Why is there only one common ancestor? Why is there no evidence of other "common ancestors" which were not as successful? Was there no competition to be the "first" common ancestor?
In 2010, based on "the vast array of molecular sequences now available from all domains of life," a formal test of universal common ancestry was published. The formal test favored the existence of a universal common ancestor over a wide class of alternative hypotheses that included horizontal gene transfer. While the formal test overwhelmingly favored the existence of a single LUA, this does not imply that the LUA was ever alone. Instead, it was a member of the early microbial community.
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These lines of chemical evidence, taken into account for the formal statistical test by Theobald (2010), point to a single cell having been the LUA in that, although it was a member of the early microbial community, only its descendents survived beyond the Paleoarchean Era.(LUA = Last Universal Ancestor)
https://en.wikipedia.org/wiki/Last_universal_ancestor?wprov=sfla1
http://www.nature.com/nature/journal/v465/n7295/full/nature09014.html