ScienceWeek

SCIENCEWEEK

November 17, 2006

Vol. 10 - Number 46

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And the end of all our exploring Will be to arrive where we started And know the place for the first time.

-- T.S. Eliot (1888-1965)

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Contents:

1. Anthropology: On Analysis of Tooth Enamel. Seasonal variations in temperature, rainfall, and food availability drive many animals to hibernate or migrate. Animals that are tethered to their home ranges and remain active in all seasons may need flexible adaptive strategies for survival, especially in arid African savannas, where seasonal and annual rainfall can vary widely. About 2.4 to 1.4 million years ago...

2. Ecology: On the Ecosystem Deep Beneath the Seafloor. Over the past 20 years, scientific drilling into sediments and basaltic crust all over the world ocean has revealed the omnipresence of microscopic life deep beneath the seafloor. Diverse communities of prokaryotic cells have been discovered in sediments and rock reaching a subsurface depth of 1 km. Most of these microorganisms have no cultured or known relatives...

3. Neurobiology: On Retina Repair. Each year many people lose their sight through disease of the retina. Millions are affected by macular degeneration and retinitis pigmentosa alone. In these diseases, the cone and rod photoreceptors -- the cells that convert light into neural signals -- degenerate. And once these cells are lost, they are not replaced in the mammalian retina...

4. Astronomy: What Are Galaxies? Of his 1938 discovery of two tiny satellite galaxies accompanying our own Milky Way, the astronomer Harlow Shapley wrote(1): "Presumably the gamut of galaxies had already been run. All forms had long been fully described. There were spirals, spheroidals, irregulars, with many variations on the spiral theme. The newly found organizations...

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Also Noted:

Nanotechnology. Risk, Ethics and Law. Geoffrey Hunt and Michael D. Mehta, Eds. Earthscan, London, 2006. Hardback: 320 pp. $34.95. ISBN 1844073580. More information at: http://www.amazon.com/...

Comparative Cognition. Experimental Explorations of Animal Intelligence. Edward A. Wasserman and Thomas R. Zentall, Eds. Oxford University Press, New York, 2006. Hardback: 718 pp., illus. $120. ISBN 0195167651. More information at: http://www.amazon.com/...

Embryonic Stem Cells. A Practical Approach. Elena Notarianni and Martin J. Evans, Eds. Oxford University Press, Oxford, 2006. Paperback: 360 pp., illus. £40. ISBN 0198550014. More information at: http://www.amazon.com/...

First Man. The Life of Neil A. Armstrong. James R. Hansen. Simon and Schuster, New York, 2006. Paperback: 784 pp., illus. $18, C$22. ISBN 0743257510. Reprint, 2005 ed. More information at: http://www.amazon.com/...

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Special Note: A New Book by the Editor of ScienceWeek:

JUNK SCIENCE. How Politicians, Corporations, and Other Hucksters Betray Us. Dan Agin. Thomas Dunne Books/St. Martin's Press, New York, 2006. Hardback: 336 pp., $24.95. ISBN 0312352417.

From NEW SCIENTIST (UK) (issue of 4 November 2006):

Dan Agin, an emeritus biologist at the University of Chicago, is passionate in defence of science. In JUNK SCIENCE he targets those who abuse or distort it, starting with scientists who fake results. This is neither rare nor easily uncovered, he warns. Agin lambasts the Bush administration, Big Tobacco, the pharmaceutical industry, mainstream and alternative medicine, psychotherapy, the religious right and others who deny or attack inconvenient research. Anyone who values good science will appreciate finding all this together in a cogent, powerfully argued book.

More information about JUNK SCIENCE at: http://www.amazon.com/...

A recent long review of this book appeared in the San Diego Union-Tribune, October 22, 2006. The review can be accessed at:

http://www.signonsandiego.com/uniontrib/...

