ScienceWeek

SCIENCE-WEEK

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

October 6, 2000 -- Vol. 4 Number 40

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As is your sort of mind,
So is your sort of search:
You will find what you desire.
-- Robert Browning (1812-1889)

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Please Note: Beginning with the issue of 13 October, ScienceWeek
will be transmitted on Saturdays (Central Standard Time). The
issue of 13 October will be transmitted on Saturday, 14 October.

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Section 1
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Contents of this Issue (Full reports in Section 2):

1. EARTH SCIENCES:
AN ALTERNATIVE SCENARIO FOR GLOBAL WARMING
A common view is that the current global warming rate will
continue or accelerate. Now a new and controversial report argues
that rapid warming in recent decades has been driven mainly by
non-carbon dioxide greenhouse gases such as chlorofluorocarbons,
methane, and N(sub2)O, and not by the products of fossil fuel
burning, carbon dioxide, and aerosols, the positive and negative
climate forcings of which are partially offsetting.
(Proc. Natl. Acad. Sci. US 29 Aug 00 97:9875)

2. EARTH SCIENCES:
ON MUDROCK QUARTZ AND MARINE PLANKTON
New evidence indicates that silica in mudstones may have derived
from the dissolution of skeletons of planktonic organisms rather
than from pre-existing rocks, and this suggests the sedimentary
record in mudstones may have been misinterpreted by past
researchers. There are important consequences for estimates of
paleoproductivity as well as for perceptions of the dynamics and
magnitude of global biogeochemical cycling of silica.
(Nature 31 Aug 00 406:981)

3. COMPUTER SCIENCE:
AUTOMATIC DESIGN AND MANUFACTURE OF ROBOTIC LIFEFORMS
In a significant advance in the quest to produce artificial
lifeforms, a combined computational and experimental approach
demonstrates that simple electromechanical systems can be evolved
through simulation from basic building blocks (bars, actuators,
and artificial neurons), and the "fittest" machines (defined by
their locomotive ability) then fabricated robotically using rapid
manufacturing technology. (Nature 31 Aug 00 406:974)

4. CELL BIOLOGY:
ON THE HISTORY OF CELL THEORY
Cell theory is a unifying concept that has provided direction for
the analysis of fundamental biological problems such as
reproduction, sexuality, development, heredity, evolution,
metabolism, coordination, growth, and numerous equally basic
biological phenomena. Although cursory treatments of cell theory
often focus on cell theory as discussed in the 19th century, in
actuality the theory originated in the 17th century, when
researchers first noted the existence of cells, and the theory
passed through various stages of understanding during the
following 300 years. (Science 8 Sep 00 289:1711)

5. PALEOBIOLOGY:
ON DATING THE EMERGENCE OF PHOTOSYNTHESIS
Although the advent of photosynthesis is one of the central
events in the early development of life on Earth, the origin and
evolution of photosynthesis is still unresolved. Various studies
have demonstrated that photosynthetic eukaryotes acquired
photosynthetic properties from endosymbiosis with cyanobacteria,
and this observation, coupled with other facts, supports the idea
that photosynthesis is a bacterially derived process. A new study
better defines the molecular origins of the major groups of
photosynthetic bacteria and clarifies the great antiquity of
anoxygenic photosynthesis. (Science 8 Sep 00 289:1703)

6. CELL BIOLOGY:
ON THE FORMATION OF BONE
A mineralized tissue that confers multiple mechanical and
metabolic functions to the skeleton, bone contains two distinct
cell types, bone-forming cells (osteoblasts), and bone-resorbing
cells (osteoclasts), and these two cell types participate in a
variety of important physiological processes during development
and postnatal life. The study of the biology of osteoblasts
illustrates how mammalian genetics has profoundly modified our
understanding of cell differentiation and physiological
processes. (Science 1 Sep 00 289:1501)

