma, Nevada, and Sevier
mountain-building events are examples. The Rocky Mountain
(Laramide) orogeny, however, is distinctive because it is
characterized by a lack of magmatism coupled with widespread
deformation in the continental interior.
6) The authors propose that an additional (fourth) method of
mountain building has been largely overlooked and may help
explain not only the Laramide Orogeny but also other unusual
geological features of the southwestern US. Their model involves
the interplay of the horizontal motions of traditional
subduction-related mountain-building processes with vertical
plumes of hot mantle ascending from thousands of kilometers below
the Earth's surface. The authors suggest that "together these
mechanisms may offer a convincing explanation for what long has
been a geologically puzzling part of the world and may lead to
better understanding of mountain building worldwide."
-----------
J.B. Murphy et al: Mantle plumes and mountains.
(American Scientist Mar-Apr 1999 87:146)
QY: J. Brendan Murphy [bmurphy@stfx.ca]
-----------
Text Notes:
... ... *magma: In general, any molten mass of rock.
... ... *microcontinental fragments: In general, any fragment or
remnant of continental crust up to approximately the size of
Madagascar (Malagasy).
... ... *terranes: (terrains) In general, a terrane is any
region of crust with well-defined margins which differs
significantly in apparent tectonic evolution from neighboring
regions.
... ... *magmatism: In general, the development and movement of
magma within the Earth.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 11Jun99
For more information: http://scienceweek.com/swfr.htm
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3. MATERIALS SCIENCE: ON THERMOELECTRICS
In nature, what are called "thermoelectric effects" occur
when both electric and thermal currents are present. (So-called
"transverse thermoelectric effects" occur when in addition there
is a magnetic field normal to these currents.) There are
essentially three ordinary thermoelectric effects:
The "Seebeck effect", discovered in 1822 by Thomas Seebeck
(1770-1831), relates to the electromotive force developed in a
circuit consisting of different conducting elements, not all of
whose contacts are at the same temperature.
The "Peltier effect", discovered in 1834 by Jean Peltier
(1785-1845), involves the reversible heating or cooling which
occurs at a contact between two dissimilar conductors when
electric current flows from one conductor to another.
The "Thomson effect" (Kelvin effect) discovered by Lord
Kelvin (William Thomson) (1824-1907) in 1856, refers to the
reversible heat absorption which occurs when an electric current
flows in a homogeneous conductor in which there is a temperature
gradient. All three effects are related by thermodynamics: if one
effect is known, the other two can be derived.
The so-called Seebeck and Peltier coefficients are measures
of the respective effects: the larger the coefficient, the
greater the effect. In the Seebeck effect, for example, the
generated voltage difference is proportional to the temperature
difference, with the Seebeck coefficient another name for the
proportionality factor.
The term "clathrates" refers to substances in which
molecules of one compound (the "host") form a crystalline lattice
that contains open spaces ("cages") that may be occupied by
molecules of a second compound or element (the "guest").
Ordinarily, no chemical bonds are formed between the host and the
guests, with the stability of a clathrate dependent on the
closeness with which the guest entities fit into the holes as
well as on the polarizabilities of the guest entities.
Devices based on the thermoelectric effects are widely used,
but the applications of such devices are limited by a low
efficiency. This has led to a search for new thermoelectric
materials, and various clathrate compounds have been a recent
focus of research.
... ... G.S. Nolas and G.A. Slack (2 installations, US) present a
review of current research on thermoelectric clathrates, the
authors making the following points:
1) The authors point out that having large Peltier and
Seebeck coefficients is not the only requirement for a useful
thermoelectric material. The material must be both a good
electrical conductor and an equally good thermal insulator. A
thermoelectric cooler with poor electrical conductivity, for
example, would increase in temperature through resistance heating
when power was applied. And if the material did not act as a good
thermal insulator, the heat transported by the flow of electric
current would tend to leak backward from the hot side to the cold
side.
2) Semiconductor material can be made to conduct electricity
well by supplying mobile electrons or holes derived from the
doping of the material with "impurities" that slightly disorder
the crystal lattice. In these same semiconductors, heat is
carried mostly by vibrational waves (phonons) moving through the
atomic lattice. Most semiconductors have high thermal
conductivity. But G.A. Slack pointed out more than 20 years ago
that if the propagation of phonons in a semiconductor can be
impeded by lattice collisions (scattering) due to disorder in the
crystal structure, the normally high value of thermal
conductivity will drop: the more disorderly the internal atomic
arrangement, the more poorly the material will conduct heat. An
ideal thermoelectric material, therefore, would be a well-ordered
atomic structure allowing electric charge to move freely, but an
atomic structure that at the same time scatters phonons. Various
amorphous solids (glasses) transmit heat poorly because their
internal atomic arrangements are so disorderly. An ideal
thermoelectric material should be a "phonon glass" and an
"electron crystal".
3) Investigations more than two decades ago suggested that
semiconducting clathrates might have low values of thermal
conductivity while retaining relatively high electrical
conductivity. The elements commonly used to construct
semiconductors do, in fact, form clathrates. The general idea is
that as long as the voids in a semiconductor clathrate structure
remain empty, the material will transmit heat too well to serve
as a thermoelectric device. But if the cages are filled with
small heavy atoms, the thermal conductivity of the composite
material can be made extremely low as a result of scattering of
phonons by the heavy atoms. At the same time, the surrounding
crystal structure is still highly ordered and allows a high
mobility of electrons and holes. Since 1997, the authors have
been engaged in research on a fabricated germanium clathrate.
This clathrate is a strontium-gallium-germanium compound, with
the heavy strontium atoms acting as phonon scatterers. The
authors report they have achieved thermal conductivity close to
the theoretical minimum, with all heat-carrying phonons
apparently scattered. "So far we have definitely achieved our aim
of synthesizing a crystal that acts as a phonon glass. Still, we
are a long way from complete success: The best semiconducting
clathrates we have fabricated so far... are only about half as
good as the materials now being used commercially for cooling."
-----------
G.S. Nolas and G.A. Slack: Thermoelectric clathrates.
(American Scientist Mar/Apr 2001 89:136)
QY: George S. Nolas: gnolas@marlow.com
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 2Mar01
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ON THERMOELECTRIC DEVICES
... ... Terry M. Tritt (Clemson University, US) presents a short
review of current advances in the application of the Peltier
effect to achieve cooling of materials, the author making the
following points: 1) The cooling of electronic devices is often
necessary to optimize performance. Traditional approaches to
cooling are usually based on thermodynamic cycles involving
compression and expansion of refrigerant gases (e.g., Freon).
Thermoelectric materials, in contrast, do not rely on chemicals
or gases but rather on the Peltier effect in solids. This has
distinct advantages in that it is solid-state refrigeration,
without moving parts and vibrations, and with quiet performance
and the ability for localized "spot" cooling. These aspects can
be of considerable importance for many semiconducting and other
electronic devices. 2) In the past, the application of
thermoelectric devices has been frustrated by the low efficiency
of these devices, and the applications have remained limited.
