After a 1000yrs of war, humans went extinct. Leaving animals, "without a leash". Without humans, we went into a new ecosystem. During the war, humans had released chemicals into the atmosphere. After they were gone, it lingered there. Though, it was contaminated enough a normal human would be dead just by breathing it in. So . . . what about us animals? Well, thats the werid part. Our ancestors managed to adapt and breath it in properly and still function a normal life. Though it had a side effect. It had managed to change our DNA. . . Leaving us to evolve.