7. IN FOCUS: ON CAUSAL LAWS AND PROBABILITY LAWS IN PHYSICS

8. FROM THE SCIENCEWEEK ARCHIVE:
ON THE CHASM BETWEEN SCIENTISTS AND NON-SCIENTISTS

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Section 2
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1. EARTH SCIENCES:
AN ALTERNATIVE SCENARIO FOR GLOBAL WARMING
     Earth's global surface temperature has increased by
approximately 0.5 degrees centigrade since 1975, a relative
"burst" of warming that has apparently taken the global
temperature to its highest level in the past 1000 years, and
there is a growing consensus that the warming is at least in part
a consequence of increasing anthropogenic *greenhouse gases.
These gases cause a global "climate forcing", i.e., an imposed
perturbation of the energy balance of the Earth with space. There
are many competing natural and anthropogenic climate forcings,
but increased greenhouse gases are estimated to be the largest
forcing and to result in a net positive forcing, especially
during the past few decades. Evidence supporting this
interpretation has been provided by observed heat storage in the
ocean, which is positive and which is of the magnitude of the
energy imbalance estimated from climate forcings for recent
decades.
... ... J. Hansen et al (5 authors at 3 installations, US)
present a discussion of global warming, the authors making the
following points:
     1) The authors point out that a common view is that the
current global warming rate will continue or accelerate. The
authors, however, argue that rapid warming in recent decades has
been driven mainly by non-carbon dioxide greenhouse gases such as
chlorofluorocarbons, methane, and N(sub2)O, and not by the
products of fossil fuel burning, carbon dioxide, and *aerosols,
the positive and negative climate forcings of which are partially
offsetting.
     2) The authors point out that the growth rate of non-carbon
dioxide greenhouse gases has declined in the past decade. If
sources of methane and O(sub3) precursors were reduced in the
future, the change in climate forcing by non-carbon dioxide
greenhouse gases in the next 50 years could be near zero.
Combined with a reduction of *black carbon emissions and
plausible success in slowing carbon dioxide emissions, this
reduction of non-carbon dioxide greenhouse gases could lead to a
decline in the rate of global warming, reducing the danger of
dramatic climate change.
     3) The authors suggest that such a focus on air pollution
has practical benefits that unite the interests of developed and
developing countries, although assessment of ongoing and future
climate change requires composition-specific long-term global
monitoring of aerosol properties.
-----------
[Editor's note: After its publication several weeks ago, this
paper became controversial and received considerable publicity.
The senior author, James Hansen, is noted for helping to alert
the world to global warming in 1988, and this recent paper has
been interpreted as a reversal of his ideas concerning the
dangers of fossil fuel, carbon dioxide, and aerosol emissions,
and publicized by those opposed to the Kyoto Protocol on climate
change. For an account of reaction to this paper, see:
Nature 7 Sep 00 407:7.]
-----------
J. Hansen et al: Global warming in the 21st century: An
alternative scenario.
(Proc. Natl. Acad. Sci. US 29 Aug 00 97:9875)
QY: James Hansen: jhansen@giss.nasa.gov
-----------
Text Notes:
... ... *greenhouse gases: The physical basis of the so-called
"*greenhouse effect" is essentially simple: carbon dioxide gas is
transparent to visible light but relatively opaque to infrared
radiation. The same is true of glass. Relatively high-energy
visible light radiation from the sun passes inward through the
atmosphere, warms the surface of the Earth, which then radiates
lower energy in the form of infrared radiation (heat) back to the
atmosphere. But if the atmosphere has a concentration of infrared
impenetrable gases such as carbon dioxide, the infrared radiation
cannot pass out, and the surface of the Earth underlying the
atmosphere cannot cool, and the surface of the Earth thus will
continue to grow hotter.
... ... *aerosols: The term "aerosol" refers to a dispersion in
which a finely divided solid is suspended in air and the
particles are of colloidal dimensions. The term "colloidal
dimensions" refers to the range approximately 1 nanometer to 100
nanometers in diameter.
... ... *black carbon: (carbon black) Amorphous (i.e., non-
crystalline) carbon.
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Summary & Notes by SCIENCE-WEEK http://scienceweek.com 6Oct00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
CLIMATOLOGY:
ANTHROPOGENIC ATMOSPHERIC AEROSOLS AND GLOBAL CLIMATE CHANGE
... Aerosols are an important component of atmospheric chemical
dynamics, with the contributions of aerosols to global and
regional climate far from simple.
... ... S.E. Schwartz and P.R. Buseck (2 installations, US)
present a commentary on recent research on anthropogenic
atmospheric aerosols, the authors making the following points:
     1) Most considerations of global climate change caused by
human activities have focused on the warming influence of
greenhouse gases. However, aerosols are another important
atmospheric constituent that influences climate and that has been
affected by human activities. In general, aerosol particles
increase scattering and absorption of shortwave (solar)
radiation, increase cloud reflectance, enhance cloud lifetimes,
and suppress precipitation. These phenomena are all thought to
exert a cooling influence on climate. Recent data indicate that
anthropogenic aerosols reduce cloud droplet size and suppress
precipitation downward of major urban areas and industrial
facilities, which is consistent with earlier hypotheses.
     2) The influences of aerosols on climate are more complex
than those of greenhouse gases. Bulk aerosol composition is
highly variable spatially and temporally because of different
sources and production mechanisms and short atmospheric residence
times (from less than a day to more than a month). Particles
sizes range from nanometers to microns, and within the same size
class, particles can exhibit widely different compositions and
morphologies, with different constituents present within the same
particle (e.g., 10 nanometer carbon spherules can be found
embedded within much larger sulfate particles). The
inhomogeneities in properties and geographical distribution of
aerosols make it difficult to characterize their influences on
climate and to represent these influences in models.
     3) Recent analysis of the consequence of absorption of
shortwave radiation by aerosols indicates that the heating of the
atmosphere can evaporate clouds. Clouds exert both cooling and
warming influences on climate: cooling in the shortwave (because
of their reflectance), and warming in the longwave (because of
absorption and re-emission of thermal infrared radiation). The
shortwave component dominates, so a reduction in cloud coverage
would result in a net warming influence.
     5) The authors conclude: "Recent studies demonstrate both
the importance of aerosol effects on climate and the complexity
of aerosol-cloud interactions. Unfortunately for those would like
a quick and accurate assessment of anthropogenic climate forcing
over the industrial period, the studies also demonstrate that
there is much to be learned before such an assessment can
confidently be given."
-----------
S.E. Schwartz and P.R. Buseck: Absorbing phenomena
(Science 12 May 00 288:989)
QY: S.E. Schwartz [ses@bnl.gov]
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Summary by SCIENCE-WEEK http://scienceweek.com 23Jun00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ON CLIMATE FORCINGS IN THE INDUSTRIAL ERA
A "climate forcing" is an imposed perturbation of the Earth's
energy balance with space, for example, a change of the solar
radiation incident on the planet, or a change of carbon dioxide
in the Earth's atmosphere. The unit of measure of climate forcing
is Watts per square meter. Thus, the forcing due to the increase
of atmospheric carbon dioxide since pre-Industrial times is
approximately 1.5 Watts per square meter. Climate change is
combination of deterministic response to forcings and *chaotic
fluctuations -- the chaos a consequence of the nonlinear
equations governing the dynamics of the system. Quantitative
knowledge of all significant climate forcings is needed to
establish the contribution of deterministic factors in observed
climate change and to predict future climate. J.E. Hansen et al,
in a review of current considerations concerning climate forcings
in the Industrial era, make the following points: 1) The forcings
that drive long-term climate change are not known with an
accuracy sufficient to define future climate change. 2)
Anthropogenic greenhouse gases, which are well-measured, cause a
strong positive (warming) force. But other, poorly measured,
anthropogenic forcings, especially changes of atmospheric
aerosols, clouds, and land-use patterns, cause a negative forcing
that tends to offset greenhouse warming. 3) One consequence of
this partial balance is that the natural forcing due to solar
irradiance changes may play a larger role in long-term climate
change than inferred from comparison with greenhouse gases alone.
Current trends in greenhouse gas climate forcings are smaller
than in popular "business as usual" or 1 percent per year carbon
dioxide growth scenarios. The authors suggest that a summary
implication of their considerations is a paradigm change for
long-term climate projections: uncertainties in climate forcings
have supplanted global climate sensitivity as the predominant
issue. The authors further suggest that climate forcing scenarios
are essential for climate predictions, but if only one forcing
scenario is used in climate simulations, as has been a recent
tendency, the scenario itself is likely to be taken as a
prediction, as well as the calculated climate change. The authors
recommend that the use of multiple scenarios will aid objective
analysis of climate change as it unfolds in coming years.
-----------
J.E. Hansen et al (6 authors at National Aeronautics and Space
Administration, US)
Climate forcings in the Industrial era.
(Proc. Natl. Acad. Sci. US 27 Oct 98 95:12753)
QY: James E. Hansen 
-----------
Text Notes:
... ... *chaotic fluctuations: The term "chaotic", in this
context, is specific. In the study of physical systems, the
term "chaotic behavior" has a specific meaning: the behavior of a
system is said to be "chaotic" if its final state is so sensitive
to the system's precise initial conditions that the behavior of
the system is in effect unpredictable and cannot be distinguished
from a random process, even though the behavior of the system is
strictly determinate in a mathematical sense. In other words, a
deterministic system characterized by extremely sensitive
instabilities, despite the system being determinate, can exhibit
behavior that is unpredictable, and the system is then called
"chaotic". During the past several decades, the analysis of such
chaotic systems has intrigued both physicists and mathematicians.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 4Dec98
For more information: http://scienceweek.com/swfr.htm