Recently, however, new thermoelectric materials for practical
refrigeration applications have been developed. 3) In general,
the key to a useful thermoelectric material depends on the
achievement of good electrical conductivity and poor thermal
conductivity [*Note #1]. This is related to a concept formulated
by G.A. Slack in the 1970s: a good thermoelectric material should
be based on a *phonon-glass/electron-crystal model. In other
words, the material should have the electronic properties of a
crystalline material and the thermal properties of glass. 4)
Slack's concept, when applied to semiconductors, centers around
minimizing the lattice thermal conductivity of a material by
inserting loosely bound atoms (called "rattlers") into the voids
or holes in the structure. These rattlers will move or bounce
around inside these voids and thus scatter phonons (quantized
lattice vibrations), the result an effective reduction and
possible minimization of lattice thermal conductivity. 5) A
variety of approaches to the development of useful thermoelectric
materials are now being followed in various laboratories, and
many of the different approaches -- thin films and bulk
materials, varieties of semiconductor materials, etc. -- are
apparently showing promise. 6) The author concludes: "The
development of the next generation of thermoelectric materials,
especially for thermoelectric cooling well below room
temperature, could tremendously alter a number of high-tech
industries. Although a clear front-runner cannot yet be
identified, the diversity of approaches suggest that we may not
need to wait another 30 years for the next generation of
thermoelectric materials."
-----------
Terry M. Tritt: Holey and unholey semiconductors.
(Science 5 Feb 99 283:804)
QY: Terry M. Tritt [ttritt@clemson.edu]
-----------
Text Notes:
... ... *Note #1: In thermoelectric materials, heat is not
primarily transported by the lattice but by the electrical charge
carriers. Applying an electric current through a thermoelectric
material thus cools one end and transports the heat to the other
end of the material or device, with an efficiency that depends on
the heat conduction properties of the lattice through which the
heat carriers are moving. If heat conduction in the lattice is
poor, the temperature gradient produced by the electrical charge
carriers can be maintained. Thus, the ideal thermoelectric
material is at the same time both a poor heat conductor and a
good electrical conductor.
... ... *phonon: Phonons are virtual particles associated with
crystal thermal vibrations.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 14May99
For more information: http://scienceweek.com/swfr.htm
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4. EVOLUTIONARY BIOLOGY:
A CRITIQUE OF THE COEVOLUTION THEORY OF THE GENETIC CODE
In general, the term "genetic code" refers to the sequences
of nucleotides in DNA and RNA that determine the various amino
acid sequences of proteins. Proteins are not synthesized directly
from information in DNA; instead a "messenger" molecule,
"messenger RNA" (mRNA), is synthesized from DNA and then directs
the synthesis of protein in special structures called
"ribosomes". Messenger RNA is composed of 4 nucleotides: adenine
(A), guanine (G), cytosine (C), and uracil (U). Three adjacent
nucleotides constitute a unit known as a "codon", and it is this
unit that codes for a particular amino acid. For example, the
sequence AUG is a codon that specifies the amino acid methionine.
There are 64 possible codons, three of which do not code for
amino acids but instead indicate the end of a protein. The
remaining 61 codons specify the 20 amino acids that make up
proteins. The AUG codon, in addition to coding for methionine, is
found at the beginning of every messenger RNA, and this codon
indicates the start of a protein. Because most of the 20 amino
acids are coded for by more than one codon, the genetic code, as
a code, is "degenerate". The code was once believed to be
identical in all forms of life, but the codon-amino acid code
differs in *mitochondria, in certain bacteria, and in certain
*ciliated protozoa. The differences, however, are rare, and in
general the genetic code is identical in nearly all species, the
same codons specifying the same amino acids.
In this context, the term "quartet" derives from the
following: The 64 possible codons can be organized into 16
"quartets" defined by the first 2 nucleotides in a triplet. Thus,
UUU, UUC, UUA, UUG is one quartet; GAU, GAC, GAA, GAG is another
quartet.
As with any other biological subsystem, researchers believe
the genetic code evolved from a more primitive form, although no
ancestral codes have yet been discovered. Attempts at an
evolutionary reconstruction of the genetic code have therefore
generally relied on a detailed analysis of the features of the
present code, and those features of the genetic code of apparent
importance in theoretical logical and probabilistic
considerations of its evolution are as follows:
1) Messenger RNA molecules consist of only 4 kinds of
nucleotide bases, and these compose chains of varying lengths and
varying sequences.
2) A codon that specifies a particular amino acid is always
a triplet consisting of a chain of 3 nucleotides.
3) The code has no "spacers" (intermediary nucleotides)
between codons (the code is "commaless" and nonoverlapping): each
codon is translated in a continuous sequence, 3 successive
nucleotides at a time, from one end of a messenger RNA "reading
frame" to the other. (A "reading frame" is a particular
nucleotide sequence coding for a polypeptide that starts at a
specific point and then partitions into codons until it reaches
the final codon of that sequence.)
4) Each codon sequence is complementary to an "anticodon"
sequence of another RNA molecule, transfer RNA (tRNA), that
carries a particular amino acid to the messenger RNA codon.
5) Except for rare instances, all living organisms share the
same coding "dictionary".
6) Ambiguities have not been found in the code: the same
codon does not specify two or more amino acids.
7) With the exception of methionine and tryptophan, all
amino acids are each designated by more than one codon.
8) The pattern of degeneracy in the code is mostly in the
3rd codon position.
9) When an amino acid is coded for by only 2 of the codons
of the four in a quartet, the third codon positions in the amino
acid "duet" are both pyridines (U and C) or both purines (A and
G), never one pyrimidine and one purine.
So there is the problem: given that no ancestor codes are
available to us, the research goal is to provide a theoretical
model that can explain the evolution of the present universal
genetic code.
In evolutionary biology, the term "coevolution" refers to
the evolution of one or more species in synchrony with another
species as a consequence of their interdependence. In the context
that concerns the evolution of the genetic code, the term
"coevolution theory" refers to a set of theories proposing a
linked evolution between the evolution of the genetic code and
the evolution of amino acid biosynthetic pathways.
... ... T.A. Ronneberg et al (3 authors at Princeton University,
US) present a critique of coevolution theories of the evolution
of the genetic code, the authors making the following points:
1) The authors point out that the idea that the canonical
genetic code evolved from a simpler primordial form that encoded
fewer amino acids originates with F. Crick (1968). The more
recent (and more influential) version of this idea is the "code
coevolution" hypothesis introduced by J. Wong in 1975, which
proposes that the genetic code coevolved with the evolution of
biosynthetic pathways for new amino acids. Coevolution theory
further proposes that a comparison of modern codon assignments
with the evolutionary conserved metabolic pathways of amino acid
biosynthesis can reveal the history of code expansion: A central
tenet of coevolution theory is that a "product" amino acid
synthesized from a precursor amino acid usurped codons previously
assigned to this precursor, such that the sequence of steps by
which the code expanded is visible within modern codon
assignments.
