The first requirement is the existence of laws of physics that guarantee the kind of regularities that can underlie the existence of life. These laws as we know them are based on variational and symmetry principles; we do not know if other kinds of laws could produce complexity. If the laws are in broad terms what we presently take them to be, the following inter alia need to be right, for life of the general kind we know to exist:
The neutron-proton mass differential must be highly constrained. If the neutron mass were just a little less than it is, proton decay could have taken place so that by now no atoms would be left at all.
Electron-proton charge equality is required to prevent massive electrostatic forces overwhelming the weaker electromagnetic forces that govern chemistry.
The strong nuclear force must be strong enough that stable nuclei exist; indeed complex matter exists only if the properties of the nuclear strong force lies in a tightly constrained domain relative to the electromagnetic force.
The chemistry on which the human body depends involves intricate folding and bonding patterns that would be destroyed if the fine structure constant (which controls the nature of chemical bonding) were a little bit different.
The number D of large spatial dimensions must be just 3 for complexity to exist.
It should not be too surprising that we find ourselves in a universe whose laws of physics are conducive to the existence of semi-intelligent life. After all, we are here. What we do not know—and will probably never know: Is this the only universe that exists? This is an important question, because if there are many universes with different laws of physics, our existence in one of them may be inevitable. If, on the other hand, this is the only universe, then the fantastic claims of the theists, or at least the deists, become more plausible.
You may wonder why I call the human race semi-intelligent. Rest assured, I am not being sarcastic or sardonic. I say “semi-intelligent” to call attention to humanity’s remarkable technological and scientific achievements while also noting our incredible ineptness at eradicating war, violence, greed, and poverty from the world. What is wrong with us?
References G.F.R. Ellis, Issues in the Philosophy of Cosmology, Philosophy of Physics (Handbook of the Philosophy of Science), Ed. J. Butterfield and J. Earman (Elsevier, 2006), 1183-1285. [http://arxiv.org/abs/astro-ph/0602280]