from my point of view, the theory of evolution seems to be the most reasonable explanation for the diversity of life on our planet.
however, for the origin of life itself you have to look at the theory of abiogenisis http://en.wikipedia.org/wiki/Abiogenisis
if one claims there must be a god because we are here on this planet, able to experience and observe the natural world and coming to logical conclusions about the universe, i have to remind this person there are millions of galxies, each filled with billions of stars, and a certain percentage of theese are yellow stars like ours, each one theoretically being able to suport a planet much like ours. there is a fairly high chance that somewhere out there is a planet on which you could find similar intelligent creatures descending from birds, squids, rats or hive-state insectoids, asking the same questions, but we are to far away to communicate with them.
i'd like to point out that the development of religion is actually a pretty good case for social evolution, we started out with localized animistic believs, unfit to rule a large body of people because "why should the oak spirit living 5 days of traveltime away be responsible for things hapening in my surroundings? so why do his priests try to push their rules on me?"
as the early high cultures became more than isolated city-states, priests noticed similaritys between the local animistic deities, for instance lightning wielding warrior deities, life giving mother deities often associated with water and so on. they unified theese localized spirits, adapting popular names, thus creating the first basis for larger religios organisations that were able to demonstrate to people that they essentially shared the same beliefs, convincing them to adapt similar rules. essentially, it made societys beyond tribes possible. examples for this would be the priest-kings of egypt and ancient babylon.
later, we saw the rising of the concept of a supreme ruler of deitys, making it easier to justify that one person ruled over a large amount of other persons, because he not only was the highest priest, he was the son of the ruler of the gods. there were exceptions to this, for instance jahwe, which was seen as supreme ruler of the semitic deitys, but not as the father of the king.
even later, the concept of "the one god" was developt in multible locations, for instance atenism in the eighteenth dynasty of egypt, which was discarded after one generation, 500 years thereafter a semitic tribe later called "jews" came to the understanding that jahwe was not only the ruler of the gods, but in fact the only god. this made it easier to distinguish themself from ocupieing foreigners, saving them from cultural assimilation into the babylonian empire (funny fact: mose is actually not a semitic name, it is the old egyptian word for "chield"). another example can be found during the punic wars in which a few philosphers claimed that zeus was the only, omnipotent god and that all other gods were just aspects of him (making it easier to unite the populus of the waring city-states to unite against persia), by the time christianity rose this was a commonly accepted belief in the helenistic area. this is by the way the reason god gets depicted as a lightning wielding old man with a white beard in western societies, the concept merged with the christian god rather quickly.
later, we saw a diversification of the monotheisthic religios system, from the more exclusive monotheism of the jews to the more open christianity, which also addapted the egyptian idea of a conscious afterlife, in distinction to the "sleeping" afterlife known in ancient judaism. theese two factors, being open to people that didnt share the same ancestors as yourself and the rather pleasant afterlife promised by it made it hugely attractive to people so that it became the roman state religion a few hundred years later, making it easier to govern a large, multi-cultural empire through cultural and religios assimilation.
in the 8th century a.d. the concept was improved even further by mohamed who called his new religion "islam", which did not only promised everything that christianity did, but also invented a shortcut to salvation, a thing we came to know as militant djihad (one has to remember that the so called "large djihad" actually is not about killing people, but being a good person within the rules of the qu'ran). essentially, mohamed gave his followers the choice of either being a good person or killing as much non-believers as possible while converting the wifes and children of said non-believers to islam. this is one of the major reasons this faith spread so quickly over a large area (the other being supreme military tactics and a for the time rather progressive society).
as demonstrated, unsuccessfull religions went extinct while more successfull replaced them and spread, much like unseccessfull animals got replaced by more succesfull. all of those were about the power to influence people to abide certain laws and ruling bodies, and that is the reason SOME religios people are rather uncomfortable with the idea of evolution and science in generell, because it essentially robs them the basis of their power to influence people.
p.s. sorry if i necro this thread, i am rather slow at typing :-(