2) The authors report they have re-examined the biochemical
basis of the code coevolution theory to test the validity of its
statistical support. The authors demonstrate that the theory's
definition of "precursor-product" amino acid pairs is unjustified
biochemically because it requires the energetically unfavorable
reversal of steps in extant metabolic pathways to achieve desired
relationships. In addition, the authors suggest coevolution
theory neglects important biochemical constraints when
calculating the probability that chance could assign precursor-
product amino acids to contiguous codons. The authors report a
conservative correction for these errors reveals a surprisingly
high 23 percent probability that apparent patterns within the
code are caused purely by chance. Finally, even this figure rests
on post hoc assumptions about primordial codon assignments,
without which the probability rises to 62 percent that chance
alone could explain the precursor-product pairings found within
the code. The authors state: "We conclude that coevolution theory
cannot adequately explain the structure of the genetic code."
-----------
T.A. Ronneberg et al: Testing a biosynthetic theory of the
genetic code: Fact or artifact?
(Proc. Natl. Acad. Sci. US 5 Dec 00 97:13690)
QY: Stephen J. Freeland: freeland@codon.princeton.edu
-----------
Text Notes:
... ... *mitochondria: 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 most biologists
believe mitochondria are cell organelles that may have originated
as separate organisms that became resident in eukaryotic cells.
Mitochondrial DNA is independent of nuclear DNA. It consists of a
circular molecule, 16,569 base pairs long in humans, with a known
nucleotide sequence.
... ... *eukaryotic cells: In general, refers to cells which
contain a nucleus and/or other membrane-bound organelles (e.g.,
mitochondria).
... ... *ciliated protozoa: A phylum (or subkingdom) comprising
unicellular and colonial animals of varied form, cells ranging
from simple to extremely complex macro-structures. "Cilia" are
short threadlike extensions, hundreds usually present on an
individual ciliated cell, the cilia undergoing synchronized
movements to produce locomotion of the protozoan.
-------------------
Related Background:
A MODEL FOR THE EVOLUTION OF THE GENETIC CODE
Eukaryotic cells are cells with discrete organelles such as
nuclei, mitochondria, etc. As organelles, the mitochondria, which
are of prime importance in the oxygen metabolism of eukaryotes,
are a special class, since it is generally believed they probably
originated as primitive cells that established themselves in
symbiotic arrangements in the interiors of larger cells. The
mitochondria, in fact, carry their own DNA and thus their own
genetics. In mathematics, a Lie algebra (named after M.S. Lie
1842-1899) is a system of vector fields on a topological space in
which independent quantities are reduced to groupings whose
relationships are then subject to algebraic operations. In the
context of this report, the term "representation theory" refers
to the study of algebraic groups by the use of their
representations, with the representation of a group given by a
virtual "mapping" of the group onto a group of another type.
Pyrimidine and purine are precursors of DNA nucleotide bases. A
codon is the elemental genetic coding unit, a triplet of 3
consecutive nucleotides that define a specific amino acid. Some
amino acids are determined by more than one codon, leading to a
degeneracy (redundancy) in the genetic code.
... ... Bashford et al (3 authors at University of Tasmania, AU)
present a model for the structure and evolution of the eukaryotic
and vertebrate mitochondrial genetic codes based on the
representation theory of a Lie superalgebra, with a key role
played by pyrimidine and purine exchange symmetries in codon
quartets. The authors suggest that the group theoretical
technique is able to give a succinct account of many of the
currently understood aspects of the evolution of the genetic code
and the observed degeneracy structure of the codon:amino-acid
correspondences, and that their particular model is susceptible
to quantitative verification.
QY: P.D. Jarvis jarvis@oberon.phys.utas.edu.au
(Proc. Natl. Acad. Sci. US 3 Feb 98) (Science-Week 20 Feb 98)
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
IN FOCUS: ON THE GENETIC CODE VS. HUMAN LANGUAGE
"The analogy between the genetic code and human language is
remarkable. Spoken utterances are composed of a sequence of a
rather small number of unit sounds, or phonemes (represented, at
least roughly, by the letters of the alphabet). The sequence of
these phonemes first specifies different words, and then, through
syntax, the meanings of sentences. By this system, the sequence
of a small number of kinds of unit can convey an indefinitely
large number of meanings. The genetic message is composed of a
linear sequence of only four kinds of unit. This sequence is
first translated, via the code, into a sequence of 20 kinds of
amino acid. These strings of amino acids fold to form three-
dimensional functional proteins. Through gene regulation, the
right proteins are made at the right times and places, and an
indefinite number of morphologies can be specified. Thus in both
systems a linear sequence of a small number of kinds of unit can
specify an indefinitely large number of outcomes. But there is
one respect in which the two systems cannot usefully be compared.
In language, the meanings of sentences depend on the rules of
syntax. These rules are formal and logical. In contrast, the
'meaning' of the genetic message cannot be derived by logical
reasoning. Thus, although the amino acid sequence of the proteins
can be simply derived from the genetic message, the way they fold
up to form three-dimensional structures, and the chemical
reactions they catalyze, depend on complex dynamic processes
determined by the laws of physics and chemistry. It does not seem
possible to draw a useful comparison between the way in which
meaning emerges from syntax, and that in which chemical
properties emerge from the genetic code."
-----------
John Maynard Smith and Eors Szathmary: _The Origins of Life_
(Oxford University Press, Oxford 1999, p.169)
-------------------
SCIENCE-WEEK http://scienceweek.com 6Aug99
For more information: http://scienceweek.com/swfr.htm
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5. ZOOLOGY: A UNIQUE EXAMPLE OF CHEMICAL DEFENSE IN INSECTS
Biological organisms vulnerable to predators have evolved an
extensive variety of defense mechanisms, and of these the
chemical defense mechanisms are of great importance. This is
particularly true of insects. Dermal glands of many insects
discharge repellant or poisonous secretions over the cuticle;
other insects are protected by poisons present in their blood and
tissues, such poisons often derived from the plants on which the
insects feed.
The term "moth" refers to any of several thousand species of
usually nocturnal insects grouped in 47 families in the order
Lepidoptera, which also includes butterflies. Moths vary greatly
in size, ranging in wing span from approximately 4 millimeters to
nearly 30 centimeters. They are highly diversified and are found
everywhere except in polar habitats. Although some moth species
are active during the day, they are generally nocturnal, have
stouter bodies than butterflies, duller coloring, smaller wings,
and feathery antennae. Also, when at rest, moths either fold
their wings or hold them extended at their sides. Like those of
other Lepidoptera, the life cycle of the moth has 4 stages: egg,
larva (caterpillar), pupa (chrysalis), and adult (imago). The
larvae and adults of most moth species are plant eaters. So-
called "tiger moths", of relevance in this report, are any
members of the family Arctiidae, which contains more than 3500
species, and when found in the tropics they are often brilliantly
colored.
