The God-like dieties decided they were a bit too hasty in killing off all the dinosaurs 65 million years ago, and decide to give them another chance for a trial period. They would be given intelligence and become anthromorphic if nessisary.
As for humans, they would be devolved to shrew-like creatures - with one exception.
One randomly selected human would be kept with their memories, devolving at a greatly reduced rate. If before he or she was completely devolved they proved that Humans were capable of redemption, the human race would be restored.
That human is your player character, and is just waking up to certain changes...