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2. EARTH SCIENCES:
ON MUDROCK QUARTZ AND MARINE PLANKTON
     In geology, the term "silt" refers to a sediment with
particles in the size range 4 to 62.5 microns, and the term
"mudrock" (mudstone) refers to a silt that has been transformed
into rock ("lithified rock")
     Mudrocks are an important archive of the history of Earth,
containing the *detritus of the physical and biogeochemical
cycles that shape and regulate global systems. A major
constituent of mudrocks is quartz [silica = SiO(sub2)], one of
the most abundant rock-forming minerals and the most resistant to
weathering. The consensus view has been that the quartz found in
ancient sediments came from the erosion and fragmentation of
quartz crystals from pre-existing rocks. That view in now being
challenged.
... ... J. Scheiber et al (3 authors at 3 installations, US)
report an analysis of quartz silt from *black shales in the
eastern US, the material dating back to the late Devonian period
(approximately 370 million years ago). The authors make the
following points:
     1) Using *backscattered electron imaging and
*cathodoluminescence imaging, the authors report their results
indicate that up to 100 percent of the quartz silt in their
samples does not originate from the continental crust. Instead,
it appears to have precipitated early in *diagenesis in algal
cysts and other pore spaces, with silica derived from the
dissolution of *opaline skeletons of *planktonic organisms such
as *radiolaria and *diatoms.
     2) The authors suggest that transformation of early diatoms
into _in situ_ quartz silt might explain the time gap between the
earliest fossil occurrence of diatoms approximately 120 million
years ago and molecular evidence for a much earlier appearance
between 266 and 500 million years ago. Furthermore, the authors
suggest that if many other mudstone *successions show similarly
high proportions of _in situ_ precipitated -- rather than
detrital --  quartz silt, the sedimentary record in mudstones may
have been misinterpreted in the past, with consequences for
estimates of paleoproductivity as well as for perceptions of the
dynamics and magnitude of global biogeochemical cycling of
silica.
... ... In a commentary on this work, Alan Kemp (University of
Southampton, UK) states: "The implications of this study are wide
ranging, from our understanding of the evolution of marine
plankton and links between biogeochemical cycles of silica and
carbon, to the interpretation of fine-grained mudrocks and their
use in reconstructing past environments."
-----------
J. Schlieber et al: Diagenetic origin of quartz silt in mudstones
and implications for silica cycling.
(Nature 31 Aug 00 406:981)
QY: Juergen Shlieber: schlieber@uta.edu
-----------
Alan Kemp: Probing the memory of mud.
(Nature 31 Aug 00 406:951)
QY: Alan Kemp: aesk@soc.soton.ac.uk
-----------
Text Notes:
... ... *detritus: In this context, the term "detritus" refers to
material containing loose finely divided rock and/or finely
divided remains of animal and/or plant tissues.
... ... *black shales: Black/dark grey mudrocks rich in organic
carbon and generally formed in bottom waters poor in oxygen.
... ... *backscattered electron imaging: In general, in this
context, "backscattering" is the deflection of radiation by
scattering processes through angles greater than 90 degrees with
respect to the original angle of travel.
... ... *cathodoluminescence imaging: The term
"cathodoluminescence" refers to luminescence produced when high-
velocity electrons bombard a material in vacuum, vaporizing small
amounts of material in an excited state, the vapor emitting
radiation characteristic of the material.
... ... *diagenesis: In general, the term "diagenesis" refers to
all the changes that occur in a sediment at low temperature and
pressure after deposition. With increasing temperature and
pressure, diagenesis grades into "*metamorphism".
... ... *metamorphism: In general, the process of changing the
characteristics of a rock in response to changes in temperature,
pressure, or volatile content. Most metamorphic changes do not
include bulk chemical changes, but merely the crystallization of
new mineral phases. Examples of the transformation of
sediments through diagenesis and metamorphism are sand to
sandstone and peat to coal.
... ... *opaline skeletons: In general, "opal"
[SiO(sub2).nH(sub2)O] is a hydrated variety of silica consisting
of spheres approximately 300 nanometers in diameter. It is
normally deposited at low temperatures from silica-bearing
waters.
... ... *planktonic organisms: The term "plankton" is a general
designation for various drifting microscopic aquatic organisms in
the upper regions of the oceans.
... ... *radiolaria: A group of symmetrically shaped marine
protozoans with radiating thread-like pseudopodia and a
starburst-shaped silicaceous external wall ("test"). The tests
are practically indestructible, and upon death of the animal, the
test (on average, 50 to 100 microns in diameter) settles to the
ocean bottom.
... ... *diatoms: Also called bacillariophytes, diatoms are
microscopic unicellular eukaryotic algae differentiated into
approximately 10,000 different species. (The term "eukaryotic" is
applied to biological cells that have internal membrane-bound
organelles such as a nucleus.) The outstanding feature of diatoms
is a two-part transparent silicaceous cell wall (test), which may
be circular or elongated.
... ... *successions: In this context, groups of rock units or
strata that succeed one another in chronological order.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 6Oct00
For more information: http://scienceweek.com/swfr.htm

