This Week in Science

Science  12 May 2000:
Vol. 288, Issue 5468, pp. 925
  1. Earlier Departure

    It has been suggested that the homonids left Africa only after sophisticated tool use had been developed. Gabunia et al. (p. 1019; see the news stories by Balter and Gibbons) describe two hominid fossil crania recovered from the Dmanisi site, Republic of Georgia, that may be representative of an early hominid migration out of Africa. The hominid crania are most similar to Homo ergaster, an African hominid, and are unlike H. erectus or other later Asian or European hominds. The Dmanisi hominids are associated with primitive tools. Geochronology, paleomagnetic measurements, and particularly the sedimentological relations and associated tools and vertebrate fossils imply that the hominids are 1.7 million years old. Thus, migration out of Africa was under way before development of sophisticated tool technology.

  2. Water Does the Trick

    The reactivity of metal oxide surfaces such as alumina is strongly influenced by their degree of hydration. The detailed characterization of hydrated mineral surfaces is technically challenging, and many aspects of the structure and reactivity of these surfaces remain poorly understood. Eng et al. (p. 1029) studied the clean and hydrated alumina surfaces by crystal rod diffraction using a synchrotron x-ray source and found structural clues for the different reactivities. In contrast to the clean surface, the hydrated surface is oxygen- rather than aluminum-terminated, and the double aluminum layer directly below the oxygen-terminating layer is highly contracted.

  3. Trapping and Separating DNA

    An alternative to gel electrophoresis for the separation of long DNA molecules in microfluidic devices is the use of an entropic trap array. In this approach, a set of constrictions and a static but spatially varying electrical field provide traps for DNA motion, and longer DNA molecules, which escape the traps more readily, have greater electrophoretic mobility. Han and Craighead (p. 1026) describe a microfabricated entropic trap array in which they could load and separate long DNA molecules (5000 to 160,000 base pairs) along 1.5-centimeter tracks in about 15 minutes.

  4. Old Cold El Niños

    The El Niño-Southern Oscillation (ENSO) comes and goes, and it has been suggested that the strength of this important cycle may depend on factors such as the intensity of solar insolation or the concentration of greenhouse gases in the atmosphere. Rittenour et al. (p. 1039; see the news story by Kerr) have assembled and analyzed a 4000-year record of New England lake varved sediment thickness, a proxy for regional climate, for the interval between 17,500 and 13,500 years before present. They observe a waxing and waning signal of climate variability that has the same frequency as ENSO, which indicates that ENSO occurred even during the very different climate at the end of the last glacial period.

  5. More Sunlight via Soot

    Aerosols are generally thought to produce climate cooling by increasing Earth's albedo, either by increasing the size of cloud droplets or by acting as condensation nuclei that can increase the amount of water in clouds or the extent of cloud cover. All of these effects reflect more solar energy back into space. Ackerman et al. (p. 1042; see the Perspective by Schwartz and Buseck) have now shown that high aerosol concentrations can also decrease albedo and promote warming. The daytime coverage of trade-cumulus clouds can be reduced by more than half by aerosol pollution, an effect that could influence atmospheric circulation by weakening deep convection in the intertropical convergence zone.

  6. Going Up

    Melting of ice sheets since the last glacial maximum has raised sea level about 150 meters. Most sea-level records have been derived from coral reefs, but these have only provided a rough view of the early part of the melting record. Hanebuth et al. (p. 1033) now provide a sea-level record from the Sunda shelf in southeast Asia, where rising seas flooded a stable continental shelf. Many drill cores into the sediment sequence and radiocarbon dates provide a high-resolution record. Melt water drove sea level up abruptly by 16 meters during a period of a few hundred years about 14,000 years ago.

  7. Noble Gases Leftover from Early Earth

    Noble gases can be used to trace the differentiation of Earth. Trieloff et al. (p. 1036; see the Perspective by Kaneoka) measured the isotopic concentrations of noble gases in lavas from the Hawaiian and Icelandic hot spot plumes that may sample the lower mantle. The neon isotopic signature is similar to the sun's, but the argon, krypton, and xenon isotopic signatures are nonsolar. These signatures, combined with other isotopic correlations, indicate that the noble gas signature of the lower mantle represents the initial concentration of neon from solar wind irradiation of planetesimals that accreted to form Earth, whereas the heavier noble gases reflect early fractionation of the mantle during its magma ocean phase.

  8. Stealing Your Iron

    Iron is an essential nutrient for most microbes and pathogens, but mammalian hosts represent a severely iron-limiting environment. Successful, or virulent, pathogens must circumvent their host's iron sequestration mechanisms. Ramanan and Wang (p. 1062) identified two high-affinity iron-permease genes in the pathogenic yeast, Candida albicans, and showed that one, CaFTR1, was important for virulence and in establishing systemic infections in mice.

