On a sunny day (Sat, 12 Dec 2009 16:55:27 -0500) it happened Yousuf Khan
wrote in :

Two separate studies are claiming that Earth's two most prominent
surface fluids, air and water, came not from within but from outer
space. I have my doubts about these studies' conclusions, it seems like
a stretch that there could be enough air and water brought from meteors
that are being randomly hurled toward the inner solar system, especially
when the vast majority of them are likely heading towards the Sun.

Yousuf Khan

It is obvious in a way, and I think this is correct.
It could also mean earth gained volume long ago, gained a lot of mass,
and the heavier creatures could not handle the gravity and died.
My speculation :-)
That could have happened much later, impacts of one or more a huge ice balls.