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641.
Laser--Raman imagery of Earth's earliest fossils 总被引:18,自引:0,他引:18
Unlike the familiar Phanerozoic history of life, evolution during the earlier and much longer Precambrian segment of geological time centred on prokaryotic microbes. Because such microorganisms are minute, are preserved incompletely in geological materials, and have simple morphologies that can be mimicked by nonbiological mineral microstructures, discriminating between true microbial fossils and microscopic pseudofossil 'lookalikes' can be difficult. Thus, valid identification of fossil microbes, which is essential to understanding the prokaryote-dominated, Precambrian 85% of life's history, can require more than traditional palaeontology that is focused on morphology. By combining optically discernible morphology with analyses of chemical composition, laser--Raman spectroscopic imagery of individual microscopic fossils provides a means by which to address this need. Here we apply this technique to exceptionally ancient fossil microbe-like objects, including the oldest such specimens reported from the geological record, and show that the results obtained substantiate the biological origin of the earliest cellular fossils known. 相似文献
642.
Hox protein mutation and macroevolution of the insect body plan 总被引:23,自引:0,他引:23
643.
Onset of Asian desertification by 22 Myr ago inferred from loess deposits in China 总被引:172,自引:0,他引:172
Guo ZT Ruddiman WF Hao QZ Wu HB Qiao YS Zhu RX Peng SZ Wei JJ Yuan BY Liu TS 《Nature》2002,416(6877):159-163
The initial desertification in the Asian interior is thought to be one of the most prominent climate changes in the Northern Hemisphere during the Cenozoic era. But the dating of this transition is uncertain, partly because desert sediments are usually scattered, discontinuous and difficult to date. Here we report nearly continuous aeolian deposits covering the interval from 22 to 6.2 million years ago, on the basis of palaeomagnetic measurements and fossil evidence. A total of 231 visually definable aeolian layers occur as brownish loesses interbedded with reddish soils. This new evidence indicates that large source areas of aeolian dust and energetic winter monsoon winds to transport the material must have existed in the interior of Asia by the early Miocene epoch, at least 14 million years earlier than previously thought. Regional tectonic changes and ongoing global cooling are probable causes of these changes in aridity and circulation in Asia. 相似文献
644.
Gilman AG Simon MI Bourne HR Harris BA Long R Ross EM Stull JT Taussig R Bourne HR Arkin AP Cobb MH Cyster JG Devreotes PN Ferrell JE Fruman D Gold M Weiss A Stull JT Berridge MJ Cantley LC Catterall WA Coughlin SR Olson EN Smith TF Brugge JS Botstein D Dixon JE Hunter T Lefkowitz RJ Pawson AJ Sternberg PW Varmus H Subramaniam S Sinkovits RS Li J Mock D Ning Y Saunders B Sternweis PC Hilgemann D Scheuermann RH DeCamp D Hsueh R Lin KM Ni Y Seaman WE Simpson PC O'Connell TD Roach T Simon MI 《Nature》2002,420(6916):703-706
The Alliance for Cellular Signaling is a large-scale collaboration designed to answer global questions about signalling networks. Pathways will be studied intensively in two cells--B lymphocytes (the cells of the immune system) and cardiac myocytes--to facilitate quantitative modelling. One goal is to catalyse complementary research in individual laboratories; to facilitate this, all alliance data are freely available for use by the entire research community. 相似文献
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648.
The BRCA2 homologue Brh2 nucleates RAD51 filament formation at a dsDNA-ssDNA junction 总被引:1,自引:0,他引:1
The BRCA2 tumour suppressor is essential for the error-free repair of double-strand breaks (DSBs) in DNA by homologous recombination. This is mediated by RAD51, which forms a nucleoprotein filament with the 3' overhanging single-stranded DNA (ssDNA) of the resected DSB, searches for a homologous donor sequence, and catalyses strand exchange with the donor DNA. The 3,418-amino-acid BRCA2 contains eight approximately 30-amino-acid BRC repeats that bind RAD51 (refs 5, 6) and a approximately 700-amino-acid DBD domain that binds ssDNA. The isolated BRC and DBD domains have the opposing effects of inhibiting and stimulating recombination, respectively, and the role of BRCA2 in repair has been unclear. Here we show that a full-length BRCA2 homologue (Brh2) stimulates Rad51-mediated recombination at substoichiometric concentrations relative to Rad51. Brh2 recruits Rad51 to DNA and facilitates the nucleation of the filament, which is then elongated by the pool of free Rad51. Brh2 acts preferentially at a junction between double-stranded DNA (dsDNA) and ssDNA, with strict specificity for the 3' overhang polarity of a resected DSB. These results establish a BRCA2 function in RAD51-mediated DSB repair and explain the loss of this repair capacity in BRCA2-associated cancers. 相似文献
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Semaw S Simpson SW Quade J Renne PR Butler RF McIntosh WC Levin N Dominguez-Rodrigo M Rogers MJ 《Nature》2005,433(7023):301-305
Comparative biomolecular studies suggest that the last common ancestor of humans and chimpanzees, our closest living relatives, lived during the Late Miocene-Early Pliocene. Fossil evidence of Late Miocene-Early Pliocene hominid evolution is rare and limited to a few sites in Ethiopia, Kenya and Chad. Here we report new Early Pliocene hominid discoveries and their palaeoenvironmental context from the fossiliferous deposits of As Duma, Gona Western Margin (GWM), Afar, Ethiopia. The hominid dental anatomy (occlusal enamel thickness, absolute and relative size of the first and second lower molar crowns, and premolar crown and radicular anatomy) indicates attribution to Ardipithecus ramidus. The combined radioisotopic and palaeomagnetic data suggest an age of between 4.51 and 4.32 million years for the hominid finds at As Duma. Diverse sources of data (sedimentology, faunal composition, ecomorphological variables and stable carbon isotopic evidence from the palaeosols and fossil tooth enamel) indicate that the Early Pliocene As Duma sediments sample a moderate rainfall woodland and woodland/grassland. 相似文献