Concerning chemical defense mechanisms, certain butterflies
and moths possess repellent or toxic substances that provide
protection against predators. Sometimes these substances are
secured directly from the plant on which the larva feeds; more
often, the toxin is secreted by the insect itself. Defensive
toxins may occur in the blood (e.g., hydrogen cyanide in zygaenid
moths) or in the gut, or may be the product of special glands
that release the toxin on the occasion of an attack. Many species
of tiger moths are known to give off bubbling drops of repellent
from glands on the anterior segment of the thorax (prothorax).
The term "alkaloids" refers to any of a class of naturally
occurring organic nitrogen-containing bases. The principal
classes of alkaloids are the pyrrolidines, pyridines, tropanes,
pyrrolizidines, isoquinolines, indoles, quinolines, terpenoids,
and steroids. In nature, alkaloids are found primarily in plants:
more than 3000 different types of alkaloids have been identified
in a total of more than 4000 plant species. The function of
alkaloids in plants in not clear, but it is believed they protect
some plants from destruction by insects.
This report concerns a species of tiger moth (extant in
central Florida [US] and of a brilliant red color) in which the
male adult moth secures pyrrolizidine alkaloids from plants, then
uses the alkaloid to protect both itself and its female partner
from predatory spiders.
... ... W.E. Conner et al (6 authors at 2 installations, US)
present a study of the moth Cosmosoma myrodora (Arctiides), the
authors making the following points:
1) The authors report that males of the moth C. myrodora
acquire pyrrolizidine alkaloid by feeding on the excrescent
fluids of certain plants (e.g., Eupatorium capillifolium). The
male moths incorporate the alkaloid systemically and as a result
are protected against spiders. The males have a pair of abdominal
pouches, densely packed with fine cuticular filaments, which in
alkaloid-fed males are alkaloid laden. The male discharges the
filaments on the female in bursts during courtship, embellishing
the female with alkaloid as a result. This topical investiture
protects the female against spiders. Alkaloid-free filaments,
from alkaloid-deprived males, convey no such protection. The
males also transmit alkaloid to the females by seminal infusion.
The systemic alkaloid thus received by the female, which itself
may contribute to the female's defense against spiders, is
bestowed in part by the female on the eggs.
2) The authors state that the defensive role of
pyrrolizidine alkaloids in the life of C. myrodora seems
established, certainly in defense against spiders. Adult male
moths, after acquiring alkaloid, were rejected by the spider N.
clavipes, as were female moths after receipt of alkaloid from
their mates. The eggs too can be expected to be protected by
their alkaloidal endowment. The amount of alkaloid contained in
the eggs, on the order of 1 microgram per egg, was demonstrated
to be effective in egg defense against ants, ladybird
(coccinellid) beetles, and the larvae of another moth species.
3) The authors point out that although paternal contribution
to egg defense has previously been demonstrated for several
species of arctiid moths, "protective nuptial festooning of a
female by its mate, such as is practiced by C. myrodora, appears
to be without parallel among insects."
-----------
W.E. Conner et al: Chemical defense: Bestowal of a nuptial
alkaloidal garment by a male moth on its mate.
(Proc. Natl. Acad. Sci. US 19 Dec 00 97:14406)
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 2Mar01
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ANTAGONISTIC SYMBIOSIS: ABDUCTION OF ONE SPECIES BY ANOTHER
McClintock and Baker (2 installations, US) review chemical
ecology in antarctic seas. Many interactions between species
occur on a covert level and are difficult to perceive. These
interactions are chemical, not physical, and rely on substances
such as pheromones that attract mates, as well as toxins that
repel or kill predators, competitors, and other enemies. Chemical
interactions can profoundly alter the conventional scenarios
posited by ecologists studying predators and their prey. The
research of the authors has focused on secondary metabolites,
chemicals that do not seem to be required for any of the primary
metabolic processes such as energy production, respiration, or
photosynthesis. They have found that sessile and sluggish
organisms on the antarctic ocean floor are much threatened by
invertebrate predators and competitors, and the threatened
organisms have developed chemical defenses to ward off their
enemies. In one unusual adaptation, the amphipod crustacean
Hyperiella dilatata captures the sea butterfly Clione antarctica
(a snail without a shell). The sea butterfly is held alive in a
sustained forced attachment on the back of the crustacean, and
experiments and analysis reveal that the sea butterfly secretes a
previously undescribed beta-hydroxyketone that turns away fish
that are normally predators of the crustacean. The authors
suggest this unique association -- the abduction of one species
by another -- is unprecedented in the annals of behavioral and
chemical ecology.
QY: James B. McClintock: mcclinto@uab.edu
(American Scientist May/Jun 1998) (Science-Week 1 May 98)
-------------------
Related Background:
INITIAL CHEMICAL SIGNAL IN INSECT-PLANT-INSECT TROPHIC TRIANGLES
Both corn and cotton plants, when attacked by plant-eating
insects, release a volatile substance that specifically attracts
other insects that are the natural predators of the plant-eating
insects. A group led by J. H. Tumlinson (U.S. Department of
Agriculture), studying the trophic triangle of the beet armyworm
caterpillar (Spodoptera exigua Hubner), corn seedlings (Zea mays
L.), parasitic wasp (Cotesia marginiventris), have isolated and
synthesized the chemical substance responsible for the initial
signal. They have named the substance volicitin. It is present
in the oral secretions of the caterpillar, and it induces the
damaged corn seedlings to release a volatile blend of terpenoids
and indole, which calls in the parasitic female wasps that are
the natural enemies of the caterpillars. The wasps lay eggs in
the caterpillars, and the hatched larvae destroy the
caterpillars by eating them. Mechanically damaged plants exposed
to synthetic volicitin, in the absence of caterpillar attack,
release the usual volatiles that attract the wasps. Plants
mechanically damaged but not exposed to volicitin do not release
the volatiles. (Science 9 May 97) (ScienceWeek 15 May 97)
For more information: http://scienceweek.com/swfr.htm
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6. ANTHROPOLOGY:
DIET AND THE EVOLUTION OF THE EARLIEST HUMAN ANCESTORS
In general, the term "hominids" refers to any primate in the
human family, and the "Pliocene epoch", the geological epoch with
the time-frame 5.2 to 1.64 million years ago, is the epoch during
which the first hominids appeared. There are presently 12 species
of hominids recognized, based on fossil evidence, but some of the
classifications are controversial. Of the 12 hominid species, 6
belong to the early hominids generally known as
"australopithecines" and 6 species to the later hominid genus
"Homo".
The geological epoch preceding the Pliocene, was the
Miocene, with the time-frame 23.3 to 5.2 million years ago, and
it is during the Miocene that the precursors to the hominids, the
"hominoids", first appeared. In general, the hominoids are a
primate superfamily that includes the gibbons, the great apes
(Pongidae) and the hominids.
