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The old argument was that although the beginning of life may be highly improbable, given an infinite amount of time anything, no matter how unlikely, can occur. However, since most cosmologists are rather stingy with time these days when estimating the age of the universe, this sort of argument just won't do.

Here's Alvin Plantinga shredding of Daniel Dennett's use of an "infinite universe" hypothesis (Dennet was here attacking a version of the theistic Argument from Design):

Well, why is this [belief in an anthropomorphic God] childish? Dennett's answer, as far as I can make it out, is that the traditional arguments for the existence of God don't work. He mentions only one argument, the so-called argument from design: the universe and many of its parts give the appearance of having been designed by an extraordinarily knowledgeable and powerful designer, so probably there is an Intelligent Designer. Dennett thinks Darwinian considerations suffice to dispose of this argument; they show how it could be that all this apparent design in the living world arises without the aid of an intelligent Designer. Nowadays, however, the most popular version of the argument from design involves the exquisite fine tuning of the laws or regularities of nature. The fundamental constants of physics--the speed of light, the gravitational constant, the strength of the weak and strong nuclear forces--must apparently have values that fall within an extremely narrow range for life to be so much as possible. If these values had been even minutely different (if, for example, the gravitational constant had been different in even the most minuscule degree) habitable planets would not have developed and life (at least life at all like ours) would not have been possible. This suggests or makes plausible the thought that the world was designed or created by a Designer who intended the existence of living creatures and eventually rational, intelligent, morally significant creatures. Like its 17th and 18th century predecessors, this version of the argument is probabilistic rather than deductive: given the nature of the world, it is likely that it was fashioned by an intelligent Designer. The premises don't entail the conclusion, but are supposed to give you some reason to accept it.

Dennett's rejoinder to the argument is that possibly, "there has been an evolution of worlds (in the sense of whole universes) and the world we find ourselves in is simply one among countless others that have existed throughout all eternity." And given infinitely many universes, Dennett thinks, all the possible distributions of values over the cosmological constants would have been tried out; [ 7 ] as it happens, we find ourselves in one of those universes where the constants are such as to allow for the development of intelligent life (where else?).

Well, perhaps all this is logically possible (and then again perhaps not). As a response to a probabilistic argument, however, it's pretty anemic. How would this kind of reply play in Tombstone, or Dodge City? "Waal, shore, Tex, I know it's a leetle mite suspicious that every time I deal I git four aces and a wild card, but have you considered the following? Possibly there is an infinite succession of universes, so that for any possible distribution of possible poker hands, there is a universe in which that possibility is realized; we just happen to find ourselves in one where someone like me always deals himself only aces and wild cards without ever cheating. So put up that shootin' arn and set down 'n shet yore yap, ya dumb galoot." Dennett's reply shows at most ('at most', because that story about infinitely many universes is doubtfully coherent) what was never in question: that the premises of this argument from apparent design do not entail its conclusion. But of course that was conceded from the beginning: it is presented as a probabilistic argument, not one that is deductive valid. Furthermore, since an argument can be good even if it is not deductively valid, you can't refute it just by pointing out that it isn't deductively valid. You might as well reject the argument for evolution by pointing out that the evidence for evolution doesn't entail that it ever took place, but only makes that fact likely. You might as well reject the evidence for the earth's being round by pointing out that there are possible worlds in which we have all the evidence we do have for the earth's being round, but in fact the earth is flat. Whatever the worth of this argument from design, Dennett really fails to address it.

http://www.veritas-ucsb.org/library/plantinga/Dennett.html

The whole article by Plantinga is worth reading.