In any discussion of ideas about how life began, inevitably the conversation will progress into two opposing arguments over whether life was created, or whether it evolved. In the United States in particular, those who say life was created claim that it was created by the God of the Christian Bible. However, modern people are finding it increasingly difficult to accept that the earth really could have been created in seven literal days, so those who believe the Christian God created all life have adapted their argument into a concept referred to as Intelligent Design. Certain Christian groups are making progress introducing Intelligent Design elements into the public school science classroom, claiming that ID is just as scientific as evolution is, if not more so.
There's just one problem: The theory of evolution does not teach how life began. Anyone who's made even a cursory examination of the literature of evolutionary theory knows this. Unfortunately, those who believe evolution tells us all we need to know about the origins of life have overlooked this salient fact. In the end, they appear uneducated about their own pet theory and furthermore seem to have an axe to grind against religion in general, which reduces their credibility in the eyes of religious people. This has probably done at least half as much to derail science education in the United States as anything ID advocates might have done.
But if evolution does not teach how life began, this leaves the door wide open for multiple ideas about what set life in motion. All the theory of evolution teaches us is how life developed. So something or someone would have had to create the first cells that existed in this world, or would have had to set the chemical processes in motion that led to the development of the first cells. One could argue that it could just as easily been a Someone who did that as a something.
What possible purpose could a God have for using evolution to shape the development of life, though? The Bible says all God has to do is speak and things happen as He wishes them to happen. We've grown used to the idea that these things happen instantaneously-after all, God is conceived of in human minds as all-powerful. We like to think that if we had that much power, we could do anything in the blink of an eye-even create a new planet and put life on it.
But if we read Genesis then we understand that for whatever reason, God did not create the Earth instantaneously, if we are to take Genesis literally. The Bible says God is all-powerful, but God took six days to create the world and all the life in it (on the seventh day, He rested). This is a pretty convincing argument in favor of the idea that sometimes God likes to take things more slowly.
James Lovelock's Gaia Hypothesis tells us that the earth, or at least the biosphere (the layer of the planet that is made up of all living things), is a gigantic living system or organism that is self-sustaining. As with a human body, if anything in the biosphere gets out of balance, the rest of the biosphere takes steps to bring it back into balance. For instance, if a species population grows too quickly, it runs out of food, causing it to die back to more sustainable numbers. Or, on a larger scale, when carbon dioxide levels increase in the lower atmosphere, as long as there are enough green plants to absorb it, they restore the atmosphere to its proper oxygen/carbon dioxide balance. These kinds of things are more likely to occur in a living system than among groups of living things which just live together by chance.
Although the Gaia Hypothesis has been used by some religious groups to justify worshiping the Earth as a Goddess (for good or ill), it can also be used to illustrate why God would use evolution. If all life on Earth began with a single cell or a small related group of cells, and if those cells then evolved and specialized into different organisms, those organisms would still be related to one another, if distantly. Because they're related, they support one another's existence; because they're becoming more different from one another, each species can spread out and take over a different area of the world than its relatives do. This would have caused one small group of cells to eventually spread out and take root all over the planet, much like a fertilized egg eventually divides and specializes so much that it grows into a large, complex human body.
What purpose would this serve? As various life forms spread over the planet and gradually changed, the evidence shows that they changed the air and the water even as they changed. Once upon a time the air and water were unfit for supporting the existence of life as we know it today. Over millions of years, life gradually made changes to the sky and sea so that there was more oxygen in the air and less iron in the seawater, and more life forms could live in the ocean and on land. And now, we modern forms of life take advantage of the changes that were made, and at the same time we support one another's existence simply by being alive ourselves. Plants, for instance, provide most of the oxygen we breathe. In turn, we provide them with carbon dioxide so they can make food, and with wastes that decay and give them nutrients for growth. There are thousands of other examples of different ways that species on this planet co-exist; those are two of the most obvious.
So why would God be an evolutionist? Maybe He was setting up our life-support system all those millions of years. It is not something that science will ever be able to prove or disprove unless we find a way to see God directly (don't laugh-two hundred years ago we couldn't see bacteria, but they exist!), but it is as good an explanation as any.