Perhaps the most important characteristic of the hominids,
at least as revealed by extant fossils, is the apparent existence
of "bipedalism", walking upright on the hind limbs. The
appearance of bipedalism would have led to anatomical changes in
all parts of the body system, and many of these changes are
distinguishing characteristics in hominid fossils. The evidence
for the evolution of bipedalism includes the oldest hominid
skeletons, dated as 3.6 to 4.4 million years old, and a trackway
of footprints in volcanic ash dated as 3.75 million years old.
Another important characteristic in the anthropology of the
hominids concerns dentition and its relation to diet. In general,
hominid dental features show a reduction of the anterior teeth
(canines and incisors) that is interpreted as indicating a
dietary shift of significance. Recent research with the scanning
electron microscope has revealed tiny pits and scratches on the
surfaces of tooth enamel ("microwear patterns") that provide
evidence of dietary conditions. Grasses, for example, leave
linear scratches on teeth; leaves produce a polished effect; and
the bone crunching involved in eating meat gouges out tiny pits
in the enamel.
Concerning the early hominids, Ardipithecus ramidus is the
oldest, and is currently considered the "oldest human". This
species was found in Ethiopa in 1994 and dated as 4.4 million
years old.
Australopithecus anamensis, found in Lake Turkana in Kenya,
is dated as 4.1 to 3.9 million years old, and is considered to be
an intermediate between A. ramidus and later hominids.
The hominid Australopithecus afarensis has provided many
fossils, all in Africa, and has been dated as 3.2 to 2.9 million
years ago.
The fossil Australopithecus africanus has been dated as
approximately 3 to 1.5 million years ago.
During the last 50 years, ever-increasing numbers of hominid
fossils have been found, and their taxonomy and apparent place in
the pattern of human evolution continues to involve detailed
controversy.
... ... M.F. Teaford and P.S. Ungar (2 installations, US) present
a report on diet and the evolution of the early hominids, the
authors making the following points:
1) The authors point out that since the discovery of A.
afarensis, many researchers have emphasized the importance of
bipedality in scenarios of human origins, but less attention has
been focused on the role played by diet in the ecology and
evolution of the early hominids. Recent work in a broad range of
disciplines, such as paleo-environmental studies, behavioral
ecology, primatology, and isotope analyses has renewed interest
in early hominid diets. In addition, important new fossils from
the early Pliocene raise major questions about the role of
dietary changes in the origins and early evolution of the
hominids. The authors state: "We need to focus not just on how
the early hominids moved between food patches, but also on what
they ate when they got there."
2) The authors present a review of the fossil evidence for
the diets of the Pliocene hominids A. ramidus, A. anamensis, A.
afarensis, and A. africanus. These hominids offer evidence for
the first half of human evolution, from our split with the
prehistoric apes to the earliest members of our own genus. The
authors view the taxa under consideration as a roughly linear
evolutionary sequence from A. ramidus to A. africanus, spanning
the time-frame 4.4 to 2.5 million years ago. As such, these
species provide a unique opportunity to examine changes in
dietary adaptations of human ancestors over nearly 2 million
years. The authors also trace what has been inferred concerning
the diets of the Miocene hominoids in order to place changes in
Pliocene hominid diets into a broader temporal perspective.
3) The authors report that from their analysis it becomes
clear that the dietary capabilities of the early hominids changed
dramatically in the time period between 4.4 to 2.3 millions years
ago. Most of the evidence has come from five sources: analyses of
tooth size, tooth shape, enamel structure, dental microwear, and
jaw biomechanics. Taken together, the evidence suggests a dietary
shift in the early australopithecines, a shift to increased
dietary flexibility in conjunction with apparent climate
variability. Moreover, changes in diet-related adaptations from
A. anamensis to A. afarensis to A. africanus suggest that hard
abrasive foods became increasingly important through the
Pliocene, perhaps as critical items in the diet.
-----------
M.F. Teaford and P.S. Ungar: Diet and the evolution of the
earliest human ancestors.
(Proc. Natl. Acad. Sci. US 5 Dec 00 97:13506)
QY: Mark F. Teaford: mteaford@jhmi.edu
-------------------
Related Background:
HUMAN EVOLUTION:
FOSSIL EVIDENCE OF EARLY HOMINID KNUCKLE-WALKING
There are 185 species of living primates, of which modern
humans, Homo sapiens, are one species. All primates, from bush
babies and tarsiers to gorillas and humans are characterized by
approximately 30 characters that relate to 3 major sets of
adaptations: a) agility in the trees; b) acute brain and visual
functions; c) parental care.
The consensus view in paleoanthropology is that humans are a
very young group that has had a separate evolutionary history for
only 5 to 7 million years, with two sets of characters that
apparently set humans apart from other apes: locomotion on two
legs (bipedalism) and large brain size. Fossil evidence indicates
that by 4.1 million years ago, and perhaps earlier, *hominids
exhibited adaptations to bipedal walking. At present, however,
the fossil record offers little information about the origin of
bipedalism, and despite nearly a century of research on existing
fossils and on comparative anatomy, there is apparently still no
consensus concerning the mode of locomotion that preceded
bipedalism (*Note #1).
... ... B.G. Richmond and D.S. Strait (George Washington
University, US) now present evidence that fossils attributed to
*Australopithecus anamensis and *Australopithecus afarensis
retain specialized wrist morphology associated with knuckle-
walking. This morphology differs from that of later hominids and
non-knuckle-walking anthropoid primates, suggesting that knuckle-
walking is a derived feature of the African ape and human *clade.
The authors suggest this removes key morphological evidence for a
chimpanzee(Pan)-gorilla(Gorilla) clade, and indicates that
bipedal hominids evolved from a knuckle-walking ancestor that was
already partly terrestrial.
... ... In a commentary in the same journal on the work of
Richmond and Strait, M.Collard and L.G. Aiello (University
College London, UK) state: "Richmond and Strait's study... bears
on the long-standing debate over the evolutionary history -- the
'*phylogenetic' relationships -- of modern humans (Homo),
chimpanzees (Pan), and gorillas (Gorilla). [Molecular] genetic
analyses overwhelmingly indicate that chimpanzees and humans are
more closely related to one another than either is to gorillas.
Until the study by Richmond and Strait, however, the anatomical
evidence largely ran counter to this conclusion... Richmond and
Strait's results offer strong support for the idea that knuckle-
walking characters were present in the common ancestor of modern
humans and the African apes, and provide grounds for
reconciliation between the molecular and anatomical evidence."
-----------
B.G. Richmond and D.S. Strait: Evidence that humans evolved from
a knuckle-walking ancestor.
(Nature 23 Mar 00 404:382)
QY: B.G. Richmond [brich@gwu.edu]
-----------
M. Collard and L.C. Aiello: From Forelimbs to Two Legs.
(Nature 23 Mar 00 404:339)
QY: M. Collard [m.collard@ucl.ac.uk]
-----------
Text Notes:
... ... *hominids: In general, any primate in the human family.
