ROFL. This would be true if the Earth were one uniform environment. I suggest you open your door, and look outside a bit. You'll notice various things called "rivers", "plains," "trees", "caves", "oceans," etc. And the temperature and weather, amazingly enough, varies depending on what part of the earth you happen to be in.
Thus, it would be implausible for everything to evolve into "one animal" since it wouldn't be adapted for every possible earth environment.