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1. ANTHROPOLOGY: ON ANALYSIS OF TOOTH ENAMEL

The following points are made by Stanley H. Ambrose (Science 2006 314:930):

1) Seasonal variations in temperature, rainfall, and food availability drive many animals to hibernate or migrate. Animals that are tethered to their home ranges and remain active in all seasons may need flexible adaptive strategies for survival, especially in arid African savannas, where seasonal and annual rainfall can vary widely. About 2.4 to 1.4 million years ago, our earliest stone tool-making ancestors, Homo habilis and H. erectus, shared African savannas with their close relatives, commonly referred to as "robust" australopithecines or Paranthropus species (1). How variable were their environments? How much did their diets overlap in different seasons? And how did these two bipedal hominins manage to coexist for 1 million years? New work (2) documents the seasonal variation in diet and climate of four robust australopithecines from Swartkrans Cave in South Africa. The authors use laser ablation of tooth enamel -- a method that causes minimal damage to the precious fossils --followed by advanced methods of isotope analysis. They are literally blazing a new trail to answers to fundamental questions about early hominin paleoecology and evolution.

2) With their huge molar teeth and massive jaw muscles, robust australopithecines are considered dietary specialists that fed mainly on small, hard, tough, fibrous plant foods. Their extinction between 1.0 and 1.4 million years ago is often attributed to their low-nutrient, high-fiber diets. However, systematic assessments of the cranial and dental anatomy (1) and dental microwear (3) suggest that their diets were less specialized than previously thought and more similar to those of their ancestors and hominin competitors.

3) Dietary niche separation between closely related species is usually greatest when resources are scarce. For example, chimpanzees and lowland gorillas that live in the same area eat similar amounts of fruit for most of the year, but during the leanest season, gorillas rely entirely on herbaceous vegetation (4). The powerful teeth and jaws of Paranthropus may have been essential for survival only when they resorted to tough "fallback" foods to mitigate competition with Homo.

4) How can stable-isotope variations in teeth provide insight into seasonality in diet and climate? The answer lies in the different 13C/12C ratios of different types of plants (5). Tropical grasses (and a few herbaceous broadleaf plants) fix atmospheric CO2 using the C4 photosynthetic pathway; these plants have high 13C/12C ratios. Conversely, most broadleaf plants, including trees, shrubs, and herbs, use the C3 pathway and have low 13C/12C ratios. The isotope ratio of the diet controls that of the consumer, such that grazing (grass-eating) and browsing (broadleaf-eating) herbivores -- and the carnivores that prey on them -- preserve the isotopic difference at the base of the food web. The carbon-isotope ratios of mixed feeders reflect the proportions of C3 and C4 plants in their diets.

References (abridged):

1. B. Wood, D. Strait, J. Hum. Evol. 46, 119 (2004).

2. M. Sponheimer et al., Science 314, 980 (2006).

3. R. S. Scott et al., Nature 436, 693 (2005).

4. C. B. Stanford, J. B. Nkurunungi, Int. J. Primatol. 24, 901 (2003).

5. T. E. Dawson, Annu. Rev. Ecol. Syst. 33, 507 (2002).

Science http://www.sciencemag.org

ScienceWeek http://scienceweek.com

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2. ECOLOGY: ON THE ECOSYSTEM DEEP BENEATH THE SEAFLOOR

The following points are made by B.B. Jørgensen and S. D'Hondt (Science 2006 314:932):

1) Over the past 20 years, scientific drilling into sediments and basaltic crust all over the world ocean has revealed the omnipresence of microscopic life deep beneath the seafloor. Diverse communities of prokaryotic cells have been discovered in sediments and rock reaching a subsurface depth of 1 km. Most of these microorganisms have no cultured or known relatives in the surface world and are still only characterized by the genetic code of their DNA. Recent studies (1-4) have shed light on the ways in which they differ from microorganisms in the surface world and on the energy sources that support life in this buried ecosystem.

2) About 20 years ago, R. John Parkes and Barry Cragg started to systematically enumerate microorganisms in deep cores (5). Much later, rigorous contamination tests performed on the drill ship showed that the cells detected were indeed indigenous to the deep subsurface. The cell counts were used for a bold extrapolation to the global ocean floor. The astonishing conclusion was that this "unseen majority" of microorganisms accounts for 55 to 85% of Earth's prokaryotic biomass and about 30% of the total living biomass.

3) The first drilling expedition focused entirely on deep biosphere exploration was launched in 2002 by the Ocean Drilling Program (ODP, Leg 201) (1). The target was the eastern tropical Pacific, with sites ranging from the continental shelf to ocean depths of 5000 m. By drilling through the seafloor down to the basaltic crust, sediments with ages up to 35 million years old could be sampled. At all sites, prokaryotic cells (bacteria and archaea) were detected below the seafloor. Their numbers dropped from more than 10^(8) cm^(-3) at the sediment surface to less than 10^(6) cm^(-3) just above the ocean crust, with an average density much greater than in the ocean above. Occasional high cell numbers [up to 10^(10) cm^(-3)] coincided with sediment horizons in which more energy was available from counterdiffusing methane and sulfate.

