This month in Science Roundup:
Exploring the RNA World
Our Evolving Brains
Disconnecting During Sleep
Chimp and Human Expression
Deciphering Dengue
Power Walking
Reconstructing Ancient Language
Voyage of Discovery
Comet Collision
Stronger Hurricanes Ahead?
Shockingly Strong Metal
The Rise of Oxygen and Mammals
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RNA has long lived in the shadow of its more famous chemical cousin DNA, but awareness of its biological diversity and importance is rapidly coming to light. In a special collection of articles published with the 2 Sep 2005 issue, Science Magazine and its online Knowledge Environments wove through the multilayered world of RNA. In Science, Review and Viewpoint articles explored the relationship between RNA structure and function, the complex lives of messenger RNAs (mRNAs), the emerging "big world" of small RNA molecules, the hidden functions of noncoding RNAs, and the discovery of a new RNA enzyme. Eleven research papers presented new findings in various other aspects of RNA research including plant and mammal transcriptome analyses and structural studies of catalytic RNA molecules. Science's Signal Transduction Knowledge Environment looked at the roles for small RNAs in regulation of gene expression and how mRNAs can be selectively activated from cytoplasmic macromolecular structures called RNA granules, to contribute to such processes as synaptic plasticity. Finally, a Perspective in the Science of Aging Knowledge Environment examined at how RNA interference has transformed aging-related research in worms.
The human brain is proportionally larger than that of any other animal, and our advanced cognitive powers are seemingly unmatched. Now, two new studies suggest that our brains may still be evolving. In the 9 Sep 2005 Science Evans et al. ( http://www.sciencemag.org/cgi/content/short/309/5741/1717 ) and Mekel-Bobrov et al. ( http://www.sciencemag.org/cgi/content/short/309/5741/1720 ) concluded that two genes thought to regulate human brain growth have continued to evolve under natural selection until very recently -- and perhaps are doing so today. The studies focus on two genes -- microcephalin and ASPM-- implicated in microcephaly, a condition in which the brain is severely reduced in size. The teams report that in for each gene, they found one genetic variant with a surprisingly high frequency in human populations. Statistical tests showed that these frequencies are unlikely to be due to random genetic drift or population migration, suggesting that the variants were instead favored by natural selection because they confer some unknown benefit.
In a related Brevia article, Hayakawa et al. ( http://www.sciencemag.org/cgi/content/short/309/5741/1693 ) reported further evidence of the action of natural selection in people: A gene expressed in microglia, immune cells of the nervous system, produces a protein found in humans, but not in chimpanzees. An ancestral nonfunctioning form of the gene is present in nonhuman primates, however, which suggests that this gene may have been a target of selection during human evolution. A News story by M. Balter ( http://www.sciencemag.org/cgi/content/short/309/5741/1662 ) discussed the new findings.
When we fall asleep, consciousness fades yet the brain remains active. Now a Report by Massimini et al. ( http://www.sciencemag.org/cgi/content/short/309/5744/2228 ) in the 30 Sep 2005 Science provides insight into this conundrum. Using magnetic pulses to stimulate the brains of waking and sleeping subjects, the researchers assessed the degree of connectivity between different parts of the brain's cortex. When the subjects were awake, the magnetic pulses elicited waves of neurological activity that spread to distant areas of the cortex on both the left and right sides of the brain. However, in subjects who had entered non-rapid eye movement (non-REM) sleep -- during which people often report little or no conscious experience -- neural activity remained locally confined. The results suggest that the fading of consciousness during certain stages of sleep may be related to a breakdown in communication between different regions of the cortex. As noted in an accompanying News story by G. Miller ( http://www.sciencemag.org/cgi/content/short/309/5744/2148a ), it will be important to find out whether cortical connectivity recovers during REM sleep, after which people report conscious-like experiences in the form of vivid dreams.
The recent sequencing of the chimpanzee genome is a notable accomplishment, but the hard work of understanding how sequence information contributes to the differences between chimps and humans still lies ahead (see the News story by E. Culotta in the 2 Sep 2005 issue; http://www.sciencemag.org/cgi/content/short/309/5740/1468 ). In a Report in the 16 Sep 2005 Science (published online 1 Sep 2005), Khaitovich et al. ( http://www.sciencemag.org/cgi/content/short/309/5742/1850 ) used the new genome data to investigate the role that changes in genetic material over time have played in the emergence of humans and chimps from their common ancestor. The team compared both gene sequences and gene expression levels in the brain, heart, liver, kidneys, and testes of humans and chimpanzees and found the patterns of differences to be remarkably similar. They also found that genes expressed in the testes have evolved considerably in both species and that genes active in brain have accumulated more changes on the human than the chimpanzee lineage.
