ScienceWeek

SCIENCEWEEK

January 19, 2007

Vol. 11 - Number 3

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Dissent is the native activity of the scientist, and it has got him into a good deal of trouble in the last years. But if that is cut off, what is left will not be a scientist. And I doubt whether it will be a man.

-- Jacob Bronowski (1908-1974)

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Contents (full text below):

1. Astronomy: A Supernova Riddle

2. Anthropology: On The Emergence Of Modern Humans

3. Paleontology: A Problem With Embryonic Fossils

4. Evolutionary Biology: Genes and the Puzzle of Homosexuality

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Also Noted:

After Eden. The Evolution of Human Domination. Kirkpatrick Sale. Duke University Press, Durham, NC, 2007. Paperback: 200 pp., illus. ISBN 0822339382. More information at: http://www.amazon.com/exec/obidos/ASIN/0822339382/scienceweek

Born on a Blue Day. Inside the Extraordinary Mind of an Autistic Savant. A Memoir. Daniel Tammet. Free Press (Simon and Schuster), New York, 2007. Hardback: 238 pp. ISBN 1416535071. http://www.amazon.com/exec/obidos/ASIN/1416535071/scienceweek

Death by Black Hole. And Other Cosmic Quandaries. Neil deGrasse Tyson. Norton, New York, 2007. Hardback: 384 pp. ISBN 0393062244. http://www.amazon.com/exec/obidos/ASIN/0393062244/scienceweek

Extragalactic Astronomy and Cosmology. An Introduction. Peter Schneider. Springer, Berlin, 2006. Hardback: 473 pp., illus. ISBN 3540331743. http://www.amazon.com/exec/obidos/ASIN/3540331743/scienceweek

The Future of the Universe. Jack Meadows. Springer, London, 2007. Hardback: 185 pp., illus. ISBN 1852339462. http://www.amazon.com/exec/obidos/ASIN/1852339462/scienceweek

Oppenheimer. The Tragic Intellect. Charles Thorpe. University of Chicago Press, Chicago, 2006. Hardback: 433 pp., illus. ISBN 0226798453. http://www.amazon.com/exec/obidos/ASIN/0226798453/scienceweek

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1. ASTRONOMY: A SUPERNOVA RIDDLE

The following points are made by Douglas C. Leonard (Science 2007 315:193):

1) Roughly once per second in the observable universe, a star explodes and announces its death with an optical display that for weeks rivals the brilliance of its parent galaxy. These supernova events are classified into several types, but among the most interesting are those called type Ia supernovae (SNe Ia). The love affair of astronomers with these beacons began in earnest about a decade ago, when two groups put them to work as distance indicators and precisely mapped the recent expansion history of the universe. Before this work, most researchers expected gravity to be slowing the expansion. Instead, the data revealed a universe expanding at an accelerating rate, a finding heralded by the journal SCIENCE as the "Breakthrough of the Year in 1998" (1) and one that has since survived intense scrutiny and complementary experimental checks. Yet for all the fanfare and empirical success, it must be acknowledged that we are fundamentally ignorant: We do not know how these stars explode. New work (2) identifies a suggestive trend in an impressive set of SN Ia data that may point the way toward a deeper understanding of these enigmatic cosmic blasts.

2) Despite an embarrassing scarcity of direct observational evidence, the first part of the story of SNe Ia is largely considered settled. Each future SN Ia begins as a carbon-oxygen white dwarf -- the compact corpse of a low-mass star like our Sun after its nuclear-burning life is over -- accreting matter through some mechanism (mass flow from the envelope of a close companion star seems most likely) until a critical central density is achieved and a thermonuclear runaway is triggered. There is general agreement that, once initiated, the burning front progresses through the star for a time as a subsonic deflagration. But at this point in the story, harmony ends and pitched battles begin, with some favoring an enduring deflagration front and others insisting on a transition to a supersonic detonation.

3) The most recent "delayed detonation" models appear to better match observed SNe Ia: The events produced in these simulations are bright enough (a perennial problem for deflagration models) and have the proper ejecta composition and stratification (3). The mechanism that triggers the deflagration-detonation transition remains a mystery, however, and so the pure deflagration model still retains its share of adherents. In any event, a complete comparison of the observable distinctions predicted by the two scenarios still awaits full, three-dimensional radiation transport simulations carried out at high enough resolution to resolve physical processes at very small scales. Into this fray, Wang et al (2) now step, armed with an upstart and potentially powerful observational tool: the ability to study the geometry of the supernova ejecta by analyzing the polarization properties of the light coming from the star shortly after explosion.

