A minimal estimate for the gene content of the last universal common ancestor--exobiology from a terrestrial perspective.

Using an algorithm for ancestral state inference of gene content, given a large number of extant genome sequences and a phylogenetic tree, we aim to reconstruct the gene content of the last universal common ancestor (LUCA), a hypothetical life form that presumably was the progenitor of the three domains of life. The method allows for gene loss, previously found to be a major factor in shaping gene content, and thus the estimate of LUCA's gene content appears to be substantially higher than that proposed previously, with a typical number of over 1000 gene families, of which more than 90% are also functionally characterized. More precisely, when only prokaryotes are considered, the number varies between 1006 and 1189 gene families while when eukaryotes are also included, this number increases to between 1344 and 1529 families depending on the underlying phylogenetic tree. Therefore, the common belief that the hypothetical genome of LUCA should resemble those of the smallest extant genomes of obligate parasites is not supported by recent advances in computational genomics. Instead, a fairly complex genome similar to those of free-living prokaryotes, with a variety of functional capabilities including metabolic transformation, information processing, membrane/transport proteins and complex regulation, shared between the three domains of life, emerges as the most likely progenitor of life on Earth, with profound repercussions for planetary exploration and exobiology.

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