Let's say that life arose independently on three separate occasions, and each of these groups evolved as far as a fish completely independently of one another, and these three fish populations were by some miracle similar enough genetically to produce fertile offspring, and they mated, and we're the result. Each of the single-celled organisms that eventually gave rise to these fish would still be a common ancestor of humans, in that all of today's humans are descended from it. Whichever of these organisms appeared first would win the crown of "earliest" common ancestor.