Understanding the contribution of genomic DNA to human and great ape evolution is just one step toward answering the question "what makes us human." In a pair of Perspectives in the 2 Sep issue, M. D. Hauser ( http://www.sciencemag.org/cgi/content/short/309/5740/1498 ) and McConkey and Varki ( http://www.sciencemag.org/cgi/content/short/309/5740/1499 ) looked at the knowledge we've gained thus far from studying chimps an other great apes and highlighted the need for conserving these species. The call for conservation was echoed in a related Editorial by A. Jolly ( http://www.sciencemag.org/cgi/content/short/309/5740/1457 ).
Science's yearlong Global Voices of Science essay series continued in the 2 Sep 2005 issue with an essay by Cuban virologist María G. Guzmán ( http://www.sciencemag.org/cgi/content/short/309/5740/1495 ). Since the late 1970s, Cuba has experienced multiple epidemics of dengue, a potentially lethal viral disease transmitted by mosquitoes that now has a global distribution. In her essay, Guzmán chronicles the multi-pronged research that she and her colleagues at the Tropical Medicine Institute Pedro Kourí (IPK), in Havana, Cuba have been conducting in an effort to expose the virus's molecular details, and understand the epidemiology of the disease. Guzman's work has also contributed to our understanding of the risk factors for developing dengue hemorrhagic fever and dengue shock syndrome, which include secondary infection, age, and preexisting chronic disease. The great strides that she and her coworkers have been able to take in this research arena stem from commitments that Cuba has made to public health and education since the late 1960s. An accompanying slideshow ( http://www.sciencemag.org/sciext/globalvoices/ ) highlighted the essay.
Humans have become increasingly dependent on mobile electronic devices from medical equipment and GPS instruments to cell phones. At present, all of these devices are powered by batteries, which have a limited energy storage capacity and add considerable weight. Given that we spend much of daily energy walking, finding a way to harvest that mechanical energy could offer a valuable alternative to powering our gadgets. In a Report in the 9 Sep 2005 Science, Rome et al. ( http://www.sciencemag.org/cgi/content/short/309/5741/1725 ) described a new backpack device that can do just that. The new backpack has a vertically moveable weight that rises about 5 centimeters with each step and then turns a gear connected to a generator as it falls. The mechanical energy from the vertical movement of the backpack's cargo can thus be converted to electricity during normal walking -- a load of 38 kilograms can produce up to 7.4 watts of electricity. Although many refinements must be made, such an apparatus could generate power during journeys beyond the reach of power grids. As noted in an accompanying Perspective by A. D. Kuo ( http://www.sciencemag.org/cgi/content/short/309/5741/1686 ), the energy-harvesting backpack is novel because it generates useful amounts of electrical power while costing less metabolic energy than expected.
Reconstructing Ancient Language
Studying language family trees has traditionally depended on recognizing "cognate sets" of word pairs -- words in different languages with the same ultimate source in an ancient language -- and reconstructing the changes in their sounds and meaning. But because vocabularies change so rapidly, most linguists believe that after about 8000 to 10,000 years it is impossible to differentiate between true word homology and chance resemblance or borrowings between languages. Now, a Report by Dunn et al. ( http://www.sciencemag.org/cgi/content/short/309/5743/2072 ) in the 23 Sep 2005 Science describes a new method that uses language structure, rather than vocabulary, to construct language phylogenies, and allows a much deeper sampling of linguistic time. Using features such as the ordering of sentence elements or the grammatical elements of gender or tense, the team constructed phylogenies of Papuan languages in Island Melanesia that may have been separated since the late Pleistocene. As noted in an accompanying Perspective by R. Gray ( http://www.sciencemag.org/cgi/content/short/309/5743/2007 ), the approach is likely to be emulated by researchers studying languages in other regions of the world and offers new hope for uncovering ancient linguistic connections.
Voyage of Discovery
http://www.sciencemag.org/cgi/content/summary/309/5743/2015
Voyager 1 was launched on 5 September 1977. Eighteen months later, when it flew by Jupiter and its moons, it began to change our understanding of our solar system. The unmanned spacecraft has since flown by Saturn, Uranus and Neptune and returned nearly 80 thousand images and more than 5 trillion bits of data. Three decades later, it is still going strong. In December 2004, Voyager 1 crossed the "termination shock" where the supersonic flow known as the solar wind abruptly slows as it approaches the interstellar medium that bounds the solar system. In a special section of the 23 Sep 2005 Science, four Reports and a Viewpoint article described this landmark crossing as well as data from the heliosheath -- a vast, turbulent expanse between the termination shock and interstellar space. New observations provide details about the particles and plasmas in the heliosheath and the effects of the shock on the solar magnetic field. They also challenge notions about the origin of certain cosmic rays thought to be produced in this region. Stay tuned as Voyager 1 continues its journey toward the edge of our solar system and into the great unknown.