4) These large population sizes remain the greatest mystery of the deep biosphere. Although marine sediments harbor Earth's largest reactive carbon pool, the organic matter becomes increasingly unreactive with depth and age and would seem to be practically inaccessible for microorganisms several million years after its burial. How, then, can there be sufficient energy for all these organisms to metabolize and grow? The metabolic activity of the subsurface populations can be calculated by transport-reaction modeling of pore water solutes that are consumed or excreted by the microorganisms. Because microbial cells must metabolize a certain minimum amount of substrate before they can double their cell size and divide into two daughter cells, their minimum doubling time can also be calculated. On the basis of this calculation, the mean generation time of deep subseafloor microorganisms is more than 1000 years. This extremely slow growth cannot be reconciled with our understanding of the minimum energy requirements for life. All actively growing organisms must keep their enzymatic machinery going above a critical level to maintain vital cell functions such as replacement of degraded enzymes, repair of DNA damaged by high-energy radiation from natural radionuclides, and, presumably, the maintenance of an electrochemical gradient across the cell membrane. A possible explanation for the low apparent rates of deep subsurface metabolism could be that most subseafloor cells are not active but dormant or even dead. However, when a highly sensitive fluorescence technique (catalyzed reporter deposition-fluorescence in situ hybridization or CARDFISH) was used to detect the presence of ribosome -- a component of all living and active cells that is rapidly degraded upon their death -- the results showed that many of the subsurface cells were alive.

References (abridged):

1. S. D'Hondt et al., Science 306, [2216] (2004).

2. F. Inagaki et al., Proc. Natl. Acad. Sci. U.S.A. 103, 2815 (2006).

3. J. F. Biddle et al., Proc. Natl. Acad. Sci. U.S.A. 103, 3846 (2006).

4. J. P. Amend, A. Teske, Palaeogeogr. Palaeoclimatol. Palaeoecol. 219, 131 (2005).

5. R. J. Parkes, B. A. Cragg, P. Wellsbury, Hydrogeol. J.. 8, 11 (2000).

Science http://www.sciencemag.org

ScienceWeek http://scienceweek.com

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3. NEUROBIOLOGY: ON RETINA REPAIR

The following points are made by Thomas A. Reh (Nature 2006 444:156):

1) Each year many people lose their sight through disease of the retina. Millions are affected by macular degeneration and retinitis pigmentosa alone. In these diseases, the cone and rod photoreceptors -- the cells that convert light into neural signals -- degenerate. And once these cells are lost, they are not replaced in the mammalian retina. Treatments are being developed that may delay or prevent the loss of retinal neurons in these disorders. But for those who have already lost their sight, the possibility of retinal transplantation may be the best prospect for the restoration of vision through cell-replacement therapy. Despite many attempts, such transplants have yet to produce better vision in mammals because the transplanted cells do not wire up to the brain properly. New work (1) shows in mice that the trick may be to use cells at a particular stage in their development.

2) Retinal transplantation has a long history. Classic studies in the 1920s showed that transplantation of the eyes of normal salamanders could give vision to blind, cave-dwelling salamanders (2). This work allowed developmental biologists to explore the mechanisms that enable the correct connections to form between the eyes and the nervous system. The first successful transplant of a mammalian retina dates back to 1959, when Royo and Quay (3) transplanted fetal rat retinas into the eyes of adults of the same strain. Although the transplanted retinas did not seem to connect with the host retinas, they survived for months.

3) Since then, intact retinal sheets from embryonic mice and rats have been transplanted to the sub-retinal space. There they develop many characteristics of a normal retina (4,5), and grow to form a second retinal layer underneath the host retina. Also, transplants of "microaggregates" -- clumps of a few retinal neurons -- from newborn mice developed most characteristics of rod photoreceptors, including the expression of the rhodopsin protein and even the characteristic structures called outer segments. Unfortunately, both the intact embryonic retinal sheets and the microaggregates of photoreceptors keep to themselves, without interacting or integrating very effectively with the host retinal neurons.

