[Cold Heat]

Really, I think the trick was to make a monkey walk erect.

That was a trick indeed.

[Truth666]

You picked the right path. I will correct just one thing with the starting point : not a monkey but the artist's view for any part of the missing link.
So to start with, we have an imaginary human ape walking, like this :

Now it should not be too difficult to find out the right answer ...