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  • 1
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    In situ evidence for an ancient aqueous environment at Meridiani Planum, Mars (2004)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 2004-12-04
    Description: Sedimentary rocks at Eagle crater in Meridiani Planum are composed of fine-grained siliciclastic materials derived from weathering of basaltic rocks, sulfate minerals (including magnesium sulfate and jarosite) that constitute several tens of percent of the rock by weight, and hematite. Cross-stratification observed in rock outcrops indicates eolian and aqueous transport. Diagenetic features include hematite-rich concretions and crystal-mold vugs. We interpret the rocks to be a mixture of chemical and siliciclastic sediments with a complex diagenetic history. The environmental conditions that they record include episodic inundation by shallow surface water, evaporation, and desiccation. The geologic record at Meridiani Planum suggests that conditions were suitable for biological activity for a period of time in martian history.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Squyres, S W -- Grotzinger, J P -- Arvidson, R E -- Bell, J F 3rd -- Calvin, W -- Christensen, P R -- Clark, B C -- Crisp, J A -- Farrand, W H -- Herkenhoff, K E -- Johnson, J R -- Klingelhofer, G -- Knoll, A H -- McLennan, S M -- McSween, H Y Jr -- Morris, R V -- Rice, J W Jr -- Rieder, R -- Soderblom, L A -- New York, N.Y. -- Science. 2004 Dec 3;306(5702):1709-14.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Department of Astronomy, Cornell University, Ithaca, NY 14853, USA. squyres@astrosun.tn.cornell.edu〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/15576604" target="_blank"〉PubMed〈/a〉
    Keywords: Exobiology ; Extraterrestrial Environment ; Ferric Compounds ; Geologic Sediments ; Life ; *Mars ; Minerals ; Silicates ; Spacecraft ; Spectrum Analysis ; Sulfates ; Sulfur ; *Water
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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  • 2
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    Proterozoic and early Cambrian protists: evidence for accelerating evolutionary tempo (1994)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: In rocks of late Paleoproterozoic and Mesoproterozoic age (ca. 1700-1000 million years ago), probable eukaryotic microfossils are widespread and well preserved, but assemblage and global diversities are low and turnover is slow. Near the Mesoproterozoic-Neoproterozoic boundary (1000 million years ago), red, green, and chromophytic algae diversified; molecular phylogenies suggest that this was part of a broader radiation of "higher" eukaryotic phyla. Observed diversity levels for protistan microfossils increased significantly at this time, as did turnover rates. Coincident with the Cambrian radiation of marine invertebrates, protistan microfossils again doubled in diversity and rates of turnover increased by an order of magnitude. Evidently, the Cambrian diversification of animals strongly influenced evolutionary rates, within clades already present in marine communities, implying an important role for ecology in fueling a Cambrian explosion that extends across kingdoms.
    Keywords: Exobiology
    Type: Proceedings of the National Academy of Sciences of the United States of America (ISSN 0027-8424); Volume 91; 15; 6743-50
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  • 3
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    The limits of palaeontological knowledge: finding the gold among the dross (1996)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: Palaeontological interpretation rests on two interwoven sets of comparisons with the modern world. Palaeobiological interpretation relies on the placement of fossils within a phylogenetic and functional framework based primarily on the comparative biology of living organisms. Analogy to currently observable chemical, physical and taphonomic processes enables palaeoenvironmental inferences to be drawn from geological data. In older rocks, comparisons with the modern Earth can become tenuous, limiting palaeontological interpretation. The problem reaches its apogee in Archaean successions, yet pursuit of multiple lines of evidence establishes that complex microbial communities, fuelled by autotrophy and, likely, photoautotrophy, existed 3500 million years ago. Although Archaean palaeontology has to date focused on silicified coastal sediments, improved understanding of Earth's earliest biosphere may depend on the development of alternative environmental and taphonomic analogies. Spring precipitates and hydrothermal metal deposits are promising candidates. Terrestrial organisms may be of limited value in interpreting such fossils as may be found on Mars, although some points of comparison could prove general. Given limited opportunities for exploration, proper choice of environmental analogy is critical. Spring precipitates constitute excellent deposits for addressing questions of biology on another planet.