4) Are supernovae round? Simple to pose, this question belies a menacing observational challenge, given that all extragalactic supernovae remain point-like in the night sky throughout the critical early phases of their evolution. Fortunately, geometric information is encoded in the polarization properties of supernova light. The essential idea is that photons become polarized when they scatter off of free electrons, and hot, young supernova atmospheres contain an abundance of free electrons. Indeed, if we could view such an atmosphere as an extended source rather than as an unresolvable point of light, we would expect to measure changes in both the direction and strength of the polarization as a function of position in the atmosphere. For a spherical, unresolved source, the directional polarization components cancel exactly and yield zero net polarization. Any deviation from perfect symmetry or roundness of the source in the plane of the sky, however, gives rise to a net polarization.

5) Wang et al (2) analyzed spectropolarimetry data of 17 SNe Ia. Bright events show systematically weaker line polarization than dim ones do. This trend is consistent with the idea that different SNe Ia make the transition from deflagration to detonation at different times. The sooner it happens, the brighter the supernova and the more completely scoured the ejecta will be of the clumps left behind by the deflagration front. The agreement between model predictions and observations strengthens the case for a detonation phase.(4,5)

References (abridged):

1. J. Glanz, Science 282, 2156 (1998).

2. L. Wang, D. Baade, F. Patat, Science 315, 212 (2007).

3. V. N. Gamezo, A. M. Khokhlov, E. S. Oran, Astrophys. J. 632, 337 (2005).

4. D. C. Leonard et al., Astrophys. J. 632, 450 (2005).

5. L. Wang et al., Astrophys. J. 591, 1110 (2003).

Science http://www.sciencemag.org

ScienceWeek http://scienceweek.com

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2. ANTHROPOLOGY: ON THE EMERGENCE OF MODERN HUMANS

The following points are made by Ted Goebel (Science 2007 315:194):

1) Current interpretations of the human fossil record indicate that fully modern humans emerged in sub-Saharan Africa by 195,000 years ago (1). By 35,000 years ago, modern humans thrived at opposite ends of Eurasia, from France to island southeast Asia and even Australia. How they colonized these and other drastically different environments during the intervening 160,000 years is one of the greatest untold stories in the history of humankind. New work (2-4) interpret some of the chapters of this story.

2) To understand the dispersal of modern humans, we must know when these populations expanded from Africa into Eurasia. For the past 20 years, many researchers in this field have been under the impression that this event could have occurred as early as 100,000 years ago (5), but new genetic evidence indicates that the spread out of Africa occurred much more recently, closer to 60,000 to 50,000 years ago. However, independent corroborating evidence of this recent-dispersal hypothesis is required. Grine et al (2) provide a first important test through the analysis of the modern human skull from Hofmeyer, South Africa. This skull was originally discovered in 1952, but it came from an eroded context and not an archaeological excavation and did not yield sufficient collagen for accurate radiocarbon dating. Using a combination of other dating techniques, Grine et al. show that sediment within the skull's endocranial cavity was deposited about 36,000 years ago.

3) Thus, here is the first skull of an adult modern human from sub-Saharan Africa that dates to the critical period, and one that can speak to the relationship of early moderns from Africa and Europe. The Hofmeyer skull is morphometrically more similar to modern humans of Upper Paleolithic Europe than to recent South Africans or Europeans, and it has little in common with Neandertals. Thus, 35,000 years ago, modern populations of sub-Saharan Africa and Europe shared a very recent common ancestor, one that likely expanded from east Africa 60,000 years ago. This population not only spread south into South Africa but also east into Eurasia, navigating across the Bab el-Mandab Strait of the Red Sea from the Horn of Africa to southern Arabia.