4) Integration with the host retina is much better when transplanting retinal progenitor cells -- the immature cells that are responsible for producing all the retinal cells during embryonic development. These progenitors (also called retinal stem cells by some) are derived from fetal or newborn mice or rats, or from human fetuses. They can be maintained in cell culture and continue to proliferate and generate new neurons and specialized retinal support cells called Mueller glia. When these cells are transplanted into either normal or degenerated (dystrophic) retinas of rats and mice, they can migrate into all retinal layers and develop morphological characteristics of various retinal cell types. However, the cells do not seem to integrate efficiently into the outer nuclear layer, where the rods and cones reside, and, for the most part, evidence showing expression of photoreceptor-specific genes in the transplanted cells has been lacking.

References (abridged):

1. MacLaren, R. E. et al. Nature 444, 203-207 (2006).

2. Stone, L. S. Invest. Ophthal. 3, 555-565 (1964).

3. Royo, P. E. & Quay, W. B. Growth 23, 313-336 (1959).

4. Seiler, M., Aramant, R., Ehinger, B. & Adolph, A. R. Exp. Eye Res. 51, 225-228 (1990).

5. Ghosh, F. & Ehinger, B. Ophthalmologica 214, 54-69 (2000).

Nature http://www.nature.com/nature

ScienceWeek http://scienceweek.com

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4. ASTRONOMY: WHAT ARE GALAXIES?

The following points are made by Sidney van den Bergh (Nature 2006 444:158):

1) Of his 1938 discovery of two tiny satellite galaxies accompanying our own Milky Way, the astronomer Harlow Shapley wrote(1): "Presumably the gamut of galaxies had already been run. All forms had long been fully described. There were spirals, spheroidals, irregulars, with many variations on the spiral theme. The newly found organizations in [the dwarf galaxies] Sculptor and Fornax did not seem essential in order to fill in a natural sequence; they were not logically necessary. On the contrary, they introduced some doubt into the picture we had sketched -- they suggested that we may be farther than we think from understanding the world of galaxies."

2) Seven decades later, these words still ring true. We now know that "dwarf spheroidals" of the type discovered by Shapley are in fact the most common type of galaxy in the Universe. And, in a paper to be published in The Astrophysical Journal, Belokurov et al (2) report the discovery, in data from the Sloan Digital Sky Survey, of four more dwarf spheroidal satellites of the Milky Way system. One of these, the Coma dwarf galaxy, is the faintest galaxy ever observed, and is two orders of magnitude fainter than the brightest clusters of old stars known within the Milky Way. So where exactly does one draw the line between a bright star cluster and a faint galaxy?

3) The obvious distinguishing criterion, size, is not always reliable: although galaxies are usually larger than star clusters, tidal forces can strip off the large envelopes of galaxies, leaving behind only a compact rump. So-called dark matter, on the other hand, does provide a reliable way of telling a galaxy from a star cluster. All galaxies are thought to be embedded in a halo of this type of matter, which emits no light and so cannot be observed directly; no dark matter has ever been detected in a star cluster.

4) Because they have no dark matter contributing to their overall mass, star clusters that formed from fragments of disintegrating tidal arms are expected to have small ratios of mass to light emitted. This implies that they have a lower spread of internal velocities than galaxies, as this spread is intimately connected to a body's mass. Individual stars in star clusters also all formed at roughly the same time, and therefore have similar metal abundances, whereas stars in galaxies, which require a significantly longer time to assemble, can have a large range of metallicities. But applying these criteria to faint, distant clusters is difficult, as even the largest telescopes require a great deal of observing time to measure metallicities and internal velocities. The metallicity-dispersion criterion is also likely to be useless for the least luminous dwarfs, as these are expected to contain only very metal-poor stars.

References:

1. Shapley, H. in Galaxies 141 (Blakiston, Philadelphia, 1943).

2. Belokurov, V. et al. Astrophys. J. in the press.

3. Einasto, J., Saar, E., Kaasik, A. & Chernin, A. D. Nature 252, 111-113 (1974).

4. Klypin, A., Kravtsov, A. V., Valenzuela, O. & Prada, F. Astrophys. J. 522, 82-92 (1999).

5. Moore, B. et al. Astrophys. J. 524, L19-L22 (1999).

Nature http://www.nature.com/nature

ScienceWeek http://scienceweek.com

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