There are innumerable possibilities, and among them is the possibility that alien races have periodically visited this planet in ancient times and seeded some of the lifeforms here...both animal and human...well, and perhaps some plant lifeforms too.

But that's another whole discussion. ;-) It would be responded to with howls of outrage and ridicule anyway, by various individuals who have already arbitrarily decided in their own minds that it could NEVER have happened! (despite having no rational basis for such certainty)....so just forget I even mentioned it, and carry on as before, folks.