Hi Oz,
If we had evolved would we not have eyes in the back of our heads, 4 arms and retracted testicles?
I know you're just kidding, but this is a good question! It recently was covered in a Scientific American article: http://www.scientificamerican.com/article.cfm?id=evolved-eyes-back-of-head
Of course the reason has to do with a phrase Darwin used in his book, Natura non facit saltus (nature makes no jumps). See: http://en.wikipedia.org/wiki/Natura_non_facit_saltus
Here is my understanding. The theory of evolution dictates change happens, not so much as a random chance thing, but through a process of mutation, gene selection and what survives and what dies. In all cases evolution must work with the current state and in no way is purpose driven or planning towards some end. So when certain things locked in and worked all that happens is they get refined and refined. Thus forward facing 3-D bilateral eyes will be selected for in predators, and more off on the side non-3D surround vision in prey, such as rabits. Animals like us, tend to land on the predator side of this, so our eyes are forward facing. But because humans became so social, things like the white portion of our eyes became more visible -- it helps other humans know where our interests are.
Since humans evolved the ability to cover great distance and hunt in social groups, we gave up our 4 arms/hands and went instead with feet. Not too good for tree climbing but very helpful for covering distant without tiring out.
Our testicles are a bit retractable, in as much as regulating the temperature in this are of the body is critical. I've read, although not sure how well established these ideas are... Some experts speculate the changes in human sex organs, eyes, hair texture/color and breasts tend to reflect our upright walking stance. The challenge of attracting a mate face to face is somewhat unique to humans among mammals.
Cheers,
-Randy