4) Archaeological evidence of the hypothesized passage across the Red Sea still eludes us, but the fossil and archaeological records for southeast Asia and Australia indicate that moderns had arrived in these regions by 50,000 years ago. The road east likely followed the south Asian coastal margin, a route requiring few modifications in adaptation other than those mandated by the initial exodus from Africa. The spread north, however, required more time for adaptation to cope with colder temperatures, drier climates, and -- most challenging of all -- Neandertals. Despite these constraints, genetic records suggest that sets of genes, called haplotypes, carried by the first moderns into northern Eurasia existed by 45,000 years ago. Precisely where they evolved remains unknown; possibilities include southern Arabia, India, or other regions of interior western Asia. In any case, the outcome was a series of concomitant founding migrations about 40,000 years ago from western Asia to the Mediterranean, temperate Europe, Russia, and central Asia.

References (abridged):

1. I. McDougall et al., Nature 433, 733 (2005).

2. F. E. Grine et al., Science 315, 226 (2007).

3. M. V. Anikovich et al., Science 315, 223 (2007).

4. A. Olivieri et al., Science 314, 1767 (2006).

5. P. Mellars, in The Emergence of Modern Humans: An Archaeological Perspective, P. Mellars, Ed. (Cornell Univ. Press, Ithaca, NY, 1990).

Science http://www.sciencemag.org

ScienceWeek http://scienceweek.com

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3. PALEONTOLOGY: A PROBLEM WITH EMBRYONIC FOSSILS

The following points are made by Philip C. J. Donoghue (Nature 2007 445:155):

1) The origin of animals is almost as much a mystery as the origin of life itself. An abundant fossil record extends back 542 million years to the beginning of the Cambrian period, testifying to the establishment of all of today's main groups of animals by this time. However, the degree to which animal evolutionary history extends beyond the Cambrian is a controversy rich in speculation but sparse in evidence. It is no wonder that the 1998 report of fossilized animal embryos from the Doushantuo phosphorite rocks of southern China created a stir (1). At more than 580 million years old, these were the oldest unequivocal animal fossils, and, as embryos, they provided a glimpse of animal embryology at a time when today's main animal groups were emerging. These fossils have revealed ancient patterns of cell division and cell arrangement (2), and promised to reveal further secrets about developmental evolution at this crucial time. But this prospect may have been dashed by a new study (3) presenting a compelling reinterpretation of these fossils -- not as animal embryos, but as giant bacteria.

2) The fossils in question are species of Parapandorina, which were thought to be embryos, and Megasphaera, which were proposed to be fertilized eggs. Most Parapandorina fossils were preserved only in their very earliest stages of embryonic development, and were composed of 2, 4, 8, 16, 32 or 64 cells, with some examples having undergone further rounds of cell division -- a process known in embryos as cleavage. These remarkable fossils are preserved in calcium phosphate, and are present in such abundance that they are often the main constituent of the rock. But the icing on the cake is that in the Doushantuo rocks quantity goes hand in hand with quality. The detail of preservation can be staggering, with features such as cell nuclei and subcellular vacuoles -- membrane-bound compartments -- being observed (2).

3) The embryos were originally identified as colonies of green algae, but were later classified as animal embryos because of their comparatively large size (typically just under a millimetre in diameter), and because the cell walls show evidence of distortion in response to the division of their neighbours, suggesting that they are not rigid, as algal cell walls would be (1,4). More specific evolutionary relationships to sponges and arthropods have been mooted, but definitive evidence has been lacking.

4) Bailey et al (3) now show that, although Parapandorina and Megasphaera may not be algae, the features thought to be indicative of animal embryos could just as readily be those of bacteria. This is based on comparisons with Thiomargarita --giant sulphur bacteria that live in seafloor sediments along the Namibian coast and in the Gulf of Mexico. For instance, the pattern of cell division in Parapandorina is conventionally interpreted as cleavage. But this pattern is also seen in Thiomargarita, where it represents successive rounds of cell division. This results in clusters of two, four and eight cells (5) with geometries identical to those of Parapandorina (3). Thiomargarita is also as large as the Doushantuo fossils, and, like Parapandorina2, its cells are densely vacuolated (5).

