As an undergraduate, I had an opportunity to go on a number of archeological digs. So I had experience excavating, digging up remains of ancient Indian villages in the Midwest and in the Southwest.

And what I wanted to do was, I wanted to explore problems and areas where we didn't have answers. In fact, where we didn't even know the right questions to ask.

There have been some friendships lost over this. That's the most difficult for me. I find it very uncomfortable to know that I was at one time close friends with someone, and because of jealousies and misunderstandings and so on, these friendships have dissolved.

When the first fossils began to be found in eastern Africa, in the late 1950s, I thought, what a wonderful marriage this was, biology and anthropology. I was around 16 years old when I made this particular choice of academic pursuit.

Normally, we are happy to find a fragment of jaw, a few isolated teeth, a bit of an arm, a bit of a skull. But to find associated body parts is extremely rare.

The right question to ask from a Darwinian prospective is what was it about bipedalism that was so advantageous? Why did it lead to a - why did that adaptation ultimately lead to a species Homo sapiens that has come to dominate the planet today with six and a half billion people?

I think that many of my ideas are correct, but I'll bet you, before my death other discoveries will be made that will prompt me to alter various ideas I have about human evolution.