It would seem that the original sub- plot of a monolith arriving to give a tweak to the evolutionary characteristics of what was, as a result, to become humankind was set in more recent pre- history than we see on screen. Our first glimpse of life was intended to show our ancestors well past the "ape- person" stage, and more clearly recognisable as ourselves, far removed.

This, at any rate, represented what Kubrick had decided to base his film on: that farrago of nonsense called "evolution". You might have expected him, and Clarke, to know better, but at least changing his starting- point meant he could search for real actors to play the roles, requiring only comparatively simple make- up. Hence Dave's companion in that strange picture. Here is the same unnamed person photographed outside the MGM studios in Borehamwood, near London, where  2001: A Space Odyssey was filmed, with an additional study shot (it is not clear whether it is the same person after a make- up session, or another subject altogether):
 

Some consideration was given to "above the waist" filming; another idea was to use some kind of concealing material applied to those awkward but necessary features of the human body. Either of these would have made the whole thing look even more ridiculous; hence the decision to move the sequence back in time to a point where clothing and decency would no longer be issues.

Of course, this made the job of make- up far more complicated, but Kubrick hired only the best people, and Stuart Freeborn's costumes, as donned by Daniel Richter's mime actors, were a major factor in creating the most convincing scenes ever filmed of a fanciful distant past.