A recent study((1)) of sequence data from many different proteins has
suggested that contemporary prokaryotes and eukaryotes may have shared
a common ancestor as recently as 2 billion years ago (the molecular c
lock). Strong evidence from the geological record, however, indicates
that oxygen-producing microorganisms, perhaps similar to modern cyanob
acteria, existed 3.5 billion years ago. The fossil evidence, therefore
, suggests that any common ancestor of prokaryotes and eukaryotes must
have existed at least 1.5 billion years earlier than suggested by the
molecular clock evidence. The discrepancy between molecular and geolo
gical evidence for the age of modern cells is considered here, as are
aspects of gene descent in the tree of life that might help to account
for it.