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3. COMPUTER SCIENCE:
AUTOMATIC DESIGN AND MANUFACTURE OF ROBOTIC LIFEFORMS
     The study and design of artificial life is a diverse field
of research, but a common theme is an attempt to understand the
fundamental principles of life by building detailed working
models. One of the most ambitious goals of artificial-life
research is the construction of living systems from non-living
parts.
     In general, biological life is in control of its own means
of reproduction, a process involving a specific sequence of
complex autocatalyzed chemical reactions. The autonomy of design
and manufacture found in living systems has not been realized
artificially: robots are still laboriously designed and
constructed by teams of human engineers, usually at considerable
expense. Few robots are available because these costs must be
absorbed through mass production, which at present is
economically justified only for toys, weapons, and certain
industrial systems.
... ... H. Lipson and J.B. Pollack (Brandeis University, US) now
report a study and implementation of a paradigm for the automatic
design and manufacture of robotic life forms. The authors make
the following points:
     1) The authors point out that in the field of artificial
life, "life as it could be" is examined on the basis of
understanding the principles, and simulating the mechanisms, of
real biological forms. Just as airplanes use the same principles
as birds, but have fixed wings, artificial lifeforms may share
the same principles, but not the same implementations in
chemistry: stored energy, autonomous movement, and even animal
communication are replicated in toys using batteries, motors, and
computer chips.
     2) The authors propose that to realize artificial life, full
autonomy must be attained not only at the level of power and
behavior (the present goal of robotics), but also at the levels
of design and fabrication. Only then can we expect synthetic
creatures to sustain their own evolution. The goal, therefore, is
automatically designed and constructed physical artefacts that
are functional in the real world, diverse in architecture, and
automatically producible with short turnaround time, at low cost,
and in large quantities. So far, these requirements have not been
met.
     3) The authors report the results of a combined
computational and experimental approach in which simple
electromechanical systems are evolved through simulation from
basic building blocks (bars, actuators, and artificial neurons).
The "fittest" machines (defined by their locomotive ability) are
then fabricated robotically using rapid manufacturing technology.
The authors suggest they thus achieve autonomy of design and
construction using evolution in a "limited universe" physical
simulation coupled to automatic fabrication. [Editor's note:
"Evolution" here is an automated algorithmic computational
process, with consequent designs robotically fabricated.]
     4) The authors conclude: "Future work is needed primarily in
understanding how more complex modular structures might self-
organize, and how these complex structures may transfer into
reality under control of the evolutionary process. Technological
advances in microelectromechanical systems (MEMS),
nanofabrication, and multi-material rapid prototyping that can
embed circuits and actuators in bulk material, together with
higher-fidelity physical simulation and an increased
understanding of evolutionary computational processes, may pave
the way for the self-sustaining progress that Moravec has termed
'escape velocity'."
... ... In a commentary on this work, R. Brooks (Massachusetts
Institute of Technology, US) states: "Lipson and Pollack have at
last demonstrated a computational system that designs functional
machines and builds them with almost no human intervention. The
resulting machines cannot match the complexity of the rapid-
prototyping machine designed by human engineers that is required
to do the actual fabrication. Nevertheless, this is a long
awaited and necessary step towards the ultimate dream of self-
evolving machines."
-----------
H. Lipson and J.B. Pollack: Automatic design and manufacture of
robotic lifeforms.
(Nature 31 Aug 00 406:974)
QY: Hod Lipson: lipson@cs.brandeis.edu
-----------
Rodney Brooks: From robot dreams to reality.
(Nature 31 Aug 00 406:945)
QY: Rodney Brooks: brooks@ai.mit.edu
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 6Oct00
For more information: http://scienceweek.com/swfr.htm

