[My2Cents]

The first human ancestors appeared between five million and seven million years ago

That pins it down....And is that all that distinguishes humans from the rest of the animal world: the fact we walk on two legs?

The answer to the question is easy: Our ancestors became human when God breathed His spirit into them, making them living souls. This is what distinguishes us from the rest of the animal world. Any attempt to find a different definition of "human" always fails.

[aShepard]

Our ancestors became human when God breathed His spirit into them, making them living souls. This is what distinguishes us from the rest of the animal world.

How do we know that my dog, or my rabbit, doesn't have his own God?

It may not be like the human God, but then he can't tell us about his beliefs.