    Keywords: Exobiology
    Type: Ciba Foundation symposium (ISSN 0300-5208); Volume 202; 198-209; discussion 210-3
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  • 4
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    Evolution and extinction in the marine realm: some constraints imposed by phytoplankton (1989)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: The organic and mineralized remains of planktonic algae provide a rich record of microplankton evolution extending over nearly half of the preserved geological record. In general, Phanerozoic patterns of phytoplankton radiation and extinction parallel those documented for skeletonized marine invertebrates, both augmenting and constraining thought about evolution in the oceans. Rapidly increasing knowledge of Proterozoic plankton is making possible the recognition of additional episodes of diversification and extinction that antedate the Ediacaran radiation of macroscopic animals. In contrast to earlier phytoplankton history, the late Mesozoic and Cainozoic record is documented in sufficient detail to constrain theories of mass extinction in more than a general way. Broad patterns of diversity change in planktonic algae show similarities across the Cretaceous-Tertiary and Eocene-Oligocene boundaries, but detailed comparisons of origination and extinction rates in calcareous nannoplankton, as well as other algae and skeletonized protozoans, suggest that the two episodes were quite distinct. Common causation appears unlikely, casting doubt on monolithic theories of mass extinction, whether periodic or not. Studies of mass extinction highlight a broader class of insights that paleontologists can contribute to evolutionary biology: the evaluation of evolutionary change in the context of evolving Earth-surface environments.
    Keywords: Exobiology
    Type: Philosophical transactions of the Royal Society of London. Series B, Biological sciences (ISSN 0962-8436); Volume 325; 279-90
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  • 5
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    Life's expanding realm (1994)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: Geologic sediments from the Nullagine Range in Australia are used to illustrate the early existence of microbial communities in the oceans. These communities survived in oxygen-free environments. Some microbes, particularly cyanobacteria, developed the ability to synthesize energy from light, which led to the evolution of creatures with oxygen-dependent metabolism. Only recently has geologic evidence been discovered that supports the theory that animals developed only when there was enough oxygen in the atmosphere to support higher forms of life.
    Keywords: Exobiology
    Type: Natural history (ISSN 0028-0712); Volume 103; 6; 14, 16, 18-20
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  • 6
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    Comparative Earth history and Late Permian mass extinction (1996)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: The repeated association during the late Neoproterozoic Era of large carbon-isotopic excursions, continental glaciation, and stratigraphically anomalous carbonate precipitation provides a framework for interpreting the reprise of these conditions on the Late Permian Earth. A paleoceanographic model that was developed to explain these stratigraphically linked phenomena suggests that the overturn of anoxic deep oceans during the Late Permian introduced high concentrations of carbon dioxide into surficial environments. The predicted physiological and climatic consequences for marine and terrestrial organisms are in good accord with the observed timing and selectivity of Late Permian mass extinction.
    Keywords: Exobiology
    Type: Science (ISSN 0036-8075); Volume 273; 452-7
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  • 7
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    The NASA Astrobiology Roadmap (2003)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: The NASA Astrobiology Roadmap provides guidance for research and technology development across the NASA enterprises that encompass the space, Earth, and biological sciences. The ongoing development of astrobiology roadmaps embodies the contributions of diverse scientists and technologists from government, universities, and private institutions. The Roadmap addresses three basic questions: How does life begin and evolve, does life exist elsewhere in the universe, and what is the future of life on Earth and beyond? Seven Science Goals outline the following key domains of investigation: understanding the nature and distribution of habitable environments in the universe, exploring for habitable environments and life in our own solar system, understanding the emergence of life, determining how early life on Earth interacted and evolved with its changing environment, understanding the evolutionary mechanisms and environmental limits of life, determining the principles that will shape life in the future, and recognizing signatures of life on other worlds and on early Earth. For each of these goals, Science Objectives outline more specific high-priority efforts for the next 3-5 years. These 18 objectives are being integrated with NASA strategic planning.
