Well the normal human conception of God and our conceptions of Aliens isn't mutually exclusive. If our universe was a computer simulation (like the Sims) God would be an alien. I don't believe in either, and if God contacted us, I would believe there was a natural explanation for God meaning it would be an alien or from a shadow biosphere or another evolutionary line on Earth, but far more likely would be an alien. I think I understand abiogenesis well enough to say that it's highly unlikely Aliens created life. There are major problems in our understanding of abiogenesis (like whether metabolism or nucleic acids came first or why aren't amino acids racemic), but I know enough about it to believe that it is possible and as I have evidence that it did happen, I am ok with only having ideas and not facts about the details.
I am fairly certain that neither exists (I am very certain, in fact, I am certain that God doesn't exist, but I'm only slightly confident about intelligent life elsewhere). I become less and less certain about intelligent life the farther away from where we are. I highly doubt that there is intelligent life in our galaxy, and I somewhat doubt there is intelligent life in our observable universe, and I don't know about anything larger than that. Most people seem to believe there are little green men somewhere, but when I think of the path life took to achieve intelligence, it seems like probably not; couple that with the fact that technology is discovered or invented at an exponential rate and we haven't ever seen anyone, it makes me believe that we are alone. I'm pretty sure life exists elsewhere (probably even on Mars or Titian), but I highly doubt that any of it we would call intelligent.
Many of you probably do not believe in human evolution, but that won't stop me from thinking about the consequences of it. Even just going back less than half a million years, at least once our species almost went extinct. In genetics there is a well known bottleneck in human evolution in which there were probably less than two thousand of us remaining. This corresponds well with the Toba catastrophe theory, but that isn't a sure fire thing. Going back five million years ago and less, you see by how we evolved that we are not a major species and were not until very recently. We are scavengers because there was a time in which we starved. Chimpanzees seem much more similar to our last common ancestor than we do, and why is that? Evolution does not happen at a constant rate but rather depending on selection pressure, it goes slower or faster. We evolved such a ridiculous intelligence because there was an absurd selection pressure on our species to change as our environment changed drastically (we got out of the trees). This was coming from an already hyper intelligent species. But even once we had our modern brain, we didn't use it to even a part of it's capacity. There have probably been anatomically pure humans for 250,000 years, yet in the last ten thousand we invented farming at least seven independent times. Even though, we have been using tools since we've been human, it took us 240,000 years to gain the knowledge to farm (a big part of this is that farming is not necessarily good for a populace; it has advantages and disadvantages and until recently the advantages did not outweigh the disadvantages). Technology looks like a fluke within a fluke within a fluke. And that's just within human evolution. I could go on about the evolution of sociality and it's flukish properties and the evolution of lower intelligence and how flukish that was (it's probably because of a meteor or a flood basalt that intelligence has a place amongst major species). I could also talk about how flukish coming on to land is and the necessity of migrating on land for the sort of intelligence that would lead to technology.
And even if life does evolve intelligence, it doesn't seem like it lasts. In the last century we invented the power to destroy ourselves and barely didn't use it. Disease or some natural disaster could have easily destroyed us in the last 50,000 years, and a runaway greenhouse effect, AI, nano technology or a mad biochemical engineer might destroy us yet still (though I doubt any of those will because of the exponential growth of technology). I just don't see intelligent life evolving sustaining itself (as we don't even know if we will sustain ourselves) being common even if life is. But if we do survive, you can bet that we will make life somewhere else and one day arrive in ships and tell them we are God. I think our sense of humor includes that.
