Exobiology 



toward living systems with emphasis on the following areas: 



— The organic synthesis of cellular building blocks in the context of 

 carbon-dioxide-rich atmospheric and hydrothermal environments 



— The organic and inorganic chemical models for metabolic and self- 

 replicating systems compatible with existing constraints on early 

 environmental conditions 



— The nature of interactions between monomers or polymers of nucleotides 

 and amino acids that constitute the physical-chemical basis for the 

 genetic code. 



Early Evolution of Life 



The research goal for the Early Evolution of Life is to understand the relationship 

 between planetary evolution and the evolution of unicellular life from the origin of 

 the universal common ancestor to the emergence of multicelled organisms. Two 

 avenues are available for study of evolutionary history on Earth: 1) the biological 

 record preserved in the metabolic patterns of extant organisms and the sequences 

 of amino acids and nucleotides in their proteins and nucleic acids, respectively; 

 2) the geological record of fossil life and its environment preserved in ancient 

 sedimentary rocks. The discovery and study of fossil organisms in ancient 

 sedimentary rocks returned from Mars could yield unique insights into the 

 evolution of extraterrestrial life. Searches for astronomical evidence of disequilibria 

 hold promise of revealing the distribution of life forms beyond our solar system. 



Although the oldest rocks of Earth are 3.8 billion years old, the existing record of 

 biological evolution begins only at 3.5 billion years, and rocks containing fossils are 

 very sparse until 2.8 billion years ago. To obtain a more complete geological record 

 of life on Earth, the search must continue for additional unmetamorphosed 

 sedimentary rocks older than 3.0 billion years in terrestrial continental deposits. If 

 life arose on Mars over the same period of time as it did on Earth, the planet's 

 relatively low level of geological activity may have permitted more complete 

 preservation of a record of early biological evolution. The return of samples from 

 ancient Martian sedimentary environments would make available a geological rec- 

 ord that could permit the beginnings of a comparative paleontology among 

 planets. 



The earliest sedimentary rocks on Earth containing stromatolitic and microfossil 

 evidence of microbial ecosystems have been found in Western Australia and South 

 Africa (3). The sediments appear to have been deposited in shallow marine 

 hydrothermal environments on the flanks of volcanic island platforms during 

 relatively quiescent periods between cycles of volcanic eruptions. Except for the 

 effects of oxygen in the atmosphere today, this early setting resembles in many 

 respects habitable environments that exist currently or that may have existed on 

 Mars during an early period of active volcanism. The laminar structures of the 

 stromatolites, the morphology of the microfossils, and the carbon isotopic 

 composition of the associated organic matter are consistent with the presence of 

 both heterotrophs and autotrophs, with filamentous, phototactic, and probably 



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