I'm basically asking this to Farmerman and Thomas as those guys know everything (it wouldn't even surprise me if one of them had started it) but what I'm really hoping for is an answer from the Karl Pilkington of a2k - Gungasnake.
Just now noticed this thread, sorry for the delay.
Without a time machine the best tool we have as to natural history type conjectures is logic.
Logic indicates that the original creation of the RNA/DNA system which is the basis for all meaningful life had to be the work of a single pair of hands. Logic also dictates that it is highly unlikely that the original creation of DNA/RNA occurred on this particular planet.
Logic also dictates that an all-powerful and well-intentioned God would not create biting flies, mosquitos, ticks, chiggers, and the other creatures of Pandora's box, and that an omniscient God would not need to go through 50+ species of horses or elephants to get to the one he wanted. When you get to a point a few thousand or a few tens of thousands of years ago on this particular planet, what you see looks more like what you'd expect if the engineering and re-engineering of complex life forms had become a sort of a cottage industry with more than one pair of hands involved.
First life on this planet could mean the Cambrian explosion or whatever if anything existed prior to that. In either case such life was either created here or brought here (deliberate act), or simply ended up here via some random cosmic event (splash saltation). In the case of humns and/or Cenozoic mammals my guess would be that "brought here" was most likely.