Life Sciences in the Space Program 



autotrophic organisms composing the major stromatolite-building microorganisms 

 in the community. Progress in understanding the relationship between environ- 

 mental and biological evolution in these communities, however, is hindered by the 

 limited number of sedimentary sequences available for study and the difficulties in 

 preserving evidence of biological development occurring at the intracellular level. 



A major increase in the abundance of fossil microorganisms coincides with the 

 growth of continents and the emergence of wide continental margins between 2.8 

 and 2.2 billion years ago. Microfossiliferous deposits are abundant in rocks 

 between 2.5 and 0.6 billion years old (3,5). During this latter period, evidence of 

 nucleated cells, multicelled life, formation of biogenic mineral deposits, and the 

 persistence of atmospheric oxygen appears in the geological record. The availability 

 of sedimentary sequences from this period offers opportunities for establishing 

 causal relationships between the occurrence of glaciations, of oxygen in the 

 atmosphere, and of periodic increases in geological activity (such as mountain 

 building and growth of continents) and the manifestation of biological milestones, 

 such as the advent of oxygenic photosynthesis, the development of planktonic 

 eukaryotes and multicelled organisms, and the occurrence of episodes of 

 evolutionary radiation. 



Homologues of the ancient microbial ecosystems exist today in the form of 

 microbial mats, the features of which are still controlled almost exclusively by 

 unicellular life. Like the earliest ecosystems, these mat systems are also associated 

 with hot springs and hydrothermal vents, as well as shallow hypersaline marine 

 environments. The abundance, physiology, and ecology of the microorganisms in 

 these contemporary systems should be studied as models for interpreting specific 

 morphological, chemical, and isotopic features preserved in ancient rocks. Studies 

 of both ancient and modern systems will be invaluable in establishing a knowl- 

 edge base for carrying out the search for possible evidence of analogous biogenic 

 structures on Mars. 



In even the simplest of contemporary microorganisms, the complexity of the 

 mechanisms for energy transduction, metabolism, replication, and translation 

 argues for origins in much simpler apparatus. Vestiges of these primitive systems 

 may still be preserved in the structures and functions of extant life. Although the 

 biochemical and structural characterization of some enzyme systems has been 

 investigated to determine the minimum requirements tor retention of function, 

 more work in this arena is needed, including efforts directed at ribosomal RNAs. 

 Emphasis should be placed on understanding how the complex mechanisms 

 found in organisms today may have developed from the simpler machinery. 

 Studies from this evolutionary perspective hold promise of providing working 

 models for the functional components of a minimal cell toward which research on 

 prebiological chemical systems could be targeted. 



A molecular phylogenv based on homologies in the nucleotide sequences of 

 ribosomal RNA's has traced the ancestry of contemporary life back to three lines ot 

 descent from the primary kingdoms of eubacteria, archaebacteria, and eukaryotes. 



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