In the first part of a three-part series, noted anthropologist Donald Johanson probes the earliest ancestors of the human species, reaching back more than three million years to a strange ape who walked upright. Johanson takes viewers to the site in Ethiopia where he discovered the fossil remains of this missing link nicknamed "Lucy".

At what point did our distant ancestors become anatomically like us? And, more importantly, when did they begin to act like us? Anthropologist Donald Johanson looks at what it is that makes us human.