References (abridged):

1. Xiao, S., Zhang, Y. & Knoll, A. H. Nature 391, 553–558 (1998).

2. Hagadorn, J. W. et al. Science 314, 291–294 (2006).

3. Bailey, J. V., Joye, S. B., Kalanetra, K. M., Flood, B. E. & Corsetti, F. A. Nature 445, 198–201 (2007).

4. Xiao, S. & Knoll, A. H. J. Paleontol. 74, 767–788 (2000).

5. Kalanetra, K. M., Joye, S. B., Sunseri, N. R. & Nelson, D. C. Environ. Microbiol. 7, 1451–1460 (2005).

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

ScienceWeek http://scienceweek.com

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4. EVOLUTIONARY BIOLOGY: GENES AND THE PUZZLE OF HOMOSEXUALITY

The following points are made by V. Savolainen and L. Lehmann (Nature 2007 445:158):

1) For human societies at large, homosexuality is a sensitive issue. For biologists it is an intriguing one (1,2). How can genes influencing homosexual -- and so non-reproductive --behaviour be favoured by natural selection? An answer is offered by Gavrilets and Rice (3). They provide a population-genetic analysis that explains why, in theory, a gene predisposing an individual to homosexual behaviour would spread in a population. The analysis predicts the widespread occurrence of this gene in humans and other sexually reproducing species.

2) No predisposing gene for homosexual behaviour has been identified, but there is evidence that genetic controls are involved: for example, human twins are more likely both to be gay compared with non-identical brothers; and male homosexuality is more often inherited maternally, indicating that heritable maternal effects and/or genes linked to the X chromosome are in operation (2,3). However, unlike heterosexuals, who devote a significant amount of time to reproductive sex, homosexuals are involved in non-reproductive sex, hampering the direct transmission of any gene underlying this behaviour. Homosexuality has a cost to fitness -- that is, the ability of an individual to produce offspring that survive and reproduce -- and it can only evolve if it otherwise provides indirect benefits to reproduction.

3) Three main mechanisms have been proposed in which variety in genes controlling homosexuality could be maintained in a population: overdominance, sexually antagonistic selection, and kin altruism (2-4). For simplification, we will consider here male homosexuality, but these mechanisms also apply to female homosexuality. They also apply no matter how many genes contribute, but Gavrilets and Rice's analysis deals with a single theoretical gene and its two variants (alleles).

4) First, in the case of overdominance, a "gay allele" would result in homosexual behaviour in an individual who has received this allele from both parents (homozygous), but would provide an advantage to the heterozygote (where only one parent has transmitted the gay allele). This situation would be similar to the renowned example of sickle-cell anaemia in Africa, a genetically inherited disease controlled by a deficient allele. Homozygotes for this allele suffer severe disorders. But because this allele confers resistance to malaria when heterozygous, it is maintained in human populations exposed to malaria. Under this scenario, heterozygotes for the gay allele may have higher success in attracting females and/or their sperm may have some competitive advantage (5).

5) In the second case, sexually antagonistic selection, a gay allele would result in a cost when expressed in males ("feminization" and loss of fitness), which would be counterbalanced by a fitness advantage when it is expressed in females.

6) In the third hypothesis, kin altruism, homosexuals would help their own family members, increasing the fitness of their relatives and therefore the probability that a gay allele is passed on to the next generation (2,4).

7) These hypotheses have previously been speculative, but they have now been modelled and formalized by Gavrilets and Rice (3). The authors adapted the classical population-genetic equations established by J. B. S. Haldane and describe the evolution of the frequency of two alleles at one locus, in large populations for which each allele may result in sex-specific effects on fitness. Considering hypothetical straight and gay alleles, Gavrilets and Rice document the conditions of relative costs and benefits to fitness under which the gay allele can enter a population of straight alleles and be maintained subsequently. They establish the conditions under both the overdominance and sexually antagonistic-selection hypotheses for a homosexual gene that would be located on autosomes (non-sexual chromosomes) or on the X chromosome. These conditions still remain to be evaluated in the kin-altruism hypothesis.

References (abridged):

1. Bagemihl, B. Biological Exuberance: Animal Homosexuality and Natural Diversity (St Martin's Press, New York, 1999).

2. Camperio-Ciani, A., Corna, F. & Capiluppi, C. Proc. R. Soc. Lond. B 271, 2217–2221 (2004).

3. Gavrilets, S. & Rice, W. R. Proc. R. Soc. Lond. B 273, 3031–3038 (2006).

4. Wilson, E. O. Sociobiology: The New Synthesis (Harvard Univ. Press, 1975).

5. MacIntyre, F. & Estep, K. W. Biosystems 31, 223–233 (1993).

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

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