... ... *Note #1: The hominid fossil record is far from
extensive, and a common quip in paleoanthropology is that there
are more working paleoanthropologists than discovered hominid
fossils to be worked on.
... ... *Australopithecus anamensis: Found by Meave Leakey in
4.1- to 3.9-million-year-old sediments near Lake Turkana in
Kenya. The remains include two jaws, a humerus, a tibia, and
isolated teeth. It has a primitive jaw with a shallow palate and
large canine teeth. The tibia indicates that A. anamensis was a
biped.
... ... *Australopithecus afarensis: The famous skeleton of
"Lucy", discovered in the 1970s, is an example of an
Australopithecus afarensis fossil. A. afarensis specimens range
from 3.2 to 2.9 million years in age. The individuals are 1 to
1.2 meters tall, with a brain size of only 415 cubic centimeters
and a generally ape-like face. Primitive characters include long
arms, short legs, and curved finger and toe bones that imply the
use of hands and feet in grasping branches. However, A. afarensis
is fully human in some significant ways, including hind limbs and
pelvis fully adapted for a type of bipedal locomotion.
... ... *clade: In general, a "clade" is a cluster of taxa
apparently derived from a single common ancestor.
... ... *phylogenetic: The term "phylogeny" refers to the
evolutionary history of a species or group of species in terms of
their derivation and relationships.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 23Jun00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ISOTOPIC ANALYSIS OF THE DIET OF AN EARLY HOMINID
The term "hominid" refers to any primate in the human family
(Hominidae) of which Homo sapiens (modern man) is the only living
specimen. The current consensus, based on molecular evidence, is
that our closest living relatives, gorillas and chimpanzees,
probably separated from our line (or us from them) approximately
6 to 8 million years ago. Humans, gorillas, common chimpanzees
and pygmy chimpanzees, the 4 African species of "great apes",
apparently had common ancestors perhaps 10 to 12 million years
ago. The first record of human footprints, of hominids walking
upright, was discovered at *Laetoli in East Africa, and has been
dated at 3.6 million years ago. This ancestor, Australopithecus
afarensis, probably weighed 25 to 50 kilograms (60 to 120 lbs.)
as an adult (*Note #1). Apparently derived from Australopithecus
afarensis were several species, including Australopithecus
africanus, a species which is believed to have appeared
approximately 3 million years ago. Little is known about the
diets of hominids that predate the Homo genus, because these
hominids did not leave archeological traces such as "*kitchen
middens" and stone tools. Consequently, researchers have made
inferences concerning hominid diet on the basis of craniodental
morphology, gross dental wear, and dental microwear. The current
consensus is that the 3-million-year-old Australopithecus
africanus hominid subsisted on fruits and leaves, similar to the
modern chimpanzee. Early hominid diets are of some theoretical
significance, since one current view is that the emergence of the
more intelligent Homo genus depended on the consumption of high-
quality animal foods that made possible biological changes
resulting in the evolution of a larger brain.
... ... M. Sponheimer and J.A. Lee-Thorp (2 installations, US
ZA) now report a stable carbon isotope analysis of A. africanus
fossils from *Makapansgat Limeworks, South Africa. The authors
sampled 4 of the 14 Australopithecus africanus individuals that
have been unearthed at that location, and also analyzed the
dental enamel of associated 3-million year old animals (65
individual animals from 19 mammalian taxa) in order to place A.
africanus within a broader ecological context. The authors report
their results demonstrate that A. africanus ate not only fruits
and leaves, but they also ate large quantities of carbon-13
enriched foods such as grasses and sedges, or they ate animals
that ate these plants, or both. The authors suggest their results
indicate that early hominids such as A. africanus regularly
exploited relatively open environments such as woodlands or
grasslands for food, and that early hominids may have consumed
high-quality animal foods before the development of stone tools
and the origin of the genus Homo.
-----------
M. Sponheimer and J.A. Lee-Thorp: Isotopic evidence for the diet
of an early hominid, Australopithecus africanus.
(Science 15 Jan 99 283:368)
QY: Matt Sponheimer, Rutgers Univ. New Brunswick 908-932-8789.
-----------
Text Notes:
... ... *Laetoli: The discovery involved a 30-yard-long footprint
trail of three bipedal individuals, the trail made approximately
3.6 million years ago in a newly deposited layer of volcanic ash.
The ash layer also contains the prints of many other animal
forms.
... ... *Note #1: The famous skeleton of "Lucy", discovered in
the 1970s, is an example of an Australopithecus afarensis fossil.
... ... *kitchen middens: In general, a "midden" is a refuse
heap. In this context, a "kitchen midden" is a mound consisting
of shells of edible molluscs and other refuse, the mound marking
the site of a prehistoric human habitation.
... ... *Makapansgat: (Makapansgaat) A cave site northeast of
Johannesburg.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 9Apr99
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ANTHROPOLOGY: DEFINING THE HUMAN GENUS
Taxonomy is the field in biology devoted to the classification of
living organisms, and the term "taxon" is used to indicate the
members of any particular group in the classification scheme:
class, family, genus, etc [*Note #1]. The general problem is how
to incorporate information about evolutionary history and
adaptation into taxonomic classification schemes, with the
problem exemplified by attempts to define our own genus, Homo.
The definition of the genus Homo has always been somewhat
contentious, not least because it is tied, implicitly or
explicitly, to the state of "being human". A series of anatomical
characteristics is found to be apparently unique in Homo -- for
example, an increase in cranial vault height and cranial
thickness, reduced lower facial prognathism (i.e., reduced
projection of the jaw), and in dentition reduction in the size of
the premolars and molars and the length of the molar row -- but
what has always been emphasized by taxonomic definitions is the
size of the Homo brain as revealed by the size of the cranium.
According to the classical scheme, to be Homo is to be a large-
brained *hominine (the apparent human-related fossil group), a
hominine presumably more technologically accomplished than the
ancestor group, the *Australopithecines. At the present time, as
is evident in this report, the taxonomic categorizations related
to the human genus are far from fixed. ... ... B. Wood and M.
Collard (2 installations, US UK) present an extensive review of
the taxonomic situation concerning the genus Homo, the authors
making the following points:
1) The authors suggest that recent data, fresh
interpretations of the existing evidence, and the limitations of
the paleoanthropological record invalidate existing criteria for
allocating fossil species to Homo.
2) The authors suggest that regardless of any formal
definitions, in current practice fossil hominin species are
assigned to Homo on the basis of one or more out of 4 criteria:
a) absolute brain size at least 600 cubic centimeters; b)
possession of language as inferred from *endocranial casts; c)
possession of a modern human-like precision grip involving a
well-developed and opposable thumb (pollex); d) the ability to
manufacture stone tools. The authors state: "It is now evident,
however, that none of these criteria is satisfactory."
