All modern organisms on Earth share a common mechanism for replication and expression of genetic material. Given the complexity of the genetic mechanism, it seems unlikely that the same construct developed independently in different organisms. Therefore, a reasonable hypothesis is that all modern organisms on Earth are descendants of a single common ancestral organism, and the common ancestor already had the basic genetic mechanism found in modern organisms. A phylogenetic tree that illustrates the evolutionary paths of organisms also shows that all existing organisms originate from a single root that is located between the last common archaeal and bacterial ancestors. Recently published articles on the universal ancestor suggest that it was an anaerobic autotroph dependent on H2 and CO2 from geochemical sources and surrounded by a cell membrane similar to those found in modern bacteria and eukaryotes. In contrast to conflicting conclusions of in silico studies on the environmental temperature of the universal ancestor, reconstruction of ancestral protein sequences and characterization of their properties in vitro suggest that the universal ancestor was a thermophile or hyperthermophile that thrived at a very high temperature. Future research may continue to revise these predictions of features associated with the universal ancestor.
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