There must be one before there can be many. What is the difference between human and nonhuman? Can a species produce anything (reproduce) except its on species? Growth and change and improvements within a species can be expected, call that evolution if you like, but an oak tree will never make apples no matter how much its acorns change.
Common ancestry?
If Darwin's ancestors were anything but human, then we have nothing in common. James E. Gambrell