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4. CELL BIOLOGY:
ON THE HISTORY OF CELL THEORY
     The term "cell theory" refers to the proposition that all
forms of life (on Earth) are composed of cells, and that cells
are the simplest units to exhibit the functions characteristic of
living systems. Cell theory is a "theory" in the same sense that
atomic theory is a theory in the physical sciences -- the
proposition is universally accepted by biologists, and the
extensions flowing from the theory embrace all of modern biology
with widely diverse and ramifying implications. (A similar
statement can be made concerning so-called evolutionary
"theory".) Cell theory is a unifying concept that has provided
direction for the analysis of fundamental biological problems
such as reproduction, sexuality, development, heredity,
evolution, metabolism, coordination, growth, and numerous equally
basic biological phenomena.
     Although cursory treatments of cell theory often focus on
cell theory as discussed in the 19th century, in actuality the
theory originated in the 17th century, when researchers first
noted the existence of cells, and the theory passed through
various stages of understanding during the following 300 years.
The various forms of cell theory have included an elementary
conception of basic microscopic units in the mid-17th century, a
more fully articulated cell theory in the early 19th century,
which held that cells are the basic building blocks of living
organisms, a later 19th century conviction that the actions of
cellular material actually bring about organic development and
differentiation, and the idea that cells hold the key to
evolutionary development as well. Controversies and disagreements
about detail have occurred, but some version of cell theory and
the fundamental role of cells in development has persisted, and
the general idea of cells as the basic functional units of living
systems remains the foundation of modern biology.
... ... Paul Nurse (Imperial Cancer Research Fund, UK) presents a
review of the history of cell theory, the author making the
following points:
     1) The discovery of biological cells followed quickly after
the invention of the microscope in the 17th century. Robert Hooke
(1635-1703) published drawings of sections of cork in 1665, first
calling the observed walled cavities "cells". Within a few years,
Nehemiah Grew (1641-1712) and Marcello Malpighi (1628-1694)
published studies that led to the view that plant tissues are
mostly composed of aggregates of cells, and later in the 17th
century, Malpighi, Anton van Leeuwenhoek (1632-1723), and Jan
Swammerdam (1637-1680) first recognized cells in animals. These
microscopists described corpuscles in blood -- no one at that
time proposed that solid animal tissues were also made of cells:
that proposition was not made until the 19th century, and in
particular by Theodor Schwann (1810-1882) and Matthias Schleiden
(1804-1881).
     2) Although Schleiden and Schwann correctly articulated the
cell theory, their ideas concerning the formation of cells were
wrong: they believed cells arose by processes similar to
precipitation or crystallization. Others, particularly Robert
Remak (1815-1865), recognized that cells arose from preexisting
cells by a process of binary fission. This view was championed by
Rudolf Virchow (1821-1902), who popularized the phrase "all cells
come from cells."
     3) Most cells contain a single nucleus that reproduces
during mitosis and cell division. Elongated chromosomal threads,
described by Walther Flemming (1843-1905) and Eduard Strasburger
(1844-1912) in the 1880s, were observed to split lengthwise
before shortening and thickening as mitosis proceeds. The
longitudinal halves then separate into the two daughter nuclei.
Approximately the same time as these observations were made,
Edouard van Beneden (1846-1910) demonstrated that the chromosomes
in a fertilized roundworm (nematode) egg are derived in equal
numbers from the egg and the sperm. That led August Weismann
(1834-1914) to propose that the hereditary system is based on the
chromosomes.
     4) The author concludes: "The cell is the simplest unit to
exhibit life's functions. We now have both the molecular tools
and the conceptual frameworks to undertake a concerted program to
understand how cells operate. The genome projects will anchor
that foundation by identifying all the genes required for a cell
to function, yet researchers will still have to work out how the
relevant gene products act and interact to generate cellular
organization... The coming years will be exciting ones during
which new ideas and theories will help us fully understand cells
and thereby life itself."
-----------
Paul Nurse: The incredible life and times of biological cells.
(Science 8 Sep 00 289:1711)
QY: Paul Nurse, Imperial Cancer Research Fund, London, UK.
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 6Oct00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
IN FOCUS: ON THE BEGINNING OF CELL THEORY IN BIOLOGY
"When organic nature, animals and plants, is regarded as a
Whole, in contradistinction to the inorganic kingdom, we do not
find that all organisms and all their separate organs are compact
masses, but that they are composed of innumerable small particles
of a definite form. These elementary particles, however, are
subject to the most extraordinary diversity of figure, especially
in animals; in plants they are, for the most part exclusively,
cells. This variety in the elementary parts seems to hold some
relation to their more diversified physiological function in
animals, so that it might be established as a principle that
every diversity in the physiological signification of an organ
requires a difference in its elementary particles; and, on the
contrary, the similarity of two elementary particles seems to
justify the conclusion that they are physiologically similar...
The greater the number of physiologically different elementary
parts, which, so far as can be known, originate in a similar
manner, and the greater the difference of these parts in form and
physiological signification, while they agree in the perceptible
phenomena of their mode of formation, the more safely we may
assume that all elementary parts have one and the same
fundamental principle of development... The elementary parts of
all tissues are formed of cells in an analogous, though very
diversified manner, so that it may be asserted, that there is one
universal principle of development for the elementary parts of
organisms, however different, and that this principle is the
formation of cells." [*Note #1]
-----------
Theodor Schwann: _Microscopical Researches into the Accordance in
the Structure and Growth of Animals and Plants_.
(Sydenham Society, London 1847, transl. Henry Smith, from the
original German published in 1839)
-----------
Text Notes:
... ... *Note #1: The "cell theory" is probably the most
important biological generalization of the first half of the 19th
century, a generalization that has grown in importance and which
serves as a unifying principle in the continued development of
modern biology. A number of biologists had been writing about the
cellular organization of animals and plants, but it was Theodor
Schwann (1810-1882) and Matthias Schleiden (1804-1881) who most
clearly stated and summarized the case for the cell theory,
Schwann for animals and Schleiden for plants. Although a major
weakness of the theory was its proposition that the formation of
cells involved the appearance of a nucleus first and the
remainder of the cell afterward, the general idea of cell theory,
that of individual physiological entities ("cells") as the
fundamental units of biological systems, was a correct and
profound conceptual contribution. Schwann also apparently coined
the term "metabolism" to represent the overall chemical changes
occurring in living systems. He also did important work on
digestion, fermentation, and histology. He identified yeast as
consisting of tiny plant-like organisms, and he was one of the
first to propose that fermentation of sugar and starch was the
result of a life process, a proposition that provoked so much
scientific criticism in Germany that Schwann left Germany and
moved to Belgium. He became professor of anatomy at Louvain in
1838 and at Liege in 1847. It is ironic that in the last 40 years
of his life he devoted most of his energies to mysticism and
religious meditation, doing nothing to match his earlier
intellectual and scientific accomplishments in the one decade of
the 1830s.
-------------------
Notes by SCIENCE-WEEK http://scienceweek.com 17Sep99
For more information: http://scienceweek.com/swfr.htm