3) The authors present a revised definition for the genus
Homo based on criteria considered verifiable and conclude that
two species, *Homo habilis and *Homo rudolfensis, do not belong
in the genus. The authors suggest the earliest taxon to satisfy
the criteria is *Homo ergaster, or early African *Homo erectus,
which currently appears in the fossil record at about 1.9 million
years ago.
4) The authors suggest that a fossil species should be
included in Homo only if the following can be demonstrated:
... ... a) the species is more closely related to H. sapiens than
it is to the australopiths;
... ... b) the species has an estimated body mass more similar to
that of H. sapiens than to that of the australopiths;
... ... c) the species has reconstructed body proportions that
match those of H. sapiens more closely than those of the
australopiths;
... ... d) the species has a *postcranial skeleton whose
functional morphology is consistent with modern human-like
obligate bipedalism and limited facility for climbing;
... ... e) the species is equipped with teeth and jaws that are
more similar in terms of relative size to those of modern humans
than to those of the australopiths;
... ... f) the species shows evidence for a modern human-like
extended period of childhood growth and development.
5) The authors conclude by suggesting that the adoption of
the above criteria would mean the genus Homo would have both
phylogenetic and adaptive significance. "Researchers can then
explore whether this adaptive shift in hominin evolution
corresponds with changes in climate, analogous evolutionary
changes in other large mammal groups, particular innovations in
the hominin cultural record, substantial expansions in geographic
range, or changes in ecological tolerance as reflected in
reconstructions of hominin habitats."
-----------
B. Wood and M. Collard: The human genus.
(Science 2 Apr 99 284:65)
QY: Bernard Wood [bwood@gwu.edu]
-----------
Text Notes:
... ... *Note #1: The conventional hierarchy of classification in
biology is Kingdom, Phylum, Class, Order, Family, Genus, and
Species. In the literature, organisms are usually referred to by
genus and species in binomial nomenclature, with the genus
capitalized. Human beings are genus Homo, species sapiens,
binomially Homo sapiens. The convention in binomial nomenclature
is to initialize the genus; thus: H. sapiens.
... ... *hominine: The terms hominine, hominin, hominoid,
hominid, are not interchangeable, but their classification
criteria are variously in a state of flux. In general, the
hominoids are a primate superfamily, the hominid family comprises
the great apes within the hominoid superfamily, the hominini are
a "tribe" within the hominids characterized by a number of
features including bipedalism, and the hominini are further
partitioned into the genera Homo and Australopithecus. Concerning
research in human evolution, most paleoanthropologists agree that
what is important is to achieve an understanding of the
evolutionary transitions and transformations, and any
classification scheme must be secondary to this objective. In
other words, in this context, classification must ultimately
reflect phylogeny (the actual evolutionary relationships), and as
knowledge of phylogeny changes, so must the extant classification
schemes.
... ... *Australopithecines: Members of the now extinct genus
Australopithecus, believed to exist between 4.4 and 1 million
years ago, and believed to be precursors of the genus Homo. All
australopithecines are apparently characterized by an ape-like
form, rather than the human-like form of the Homo genus.
... ... *endocranial casts: In general, an "endocast" (steinkern)
is any fossil formed after dissolution of an interior molding
substance. An "endocranial cast" is an endocast involving the
cranium. The interior of the endocast can often reveal details
concerning the absent soft interior substance (in this case, the
brain).
... ... *Homo habilis: In 1964, an early fossil hominin (1.9 to
1.6 million years before the present) was found in Olduvai,
Tanzania, the brain apparently intermediate in size between the
earliest known Homo fossil *Homo erectus and the Australopithecus
group. This new fossil was denoted as a new species by its
discoverers and named Homo habilis. The original set of H.
habilis fossils included a relatively complete hand, its
structure apparently compatible with an ability to make and use
tools. (Homo habilis literally means "handy-man") Considerable
controversy in the paleoanthropology community concerning H.
habilis has continued from 1964 until the present.
... ... *Homo rudolfensis: The original H. habilis species has
more recently been divided into H. habilis and H. rudolfensis,
after a fossil of the latter group was discovered in 1993 and
related to an earlier find in 1967, both dating at approximately
2.4 million years ago. One view is that Homo habilis/rudolfensis
evolved in Africa approximately 2 million years ago and quickly
expanded into Asia to become *Homo erectus/ergaster.
... ... *Homo ergaster: H. ergaster and *H. erectus are the two
immediate precursors of H. sapiens, with H. ergaster believed to
have originated in Africa and to have given rise to H. erectus in
Asia. But as with other hominid fossil groups, precise
evolutionary sequences and geographical loci continue to be
debated.
... ... *Homo erectus: First discovered by Eugene Dubois in 1891
in Indonesia, this fossil group is currently viewed as the
closest precursor to H. sapiens. Formerly called "Anthropithecus
erectus" and "Pithecanthropus erectus". Pithecanthropus erectus
and Sinanthropus erectus ("Peking man", discovered in 1927) were
in 1951 subsumed under the single category Homo erectus, which
was then recognized as a widespread species exhibiting
significant geographical variation.
... ... *postcranial skeleton: In general, this refers to the
skeleton behind the cranium in a quadruped and below the cranium
in a biped.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 9Jul99
For more information: http://scienceweek.com/swfr.htm
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7. IN FOCUS: ON THE CURIES
[Editor's note: The physicist Pierre Curie (1859-1906) was
suddenly killed in an accident almost exactly 95 years ago. Here
is a moment of history both of people and of science, and of the
vagaries of circumstance.]
"On April 19, 1906, the 47-year-old Nobel laureate Pierre Curie
was run over by an oversize horse-drawn wagon filled with bales
of army uniforms. He was negotiating that tricky Parisian
intersection where traffic from the Rue Dauphine, the Quai Conti,
the Quai des Grand Augustins, and the Pont Neuf have created
Gallic havoc for over a century. Curie had just quit a meeting of
reform-minded university professors where he argued for
legislation to improve the lot of junior faculty and to prevent
laboratory accidents. He had planned to stop at his publisher's
office on the Quai, but the office was shut because of a strike
by equally reform-minded trade unionists. Absent-minded and
somewhat radium-sick, he turned away in the spring rain, and was
on his way to the library of the Institut when that 6-ton wagon
rumbled down the bridge from the Ile de la Cite to crush his
skull. The death brought to an end two remarkably creative
careers in physical science, his own and that of his wife, Maria
Salomea Sklodowska -- known to the world as Madame Curie. She
later recollected that on the Rue Dauphine, "I lost my beloved
Pierre, and with him all hope and all support for the rest of my
life." She was right: for although Madame Curie was to survive
her husband until 1934, her contributions to science after his
death were less than innovative; she turned her tough mind to the
application of their discoveries, to teaching young scientists,
and to construction of the Radium Institute which she turned into
a world center of physical science... The story of Pierre and
Marie Curie is a tribute to a dazzling set of discoveries jointly
made by a man and a woman of genius... Among the most memorable
photographs... is a late one of an intense Marie Curie on the
balcony of her Institute behind the Ecole Normale. Her lined face
looks forward into the future, her hands are scrumbled by the
scars of radium; it's an image that sums up the hope and the harm
of her discovery. She would have been pleased that she is shown
overlooking the street that we now call the Rue Pierre et Marie
Curie."
