In opposition to intelligent design proponents, who claim that our universe was designed specifically for life, "multiversalists" (seriously) maintain that many--perhaps an infinite number of--parallel universes were created in the intense inflationary period following the Big Bang, and life only arose in those universes with physical constants that supported it, including ours. Or, as Gribbin explains it in his book review:
There are two possibilities, often expressed in terms of an analogy with a man who buys a suit that is a perfect fit. Either the suit has been specifically tailored for the client—made to measure—or he has visited a large store with an array of suits in all possible sizes, choosing the one right for him off the peg. The best interpretation of the laws of physics as we understand them is that we live in an off-the-peg universe. A vast array of universes exist in the multiverse, many (perhaps most) of them sterile, but since life forms like us can only exist in universes like ours, it is no surprise that we live in such a universe.
Of course, we know there are 52 parallel universes, and Professor Morrison will explain them soon. I wonder if Messrs Greene and Gribbin will be reading? (And I wonder if either author discusses "hypertime," another interesting conception of parallel worlds.)
(And this post has made me very nostalgic for 52 - just reviewing the covers for this post brought back memories of the weekly anticipation, as opposed to the weekly dread of Countdown or the weekly meh of Trinity. Sigh...)