    Keywords: Exobiology
    Type: Astrobiology (ISSN 1531-1074); Volume 3; 2; 219-35
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  • 8
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    Early animal evolution: emerging views from comparative biology and geology (1999)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: The Cambrian appearance of fossils representing diverse phyla has long inspired hypotheses about possible genetic or environmental catalysts of early animal evolution. Only recently, however, have data begun to emerge that can resolve the sequence of genetic and morphological innovations, environmental events, and ecological interactions that collectively shaped Cambrian evolution. Assembly of the modern genetic tool kit for development and the initial divergence of major animal clades occurred during the Proterozoic Eon. Crown group morphologies diversified in the Cambrian through changes in the genetic regulatory networks that organize animal ontogeny. Cambrian radiation may have been triggered by environmental perturbation near the Proterozoic-Cambrian boundary and subsequently amplified by ecological interactions within reorganized ecosystems.
    Keywords: Geosciences (General)
    Type: Science (ISSN 0036-8075); Volume 284; 5423; 2129-37
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  • 9
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    The evolution of ecological tolerance in prokaryotes (1989)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: The ecological ranges of Archaeobacteria and Eubacteria are constrained by a requirement for liquid water and the physico-chemical stability limits of biomolecules, but within this broad envelope, prokaryotes have evolved adaptations that permit them to tolerate a remarkable spectrum of habitats. Laboratory experiments indicate that prokaryotes can adapt rapidly to novel environmental conditions, yet geological studies suggest early diversification and long-term stasis within the prokaryotic kingdoms. These apparently contradictory perspectives can be reconciled by understanding that, in general, rates and patterns of prokaryotic evolution reflect the developmental history of the Earth's surface environments. Our understanding of modern microbial ecology provides a lens through which our accumulating knowledge of physiology, molecular phylogeny and the Earth's history can be integrated and focussed on the phenomenon of prokaryotic evolution.
    Keywords: Exobiology
    Type: Transactions of the Royal Society of Edinburgh: earth sciences; Volume 80; 209-23
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  • 10
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    Adaptation, plant evolution, and the fossil record (1987)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: The importance of adaptation in determining patterns of evolution has become an important focus of debate in evolutionary biology. As it pertains to paleobotany, the issue is whether or not adaptive evolution mediated by natural selection is sufficient to explain the stratigraphic distributions of taxa and character states observed in the plant fossil record. One means of addressing this question is the functional evaluation of stratigraphic series of plant organs set in the context of paleoenvironmental change and temporal patterns of floral composition within environments. For certain organ systems, quantitative estimates of biophysical performance can be made on the basis of structures preserved in the fossil record. Performance estimates for plants separated in time or space can be compared directly. Implicit in different hypotheses of the forces that shape the evolutionary record (e.g. adaptation, mass extinction, rapid environmental change, chance) are predictions about stratigraphic and paleoenvironmental trends in the efficacy of functional performance. Existing data suggest that following the evolution of a significant structural innovation, adaptation for improved functional performance can be a major determinant of evolutionary changes in plants; however, there are structural and development limits to functional improvement, and once these are reached, the structure in question may no longer figure strongly in selection until and unless a new innovation evolves. The Silurian-Devonian paleobotanical record is consistent with the hypothesis that the succession of lowland floodplain dominants preserved in the fossil record of this interval was determined principally by the repeated evolution of new taxa that rose to ecological importance because of competitive advantages conferred by improved biophysical performance. This does not seem to be equally true for Carboniferous-Jurassic dominants of swamp and lowland floodplain environments. In these cases, environmental disruption appears to have been a major factor in shaping the fossil record. This does not mean that continuing adaptation was not important during this interval, but it may indicate that adaptive evolution was strongest in environments other than those best represented in the paleobotanical record.
    Keywords: Exobiology
    Type: Review of palaeobotany and palynology (ISSN 0034-6667); Volume 50; 127-49
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