In what should now be regarded as a classic of atheist literature, The Blind Watchmaker, Richard Dawkins famously argued that because theism as a proposed explanation for complexity is self-defeating, it follows that the extremely improbable but at least naturalistic event of abiogenesis followed by eons of cumulative natural selection is the only satisfactory explanation for the origin and diversity of life. As he put it, "To explain the origin of the DNA/protein machine by invoking a supernatural Designer is to explain precisely nothing, for it leaves unexplained the origin of the Designer." In other words, to infer God as best explanation for nature is question begging, and therefore nature must be its own best explanation. This amounts to the old "Who Created God?" objection.
There are many fallacies in this superficially clever argument, beginning with a non sequitur based on the argument from ignorance. Even if theism were somehow shown to be irrational or otherwise epistemically unjustified, that fact would not establish the scientific legitimacy of abiogenesis or any aspect of Darwinism, let alone the truth of atheism. To explain the origin of the DNA/protein machine by refuting a straw man theological explanation is to explain precisely nothing, for it still leaves unexplained the origin of the DNA/protein machine, which happens to be the question at hand and which happens to constitute the theoretical driving mechanism of cumulative natural selection – all of which Dawkins has assured us he has the ability to explain.
Now I don't know of any theist who would presume to explain the origin of "a supernatural designer" – the origin of God – since one of the defining characteristics of God is supernatural, eternal preexistence. Theological concepts like aseity, eternality and transcendence are attributes of God revealed in Scripture long before Charles Darwin, let alone Richard Dawkins, was born, and defended by brilliant philosophers and theologians for centuries. Ed Feser, for example, states as part of a useful primer on the cosmological argument:
So, to ask “What caused God?” really amounts to asking “What caused the thing that cannot in principle have had a cause?”, or “What actualized the potentials in that thing which is pure actuality and thus never had any potentials of any sort needing to be actualized in the first place?”, or “What imparted a sufficient reason for existence to that thing which has its sufficient reason for existence within itself and did not derive it from something else?” And none of these questions makes any sense.
In the meantime, the question before us is the origin of life, not the origin of God. They are separate questions altogether, which is why, ironically enough, Christian theism doesn't beg the question of the origin of life while the blind watchmaker "postulate" does. I could propose that my watch was created by the Seiko Watch Corporation, and a critic could retort that in itself this answer would leave the origin of the Seiko Watch Corporation completely unexplained. Indeed it would, but the fact remains that regardless of whatever its own origins may be, the Seiko Watch Corporation created my watch. To infer that my response here is question-begging is to wrongly read circularity into a valid, causally connected and perfectly true answer to the particular question of my watch's origins. Richard Swinburne calls this odd sort of reasoning the completist fallacy: "Clearly it is a fallacy. For if it were really the case that F could not explain E unless there is an explanation of F, nothing in the universe could be explained…"
God remains the most promising cosmological-explanatory stopping point, not only because God is and always has been theologically so defined, but because nature herself is demonstrably finite, contingent, and shot through with entropy. Much evidence – from the fields of theoretical physics, cosmology and thermodynamics – indicates that the physical universe has not likely existed forever. If we must choose someone or something to honor as the eternally self-existent creator of life on earth (and it appears from Dawkins' own discussion of primordial realities that we must), it only makes sense that we go with God himself rather than an impossibly restricted material substitute.
 Richard Dawkins, The Blind Watchmaker: Why the Evidence of Evolution Reveals a Universe Without Design (New York: Norton, 1996), p. 141.
 Ed Feser, "So you think you understand the cosmological argument?" http://edwardfeser.blogspot.com/2011/07/so-you-think-you-understand.html.
 Richard Swinburne, The Existence of God (New York: Oxford, 2004), p. 76.