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5. PALEOBIOLOGY:
ON DATING THE EMERGENCE OF PHOTOSYNTHESIS
Although the advent of *photosynthesis is one of the central
events in the early development of life on Earth, the origin and
evolution of photosynthesis is still unresolved. Various studies
have demonstrated that photosynthetic *eukaryotes acquired
photosynthetic properties from *endosymbiosis with
*cyanobacteria, and this observation, coupled with other facts,
supports the idea that photosynthesis is a bacterially derived
process.
... ... David J. Des Marais (NASA Ames Research Center, US)
presents a commentary on recent work in this field, the author
making the following points:
     1) Life began very early in Earth's history, perhaps before
3.8 billion years ago, and achieved remarkable levels of
metabolic sophistication before the end of the *Archean epoch
approximately 2.5 billion years ago. Although the great antiquity
of our biosphere may illustrate how easily life can arise on a
habitable planet, this antiquity also poses challenges to our
attempts to delineate our earliest ancestors.
     2) The earliest sedimentary rocks have generally undergone
extensive alteration by *metamorphism, which seriously
compromises microfossils, but traces of our distant ancestors are
recorded not only in ancient rocks but also in biological
macromolecules and pathways. The geological and biological
records are highly complementary: The geological record offers
the absolute timing of evolutionary innovations and their
environmental context, while the living biochemical record can
reveal the sequence of development of key pathways and
biomolecules.
     3) J. Xiong et al (Science 8 Sep 00 289:1724) have examined
the biological record to study the evolution of photosynthesis,
obtaining new sequence information for genes involved in
photosynthesis, and performing *phylogenetic analyses on the
major groups of photosynthetic bacteria. This work better defines
the molecular origins of these groups and clarifies the great
antiquity of anoxygenic photosynthesis. The report of Xiong et al
adds an important constraint to current perspectives, the authors
demonstrating conclusively for the first time that the major
lineages of pigments involved in anoxygenic photosynthesis arose
before the development of oxygenic photosynthesis. This indicates
that the 6 major bacterial lineages had largely developed by the
mid-Archean, approximately 3.0 to 2.8 billion years ago, and
perhaps much earlier. The study also demonstrates that the early
biosphere passed through a stage during which even its
photosynthetic populations depended exclusively on abiotic
sources of chemical reducing power.
-----------
David J. Des Marais: When did photosynthesis emerge on Earth?
(Science 8 Sep 00 289:1703)
QY: David J. Des Marais: ddesmarais@mail.arc.nasa.gov
-----------
Text Notes:
... ... *photosynthesis: From the standpoint of chemistry,
photosynthesis can be defined as the reductive carboxylation of
organic substrates carried on by chlorophyll-containing
biological cells capable of using light as their energy source.
Fully oxidized carbon atoms in the form of carbon dioxide are
covalently linked ("fixed") to organic acceptor molecules and are
subsequently reduced and rearranged into sugars and other organic
molecules, with light energy used to drive the fixation and
provide the reducing power.
... ... *eukaryotes: In general, biological cells (or systems
composed of such cells) that contain internal membrane-bound
organelles such as a cell nucleus.
... ... *endosymbiosis: In biology, the term "symbiosis" refers
in general to an intimate and protracted association of
individuals of different species, and "endosymbiosis" refers to a
symbiotic association between cells of two or more different
species in which a smaller cell inhabits a larger host cell.
... ... *cyanobacteria: A phylum of bacteria characterized by
blue-green (cyan) photosynthetic pigments, abundant in a variety
of habitats, particularly in fresh water and soil. Cyanobacteria
are responsible for generating a large portion of the free oxygen
in the Earth's atmosphere. They apparently produced stromatolite
limestone deposits, as well as the bulk of modern petroleum
deposits. (Stromatolites are laminated calcareous microbial
fossil deposits formed principally by cyanobacteria and algae.)
... ... *Archean epoch: (Archaean; Archeozoic) In general, the
earliest biotic geological era, from approximately 3.9 billion
years ago to approximately 2.6 billion years ago. This era marked
the first appearance of sedimentary rocks.
... ... *metamorphism: In general, in this context, the process
of changing the characteristics of a rock in response to changes
in temperature, pressure, or volatile content. Most metamorphic
changes do not include bulk chemical changes, but merely the
crystallization of new mineral phases.
... ... *phylogenetic: In general, 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 6Oct00
For more information: http://scienceweek.com/swfr.htm

