Exobiology 



Elucidating the evolutionary relationships of life on Earth using this approach 

 depends largely on acquiring a broad data base. Toward this end, many more 

 sequence data need to be acquired, especially among eukaryotic organisms and 

 the uncultured organisms of all three kingdoms in hot spring, hydrothermal vent, 

 and planktonic environments for which data are very sparse. Among the 

 eukaryotes, the ultrastructural diversity reflecting phenotypic diversity should be 

 studied in conjunction with the molecular studies. And integrated research on 

 phylogenetic relationships and metabolic pathways of newly discovered organisms 

 from all three kingdoms should be pursued to gain deeper insight into the 

 evolution of metabolism. Thus, coupled molecular and phenotypic studies offer a 

 quantitative means of determining the temporal sequence of early biological 

 evolution, the chronology of which may be possible to establish with data from 

 the geological record. 



The earliest cellular organism must have been far simpler in terms of size and 

 diversity of proteins, number and organization of genes, and biological specificity 

 than any that exists today. Because the root of the universal phylogenetic tree has 

 not been determined, however, the proximity of the universal ancestor of all life to 

 any of the three kingdoms is unknown. Clues to the nature of this universal 

 ancestor should be sought through comparative phylogenetic analyses of families 

 of genes representing essential cellular functions among microorganisms of the 

 three kingdoms. Common characteristics widely distributed in the earliest 

 branching organisms in these kingdoms are expected to have been attributes of 

 the universal ancestor. Identification of these traits should also yield insights into 

 the geochemical and climatic conditions in the environment of the common 

 ancestor. 



Findings for Early Evolution of Life 



• The geological record of biological evolution is lacking for the period spanning 

 the earliest evolution of life on Earth prior to 3.5 billion years ago; it is sparse 

 for the next billion years, and then increasingly accessible through the 

 Precambrian. 



• If the origin of life on Mars was contemporaneous with the rise of living 

 systems on Earth, access to the earliest geological record of Martian sediments 

 may yield the beginnings of a comparative paleontology among planets. 



• Although paleontological and geochemical evidence exists in the geological rec- 

 ord to relate the occurrence of biological radiations and innovations to 

 environmental changes due to the physical evolution of the planet, these 

 models need to be tested and refined against a broader data base. 



• Remarkable progress is being made in establishing a universal phylogeny for 

 life on Earth, and more can be expected as the extensive phylogenetic history 

 preserved in organisms continues to be deciphered by means of molecular 

 sequencing studies of their nucleic acids and phenotypic studies of their 

 structures and functions. 



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