That the last common ancestor of our mitochondria (our common matrilineal ancestor, or
Mitochondrial Eve) is not the last common ancestor of our entire genome; and that therefore while we are all descended on the maternal line from one person, she was most certainly not our only female ancestor living at the time. Better yet, our common patrilineal ancestor (
Y-chromosome Adam) is far more recent than Mitochondrial Eve; and so the talking snake story can be ruled out on genetic grounds (not that it was ever particularly plausible anyway).