Abstract
The question of whether or not all life on Earth shares a single common ancestor has been a central problem of evolutionary biology since Darwin1. Although the theory of universal common ancestry (UCA) has gathered a compelling list of circumstantial evidence, as given in ref. 2, there has been no attempt to test statistically the UCA hypothesis among the three domains of life (eubacteria, archaebacteria and eukaryotes) by using molecular sequences.