Check out this brilliant post by D. Stephen Heersink, whom I understand is himself an atheist-materialist. This is the last paragraph:
Now, it's an enormous "leap of faith" to maintain that whatever this "life" is, that another "Life" must necessarily exist for the lesser "life" to exist. No such entailment follows. But materialists have reached the limit of their explanatory powers when they try to give a materialist reason for why a pulsating body is alive one moment and is moribund the next. No material explanation as yet exists. And, given that "life" is itself a central feature of the universe we know, the inability to give a material explanation for why the alive being differs from the dead one, is not a trivial failure.
Read the whole thing.