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6. CELL BIOLOGY:
ON THE FORMATION OF BONE
     In general, the term "connective tissue" refers to tissue
which protects and supports the body and its organs, binds organs
together, stores energy reserves as fat, and provides immunity.
Connective tissue is the most abundant and widely distributed
tissue in the mammalian body, with forms ranging from the fluid
of blood to the solid substance of bone (osseous tissue). Like
other connective tissue, bone contains an abundant matrix
surrounding widely separated cells, the matrix approximately 25
percent water, 25 percent protein, and 50 percent mineral salts.
There are 4 types of cells in mammalian bone tissue:
     1) Osteoprogenitor cells (stem cells): undifferentiated
cells capable of developing into other cell types.
     2) Osteoblasts: the cells that form bone, secreting collagen
and other organic components needed in bone construction.
     3) Osteocytes: mature bone cells derived from osteoblasts,
and which are the principal cells of bone tissue. Osteocytes
maintain the ongoing cellular activities of bone tissue, such as
the exchange of nutrients and wastes with the blood.
     4) Osteoclasts: cells on the surface of bone that function
in bone resorption (destruction of matrix), which is important in
the development, growth, maintenance and repair of bone.
... ... P. Ducy et al (3 authors at Baylor College of Medicine,
US) present a review of current research in the biology of
osteoblasts, the authors making the following points:
     1) A mineralized tissue that confers multiple mechanical and
metabolic functions to the skeleton, bone contains two distinct
cell types, bone-forming cells (osteoblasts), and bone-resorbing
cells (osteoclasts), and these two cell types participate in a
variety of important physiological processes during development
and postnatal life.
     2) The term "bone formation" is sometimes used to describe
osteoblast specialization (osteoblast differentiation) during
embryonic development (skeletogenesis). Bone formation is
implicated directly or indirectly in longitudinal bone growth,
bone mineralization, and the ongoing resorption and replacement
of bone (bone remodeling) -- all functions that are not easily
studied _in vitro_. Our understanding of the molecular control of
osteoblast function has been greatly enhanced by the emergence of
gene-deletion technology.
     3) The functions of osteoblasts and osteoclasts are
intimately linked. During skeletal development and throughout
life, cells from the osteoblast lineage synthesize and secrete
molecules that in turn initiate and control osteoclast
differentiation. This is a direct and crucial interaction that
has been well established _in vivo_. Once osteoblasts and
osteoclasts are fully differentiated, there is a less direct
relationship. Bone is constantly destroyed or resorbed by the
osteoclasts and then replaced by the osteoblasts in a
physiological process called "bone remodeling". Various
experiments involving alterations in genes (genetic models)
indicate that the osteoblasts do not influence the activity of
the osteoclasts in any overt way _in vivo_. Nevertheless, bone
remodeling is tightly regulated by *local and endocrine factors.
The endocrine regulation of bone resorption has been well known
for many years, but it is only recently that bone formation has
been shown to be under endocrine control.
     4) The study of the biology of osteoblasts illustrates how
mammalian genetics has profoundly modified our understanding of
cell differentiation and physiological processes. Genetics-based
studies over the past 5 years have revealed how osteoblast
differentiation is controlled via *growth factors and
*transcription factors. Similarly, the recent identification,
using mutant mouse models, of a central nervous system component
in the regulation of bone formation has expanded our
understanding of the control of bone remodeling. This regulatory
loop, which involves the hormone leptin, may help to explain the
protective effect of obesity on bone mass in humans. In addition,
it provides a novel physiological concept that may shed light on
the etiology of *osteoporosis and help to identify new
therapeutic targets.
-----------
P. Ducy et al: The osteoblast: A sophisticated fibroblast under
central surveillance.
(Science 1 Sep 00 289:1501)
QY: Gerard Karsenty: karsenty@bcm.tmc.edu
-----------
Text Notes:
... ... *local and endocrine factors: In general, "endocrine"
mechanisms are mechanisms involving secretions of substances
(hormones) into the systemic circulation, i.e., a global (vs.
local) chemical communication pathway.
... ... *growth factors: In general, in this context, a "growth
factor" is any specific substance that must be present in a
culture medium for multiplication of the cultured cells to occur.
... ... *transcription factors: "Transcription" is the process by
which the genetic information in DNA is converted into RNA, and
transcription factors are a class of DNA-binding proteins that
regulate RNA transcription.
... ... *osteoporosis: A generalized progressive diminution of
bone density (bone mass per unit volume) that causes skeletal
weakness. The ratio of mineral to organic elements is unchanged.
The major clinical manifestations of osteoporosis are bone
fractures resulting from a reduction below the fracture threshold
of the amount of bone available for mechanical support.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 6Oct00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
IMPACT OF GENETICS ON BONE BIOLOGY
... Gerard Karsenty (Baylor College of Medicine, US) presents
a review of the recent influences of the field of genetics on
bone biology, the author making the following points:
     1) The author states that the entire field of bone biology
is dominated by the impact of bone degenerative diseases such as
*osteoporosis. Similar to research in most other organogenesis
processes, human and mouse genetic studies have been major
driving forces in redefining bone biology. Genetic studies have
opened new areas of research, elucidated at the molecular level
various known phenomena, and sometimes challenged untested
textbook assumptions. In general, genetic studies have profoundly
transformed the field of bone biology.
     2) One peculiar characteristic of bone resides in its
physiology. Bone is the only organ that contains a cell type, the
osteoclast, whose only apparent function is to constantly destroy
the organ hosting it. This destruction, or "resorption" of bone,
occurring throughout life and in healthy individuals, is
counterbalanced by new bone formation in a process called "bone
remodeling". It is through bone remodeling that bone mass is
maintained at a constant level between the end of puberty and
gonadal failure, and bone remodeling is the process affected
during osteoporosis, a disease characterized at the cellular
level by a relative increase of bone resorption over bone
formation. In recent years, we have begun to understand at the
molecular level how bone resorption is controlled, but it is
striking how little we know about the molecular mechanisms
governing bone formation.
     3) The aspect of bone biology that has made the most
progress in the last few years is the genetic control of
osteoclast differentiation and function. The osteoclast, the cell
type resorbing *mineralized bone matrix, is the last specific
cell type of the skeleton to appear during development, and the
systematic study of mouse mutants has led to the establishment of
a fairly detailed understanding of the *genetic cascade
controlling osteoclast differentiation and function. Some of this
progress has important implications not only for bone resorption,
but also for new hypotheses concerning the molecular control of
bone formation.
     4) Bones and teeth are the only tissues that mineralize
under physiological conditions; mineralization or calcification
in any other tissue is pathologic. Thus, a question could be
asked as follows: Is bone mineralization an active function
requiring active expression of one or multiple genes, or rather
is the absence of calcification in every other tissue an active
function genetically controlled? It has long been proposed that
certain proteins in the bone matrix could serve a crystal-
nucleation function at the beginning of the mineralization
process. However, many of the genes encoding these proteins have
been experimentally deleted in mice without any overt effects on
bone mineralization, which indicates that these proteins do not
alone control bone mineralization in vivo. Although the current
lack of success in identifying activators of bone mineralization
does not mean they do not exist, it is possible that bone
mineralization is a passive phenomenon involving the absence of
inhibitors of mineralization in the bone matrix, and there is
some evidence to support this idea.
     5) Bone is not made only of cells, it also contains an
extracellular matrix. This bone extracellular matrix contains
mostly *type I collagen, which accounts for 90 percent of the
protein content of the matrix, plus a variety of noncollagenous
proteins and *proteolytic enzymes. Mouse and human genetics have
shown that most of these proteins and proteolytic enzymes are
required for the integrity of bone tissue, although it is not yet
understood how this requirement is manifested at the molecular
level.
     6) The author concludes: "In terms of the approaches used,
the history of bone biology can be divided into two parts.
Initially, bone biology, like most fields, was dominated by cell
biology; however, in the past 10 years it has been ruled by mouse
genetics. Mouse and human genetics are here to stay and rightly
so. However, no single approach, whether it is cell biology,
genetics, or biochemistry, will have all answers for any field of
biology. Thus, the main challenge in the short term will be to
understand at the biochemical level how many of the new gene
products that have been identified fulfill their functions."
-----------
Gerard Karsenty: The genetic transformation of bone biology.
(Genes & Development 1 Dec 99 13:3037)
QY: Gerard Karsenty [karsenty@bcm.tmc.edu]
-----------
Text Notes:
... ... *osteoporosis: See main report.
... ... *mineralized bone matrix: The inorganic part of bone
consists largely of calcium phosphate organized into small
crystals of hydroxyapatite 0.8 to 1.5 nanometers thick, 2.4
nanometers wide, 20 to 40 nanometers long. Other anions present
are carbonate, fluoride, hydroxide, and citrate. Most of the
body's magnesium, approximately 25 percent of its sodium, and a
smaller proportion of its potassium is found in bone.
... ... *genetic cascade: The "cascade" of sequential gene
expressions occurring during development.
... ... *type I collagen: The term "collagen" refers to a group
of fibrous proteins of very high tensile strength that form the
main component of connective tissue in animals. Collagen of bones
and skin is metabolically stable, in contrast with collagen of
organs such as the liver. The collagens are products of a
superfamily of closely related genes found in multicellular
animals, the products classified into types I to XIII in the
order in which they were purified and characterized. All contain
a typical triple helical domain formed from 3 independent chains.
Type 1 collagen is the most abundant collagen, forming well-
organized fibrils.
... ... *proteolytic enzymes: In general, "proteolysis" is the
enzyme-catalyzed degradation of protein by hydrolysis of one or
more peptide bonds.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 28Jan00
For more information: http://scienceweek.com/swfr.htm

