What makes us human depends on what place on our evolutionary path we're talking about. If you go back six million years ago, what makes us human is that we were walking upright. That's all. If you go to 2.6 million years ago, it's the fact that we're designing and making stone tools.

This was the most important discovery I had ever made in my life. It was a discovery which has irrevocably changed my whole life's direction. It immediately elevated me to the status of one of the world's leading anthropologists.

In our case, finding a Lucy is unique. No one will ever find another Lucy. You can't order one from a biological supply house. It's a unique discovery, a unique specimen.

But it really wasn't until three to four years later, when we had an opportunity in the lab to make very detailed observations, and comparisons with other fossil discoveries, that we realized she was a new species of human ancestor.