-----------
Gerald Weissmann: _Darwin's Audubon: Science and the Liberal
Imagination_
(Plenum, New York 1998, p.81,85)
-------------------
SCIENCE-WEEK http://scienceweek.com 2Mar01
-------------------
Related Background:
IN FOCUS: ON PERSEVERANCE IN RESEARCH
"The School of Physics could give us no suitable premises, but
for lack of anything better, the Director permitted us to use an
abandoned shed which had been in service as a dissecting room of
the School of Medicine... Yet it was in this miserable old shed
that we passed the best and happiest years of our life, devoting
our entire days to our work. Often I had to prepare our lunch in
the shed, so as not to interrupt some particularly important
operation. Sometimes I had to spend a whole day mixing a boiling
mass with a heavy iron rod nearly as large as myself. I would be
broken with fatigue at the day's end. Other days, on the
contrary, the work would be a most minute and delicate fractional
crystallization, in the effort to concentrate the radium... Thus
the months passed, and our efforts hardly interrupted by short
vacations, brought forth more and more complete evidence. Our
faith grew ever stronger, and our work being more and more known,
we found means to get new quantities of raw material and to carry
on some of our crude processes in a factory, allowing me to give
more time to the delicate finishing treatment. At this stage I
devoted myself especially to the purification of the radium, my
husband being absorbed by the study of the physical properties of
the rays emitted by the new substances. It was only after
treating one ton of pitchblende residues that I could get
definite results. Indeed we know today that even in the best
minerals there are not more than a few decigrams of radium in a
ton of raw material. At last the time came when the isolated
substances showed all the characters of a pure chemical body.
This body, the radium, gives a characteristic spectrum, and I was
able to determine for it an atomic weight much higher than that
of the barium. This was achieved in 1902. I then possessed one
decigram of very pure radium chloride. It had taken me almost
four years to produce the kind of evidence which chemical science
demands, that radium is truly a new element. One year would
probably have been enough for the same purpose, if reasonable
means had been at my disposal."
-----------
Marie Curie: _Pierre Curie_ (1927)
(trans. Charlotte and Vernon Kellogg)
-----------
[Editor's note: Marie Curie (1867-1934) and her husband Pierre
Curie (1859-1906) shared the Nobel Prize for Physics in 1903.
Marie Curie also won the Nobel Prize for Chemistry in 1911. Three
years after receiving the Nobel Prize, Pierre Curie died at the
age of 47 in a traffic accident, run over by a horse-drawn
vehicle. During her work in the shed, Marie Curie also had to
care for her 5-year-old daughter Irene, later known as Irene
Joliot-Curie. Irene and her husband Frederic Joliot shared the
Nobel Prize in Chemistry in 1935. Marie Curie died at the age of
67 of a leukemia apparently caused by her exposure to high-energy
radiation (her notebooks are apparently still too contaminated to
handle). She died one year before her daughter was awarded the
Nobel Prize. Despite their poverty, and the chance for obvious
riches, the Curies refused to patent their radium isolation
process. Concerning the patenting of the process, Marie Curie
stated: "It would be impossible, it would be against the
scientific spirit... Physicists should always publish their
researches completely. If our discovery has a commercial future,
that is a circumstance from which we should not profit. If radium
is to be used in the treatment of disease, it is impossible for
us to take advantage of that."]
-------------------
SCIENCE-WEEK http://scienceweek.com 30Jun00
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8. FROM THE SCIENCEWEEK ARCHIVE:
ON THE DUAL-CAREER-COUPLE PROBLEM IN PHYSICS
We are in an age of changing or vanishing gender-based divisions
of labor, with more and more women working side by side with men
as professionals in scientific research. One natural consequence
of this is a substantial increase in the number of dual-research
married couples. Since research productivity depends to a large
extent upon specialization, and since the more specialized one's
field of research the less likely a position in that
specialization will be available in any specific geographical
location, there is a chronic employment problem for many dual-
research couples. It is a considerable irony that society can
commit substantial resources to the training of young scientists,
and then have those same scientists unable to work in science
because the commitment of society ends abruptly with their
training. Such problems are not unique to the physics community,
but the problems in that community are currently highlighted and
a cause of particular concern.
... ... L. McNeil and M. Sher (2 installations, US) present a
review of the problem in the physics community, the authors
making the following points:
1) The authors conducted a survey via several branches of
the American Physical Society, and received responses from 632
individuals who were members of dual-career couples. The purpose
of the survey was to gather data on the extent of the dual-career
problem in physics and consider possible solutions to the
problem. Of the respondent group, 89 percent were physicists
married to other scientists, and 45 percent were physicists
married to other physicists. The authors queried physicists whose
partners are scientists about their experiences in finding
employment and about the kinds of positions they had accepted,
and the published report consists primarily of anecdotal data.
The primary goal of the study was to obtain information about
possible approaches to the problem, and no attempt was made to
use rigorous statistical sampling techniques or sophisticated
quantitative analysis of the responses to the survey.
2) The authors report that in the 3 decades since 1966, the
percentage of women earning PhDs in physics each year has grown
from approximately 2 percent to approximately 13 percent. At
present, women constitute only 6 percent of US physicists
overall, but they make up approximately 13 percent of all
physicists under the age of 31. Approximately half of all women
physicists are married, compared to 74 percent of all male
physicists.
3) The authors report that female physicists are far more
likely to be married to physicists and other scientists than are
male physicists. Thus, the dual-career-couple problem has a
disproportionate effect on women. Although statistics are
difficult to obtain, anecdotal evidence suggests that dual-career
employment problems may cause some women to leave physics
altogether.
4) As possible solutions to the problem, the authors suggest
and discuss split positions, spousal hiring programs, alternative
academic positions, and long-distance commuting.
5) The authors conclude: "As women represent a growing
fraction of younger physicists, the number of new hires facing
the two-body problem can be expected to increase. It is in the
interests of institutions to take an active role in addressing
these dual-career situations."
-----------
L. McNeil and M. Sher: The dual-career-couple problem.
(Physics Today July 1999)
QY: Laurie McNeil, Dept. of Physics and Astronomy, University of
North Carolina Chapel Hill 919-966-3621
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 30Jul99
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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
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welcome. Editorial contact: editors@scienceweek.com
Editor/Publisher: Dan Agin
Managing Editor: Claire Haller
Associate Editor: Joan Oliner
Copyright (c) 1997-2001 SCIENCE-WEEK/Spectrum Press Inc.
All Rights Reserved
US Library of Congress ISSN 1529-1472
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