  9. A Seal Harbor for Influenza B

    Animals sensitive to influenza virus can form natural “reservoirs” that provide the source of human epidemics. Although such reservoirs have been found for influenza A, influenza B was thought to only infect humans. Osterhaus et al. (p. 1051) describe the isolation of influenza B virus from seals found in the Netherlands. Analyses of sera indicate that the virus emerged in the seal population around 1995 and genetic analysis indicates that it is closely similar to human influenza strains.

  10. Stuck Offside

    The bilateral symmetry that is such a visible characteristic of higher animals is not without its exceptions—for example, in birds and mammals, the heart loops off to one side as it develops. The signaling events that establish this asymmetry seem to begin with the earliest stages of gastrulation. García-Castro et al. (p. 1047) now demonstrate that N-cadherin, a molecule that generally mediates cell adhesion, is expressed at the right place and time in chick embryos to be one of the earliest mediators of left-right asymmetry. Thus, the processes that result in established asymmetry may include cell adhesion as well as the already known signaling events.

  11. Loop the Loop

    In simpler times, the driving gears of the circa 24-hour circadian clock were thought to consist of positively acting transcription factors that cause transcription of a gene whose protein then fed back to interfere with action of the positive elements and inhibit its own transcription. Now, Shearman et al. (p. 1013; see the cover and see the news story by Barinaga) show that the mammalian circadian clock, as was shown recently for the Drosophila clock as well, actually has two more “gears.” The negative element is CRY, which interferes with the positive action of the heterodimer CLOCK and BMAL1. The authors now show that PER2 is a positive regulator of BMAL1 and CRY is a negative regulator of PER2. The next step will be to understand how this intricate oscillator keeps circadian time.

  12. In Harm's Pathway

    The tumor supressor protein p53 causes death of cells exposed to stresses such as DNA damage. The p53 protein causes increased expression of the protein Bax, which in turn causes cell death. However, evidence indicates that expression of other genes must also contribute to p53-induced cell death. Oda et al. (p. 1053) screened for genes expressed in response to x-ray irradiation in normal cells but not in mouse cells that lack p53. They identified a gene called Noxa that appears to be directly regulated by p53. Overexpresion of Noxa caused apoptosis, and Noxa interacted with anti-apoptotic Bcl-2 family proteins at the mitochondria. Antisense RNA to Noxa prevented expression of Noxa in response to p53 and inhibited p53-induced apoptosis. Thus, Noxa appears to represent a new target regulated through p53 that contributes to stress-induced cell death.

  13. Finger on the Switch

    The process by which antibodies switch the class of their heavy chain, and therefore their effector function, is a critical step in the immune response to antigen. Signals that trigger class-switching recombination, through “switch regions” in the DNA, promote the transcription of sterile RNA transcripts (which do not code for RNA or protein products). These sterile RNAs appear to be critical for class-switching recombination and can form stable RNA/DNA hybrids at switch sequences in vitro. Tracy et al. (p. 1058; see the Perspective by Stavnezer) now show that the RNA/DNA hybrids can be isolated in vivo under conditions that stimulate switching. The hybrids are greatly reduced in transgenic mice expressing ribonuclease H, which specifically degrades RNA/DNA hybrids. Furthermore, there is also a concomitant reduction in class switching in the B cells of these mice.

  14. Northern Hemisphere Sea Ice Extent

    Comparing the observed 1953-98 decrease in Northern Hemisphere sea ice extent with simulations from two climate models, Vinnikov et al. (Reports, 3 Dec., p. 1934) found that the observed decrease was far larger than that expected from natural climate variability, and concluded that the sea ice decrease likely traces to anthropogenic global warming. Moritz and Bitz assert that the results of one of the models used, that of the Geophysical Fluid Dynamics Laboratory (GFDL), includes spurious negative values of sea ice thickness that alternate with positive values in many adjacent model grid cells. They also note that in the 50 years before the period studied by Vinnikov et al., sea ice extent increased at a rate of 130,000 km2 per decade, a trend that they suggest calls into question whether the decrease during the subsequent five decades can be attributed solely to anthropogenic factors. Vinnikov et al. respond that the supposed negative sea ice values arise from the spatial polar filter in the GFDL model, the effects of which are eliminated “before the ice thicknesses are passed to the model's atmospheric component.” The sea ice record from 1900 to 1952, meanwhile, is of highly variable quality and thus “cannot be used to extract sea ice variability from the observations.” The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/288/5468/927a

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