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7. IN FOCUS: ON CAUSAL LAWS AND PROBABILITY LAWS IN PHYSICS
"The question of replacing causal laws by statistical laws made
its appearance in the history of physics long before the times of
the theory of quanta. Since the time of Boltzmann's great
discovery, which revealed the second principle of thermodynamics
to be a statistical instead of a causal law, the opinion has been
repeatedly uttered that a similar fate may meet all other
physical laws. The idea of determinism, i.e., of strict causal
laws governing the elementary phenomena of nature, was recognized
as an extrapolation inferred from the causal regularities of the
macrocosm. The validity of this extrapolation was questioned as
soon as it turned out that macrocosmic regularity is equally
compatible with irregularity in the microcosmic domain, since the
law of great numbers will transform the probability character of
the elementary phenomena into the practical certainty of
statistical laws. Observations in the macrocosmic domain will
never furnish any evidence for causality of atomic occurrences so
long as only effects of great numbers of atomic particles are
considered. This was the result of unprejudiced philosophical
analysis of the physics of Boltzmann... Whenever we speak of
strictly causal laws we assume them to hold between idealized
physical states; and we know that actual physical states never
correspond exactly to the conditions assumed for the laws. The
discrepancy has often been disregarded as irrelevant, as being
due to the imperfection of the experiments and therefore
negligible in a statement about causality as a property of
nature. With such an attitude, however, the way to a solution of
the problem of causality is barred. Statements about the physical
world have meaning only so far as they are connected with
verifiable results; and a statement about strict causality must
be translatable into statements about observable relations if it
is to have a utilizable meaning... If we characterize physical
states in observational terms, i.e., in terms of observations as
they are actually made, we know that we can construct probability
relations between these states. For instance, if we know the
inclination of the barrel of a gun, the powder charge, and the
weight of the shell, we can predict the point of impact with a
certain probability... What then is the meaning of a statement
saying that if we knew exactly the initial conditions we could
predict with certainty the future states resulting from them?
Such a statement can be meaningfully said only in the sense of a
transition to a limit... The statement that nature is governed by
strict causal laws means that we can predict the future with a
determinate probability and that we can push this probability as
close to certainty as we want by using a sufficiently elaborate
analysis of the phenomena under consideration. With this
formulation the principle of causality is stripped of its
disguise as a principle _a priori_, in which it has been
presented within many a philosophical system. If causality is
stated as a limit of probability implications, it is clear that
this principle can be maintained only in the sense of an
empirical hypothesis."
-----------
Hans Reichenbach: _Philosophical Foundations of Quantum
Mechanics_
(Univ. of California Press, Berkeley 1944)
[Hans Reichenbach (1891-1953), mathematician and philosopher, is
noted for his work in probability theory, symbolic logic, and the
theory of relativity.]

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8. FROM THE SCIENCEWEEK ARCHIVE:
ON THE CHASM BETWEEN SCIENTISTS AND NON-SCIENTISTS
In an essay in the journal *Science* on the gap between scientist
and non-scientist, the science journalist Takashi Tachibana makes
the following points: 1) In 1959 C.P. Snow presented the idea
that the chasm between scientists and literary intellectuals was
so vast that they could not communicate with each other. After
nearly 40 years, the situation has only grown worse. 2) The
current level of basic scientific knowledge is so low that it is
difficult to interest even the brightest layman or non-science
student in what modern science is doing. The chasm between
scientist and non-scientist has widened to become a gulf. 3) The
impact of the 20th century revolutions in physics and molecular
biology has been profound: the Universe looks different, Life is
different. But this impact has not been as thorough as it might
have been, because while those with a more comprehensive
scientific education can recognize that something important has
happened, the great majority of people do not even realize that a
fundamental shift has occurred. 4) Most non-scientists who like
to think of themselves as knowledgeable about modern science
really know only about technologies -- and specifically those
technologies considered likely to bring economic profits in the
short term. This is also the mind-set of most government
officials and lawmakers who consider themselves sympathetic to
science and technology budget requests. Science for homo
economicus and homo faber is flourishing, while science for Homo
sapiens is diminishing. 5) We may one day wake up to find
barbarians at the gate in the form of an upsurge in "new" science
-- that is, not science at all -- promoted by one or the other
fundamentalist religion or occultist group ready to lead us into
a new Dark Age. The author concludes: "What we must urgently do
is renovate education and significantly raise the basic level of
scientific knowledge, for, as C.P. Snow warned four decades ago,
we must 'educate ourselves or perish'".
-----------
QY: T. Tachibana, 2-18-12 Koishikawa, Bunkyo-ku, Tokyo 112 JP.
(Science 7 Aug 98 281:778)
-------------------
SCIENCE-WEEK http://scienceweek.com 4Sep98


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