Current organisms have a higher probability of sharing a single code if the common ancestry hypothesis is true than they'd have if the hypothesis of separate ancestry were true. That is, the simpler hypothesis has the higher likelihood in the technical sense of "likelihood" used in statistics.
My Lord, I find thy face apelike and thy form misshapen. Thy beard, moreover, is an offense against decency, resembling more closely the scabrous fir which doth decorate the hinder portion of a mongrel dog than a proper adornement for a human face. Is it possible that thy mother, seized by some wild lechery, did dally at some time past with a randy goat? -Mandorallen