We know we evolved the ability to run because, well, we can run. We have structures which allow us to run because we run. We know we de-evolved the tree-climbing structures because, well, we don’t have them. And other creatures have them. So it must be true that we used to have them, because other creatures have them.
If you have a toe and I have a toe then we must have come from some other one thing with a toe. It is not possible that we both came from Some Other Thing With a Toe because, well, that would require Capital Letters and occam’s razor requires us to write in all small letters if we can get by with it.