On 4 July 2005, planetary scientists smashed a portion of the Deep Impact spacecraft into the 14-kilometer-long icy heart of comet Tempel 1 -- just to see what the inside was like. Six studies published online on Science Express this month describe what they found. In the first set of reports published on 8 Sep 2005 (see the News story by R.A. Kerr in the 9 Sep issue; http://www.sciencemag.org/cgi/content/short/309/5741/1667 ), A'Hearn et al. ( http://www.sciencemag.org/cgi/content/abstract/1118923 ) presented an overview of data collected by the instruments aboard the impactor and the flyby spacecraft that launched it. Images of the comet nucleus show that it has many impact craters (not seen on other comets) and is complexly layered. Observations of the impact-induced dust plume and indicate that the comet is highly porous, not a solid block of ice. Meech et al. ( http://www.sciencemag.org/cgi/content/abstract/1118978 ) reported observations of the comet and the impact from some 70 different Earth-based telescopes stationed around the world and Keller et al. ( http://www.sciencemag.org/cgi/content/abstract/1119020 ) reported observations from the European Space Agency's Rosetta spacecraft, which sampled the comet's pre- and post-impact dust composition.
Detailed Reports by Harker et al. ( http://www.sciencemag.org/cgi/content/abstract/1119143 ), Sugita et al. ( http://www.sciencemag.org/cgi/content/abstract/1119091 ), and Mumma et al. ( http://www.sciencemag.org/cgi/content/abstract/1119337 ) published online 15 Sep 2005 further defined Tempel 1 as a powdery, fine-grained comet containing a variety of silicates, organic materials, and organic-rich gases.
The recent devastation caused by Hurricane Katrina along the Gulf Coast of the United States is a grim reminder of how vulnerable we are to the forces of nature. Could global warming be making matters worse? Because tropical storms are heat engines that draw their energy upward from warm ocean water to drive their winds, warming oceans might be expected to spawn more frequent or more intense storms. In a Report in the 16 Sep 2005 Science, Webster et al. ( http://www.sciencemag.org/cgi/content/short/309/5742/1844 ) examined the frequency and strength of tropical storms and hurricanes worldwide during the past 35 years. Their analysis shows that although there has not been an increase in the number or frequency of storms, the number and proportion of the strongest hurricanes (categories 4 and 5) has increased significantly, particularly in the Pacific and Indian Oceans. As noted in an accompanying News Focus by R. A. Kerr ( http://www.sciencemag.org/cgi/content/short/309/5742/1807 ), the researchers caution that pinning this trend to global warming will require records reaching farther back in time than 30 years. Nevertheless, the increase in strong storms does correlate with increasing sea surface temperatures.
Extreme applications like laser-driven nuclear fusion require materials tough enough to withstand intense shock. As a typical metal deforms, atomic-scale defects can travel through individual crystalline grains, stopping once they hit the edge of the grain. But as grain dimensions approach nanometer sizes, defects instead cause grains to slip past each other, thus compromising the material's integrity. Now a Report by Bringa et al. ( http://www.sciencemag.org/cgi/content/short/309/5742/1838 ) in the 16 Sep 2005 Science shows that applying a high-pressure shock to nanocrystalline copper and other metals makes them stronger by altering how they deform. In both simulations and experiments, a shock wave from a high powered laser caused numerous defects to occur within and between grains, and these defects prevented neighboring grains from slipping past one another under intense pressure. The result: the hardness of the metal increased up to a factor of two compared with unshocked samples. A 15 Sep ScienceNOW story by R. Service ( http://sciencenow.sciencemag.org/cgi/content/full/2005/915/2 ) highlighted the Report.
The Rise of Oxygen and Mammals
Atmospheric oxygen concentration has varied considerably during the past 200 million years. In a Report in the 30 Sep 2005 Science, Falkowski et al. ( http://www.sciencemag.org/cgi/content/short/309/5744/2202 ) examined how these changes have affected the evolution of mammals. Using carbon isotopic measurements from marine carbonates and organic matter, as well as sulfur isotope records, the team reconstructed oxygen concentrations for the past 205 million years. Their analysis indicates that oxygen levels approximately doubled over this period as the supercontinent Pangea fragmented into smaller continents and the Atlantic Ocean basin opened. The evolution of phytoplankton living along the continental margins greatly increased the burial of organic matter in marine sediments, which ultimately released more oxygen to the atmosphere. In addition, there were relatively rapid increases in oxygen concentration during the Jurassic period (about 200 to 145 million years ago) and the start of the Eocene (about 55 to 40 million years ago). Comparing these trends to the fossil record, the researchers suggest that the overall increase in atmospheric oxygen was a critical factor in the evolution and radiation of large mammals, which have relatively high oxygen demands.
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