The Search for Life's Origins Progress and Future Directions in Planetary Biology and Chemical Evolution (1990) / Chapter Skim
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5. The Evolution of Cellular and Multicellular Life
5. The Evolution of Cellular and Multicellular Life
Pages 91-104
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From page 91... ...
The unfolding of terrestrial life must be understood as contingent on the particular course of this planet's development. Both the origins of life on Earth and its subsequent evolution have been influenced strongly by events in the evolution of the physical Earth and by extraterrestrial phenomena (such as halide bombardment)
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From page 92... ...
Renewed exploration of diversity in prokaryotic metabolism, spurred by the recognition of the archaebacteria as a distinct prokaryotic kingdom, has demonstrated the existence of evolutionarily important bacterial metabolisms that were unknown a decade ago. Knowledge of the early Earth has expanded in concert with this biological progress and, for the first time, evidence has accumulated that forces us to consider the role of extraterrestrial factors in determining patterns of terrestrial evolution.
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From page 93... ...
From sequence comparisons (for homologous functions) , quantitative evolutionary relationships can be inferred, and these serve as conceptual frameworks within which to relate phenotypes and their temporal evolution, as well as the ecological and geological conditions surrounding the evolutionary process.
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From page 94... ...
4. Chemical indices of global tectonic and biogeochemical systems Strontium isotopic ratios Carbon isotopic ratios Sulfur isotopic ratios Rare earth element abundances Oxidation state of weathered surfaces tary rock sequences in which fossils are found provides important clues to the environmental evolution of the Earth's surface.
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From page 95... ...
Despite these revelations, the understanding of eukaryotic evolution is limited by the small number of taxa examined to date and by the inability to bring genotypic phylogenies into juxtaposition with relationships inferred from comparisons of phenotypes and the fossil record. Several other protistan groups, including the dinoflagellates and oxymonads, may represent additional early branches in the eukaryotic line of descent, but comparative morphology cannot be solely relied upon for inferring branching order.
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From page 96... ...
A fuller understanding of eukaryotic evolution is necessary if data from this superkingdom are to be pooled with those from eubacteria and archaebacteria to make informed inferences about the nature of the "progenote." Just as electron microscopic studies have revealed a hitherto unappreciated diversity of eukaryotic phenotypes in terms of ultrastructure, so too have recent biochemical studies of prokaryotes revealed a metabolic diversity significantly broader than previously thought. The isolation and characterization of microorganisms having previously unknown metabolic capabilities (especially among the archaebacteria, cyanobacteria, and sulfurreducing prokaryotes)
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From page 97... ...
Although these primordial organisms are extinct, clues to their genetic organization, metabolic capabilities, and other phenotypic characteristics are retained in the biological traits of unicellular organisms that represent early branchings in each of the primary lines of descent. By comparing features common to these evolutionary lineages, it may be possible to infer the phenotypes of the earliest organisms; it is probable that features common to early branching groups will reflect features possessed by their common ancestors.
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From page 98... ...
, biological evolution has not proceeded independently of planetary evolution, nor has it been immune to influences from Earth's cosmic environment. The fossil record documents numerous episodes of evolutionary radiation; molecular phylogeny also suggests that evolution occurred in bursts.
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From page 99... ...
Biologists have long stressed that animal life requires certain minimum oxygen tensions to support exercise metabolism, to ensure the diffusion of oxygen to internal cells in organisms having multiple cell layers, and to complete certain biochemical reactions. Recent geochemical evidence suggests that just prior to the observed radiation of the Ediacaran fauna, significant amounts of oxygen may have accumulated in the atmosphere a consequence of abnormally high rates of organic carbon burial.
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From page 100... ...
The report concluded with the recommendation that NASA initiate a new study program designed to link existing programs in planetary biology and thereby to include in NASA's overall research effort the important evolutionary events that took place in the interval between the appearance of multicellular life and the evolution of man. During the last 600 million years of Earth history (the Phanerozoic eon)
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From page 101... ...
There have recently been a number of striking research successes in relating past global biology to solar-system~or galactic influences. Work with the marine micropaleontology of the past 700,000 years has shown rather decisively that cycles of climatic change, including the several pulses of continental glaciation, can be tied directly to Milankovich cycles of orbital change in the Earth-Moon-Sun system.
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From page 102... ...
Emphasis is given to Milankovich cycles and largebody impacts because these are areas in which there has been some preliminary success; however, totally different aspects of the cosmic and planetary environment may prove to be important to the global biology of the past, present, and future. To improve techniques for the quantitative understanding of environmental conditions on the early Earth, NASA should continue to take the intellectual lead in fostering interdisciplinary research and communication among scientists having disparate specialties.
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From page 103... ...
Geological research should be aimed not only at the elucidation of environmental evolution, but also at understanding the cosmic influences on terrestrial environments and evolution. As discussed in the final chapter of this report, much of the planning for Mars sample return missions will be spearheaded by groups outside of the exobiology research community; however, the committee views the participation of exobiologists in mission planning and execution as essential.
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From page 104... ...
104 THE SEARCH FOR LIFE'S ORIGINS Some areas of exobiological research are supported by other agencies in addition to NASA, especially NSF. NASA's continuing support is critical, however, because only it can provide the programmatic integration that promotes the necessary cross-fertilization of the various disciplines relevant to exobiology.
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