I'll come right out and say that I do not belive the science of evolution to be in conflict with Catholic doctrine. I do have qualms about various theories, and I most certainly believe that evolution must, if at all true, be a process driven by intelligence (that is, a Divine creator). What might be random in a scientific sense is not necessarily random in a metaphysical sense.
So what I'm trying to get at is how we get from an inorganic universe created in a big bang to a universe occuppied by organic beings. The latter seem to be able to thrive only by consuming other organic beings. In a universe created in a big bang, it would be difficult to claim organic beings at the offset. Therefore, there must be some kind of force present that converts inorganic matter to organic matter. Otherwise, there's no way for the evolutionary process to begin. And if such a force existed in the past, there's no explanation for why it doesn't now seem to be present. If a process were present in the past, we should be able to witness the process or effects of the process in the present in a way that would explain the origin of organisms.
Another issue is with the finite amount of organic material. If we include all carbon-based compounds, we still have to accept that there is a limited amount of such compounds available on this planet. We might be "refreshed" to a degree by some cosmic source, but for the most part, there are limited amounts of carbon-based materials, therefore limited amounts of material by which evolutionatry processes can take place.
So I see two problems with a completely atheistic concept of evolution. One doesn't explain how organisms came to be at all, and the other doesn't explain why we continue to get more and more organic material rather than having stasis.