In On the Origin of Species, Charles Darwin used a sketch of a tree of life to help describe his theory of evolution. In this metaphor, the branches of the tree represent the relationships between all living and extinct creatures. Humans, like all living creatures, are on the surface of the tree, and all extinct creatures are within the tree.
In this episode we talk with Dr. Bernard Wood who studies our part of the tree of life. Wood tells us how scientists figure out which fossil creatures are our ancestors and which were just our close relatives.  
Links:
Human Evolution: A Very Short Introduction
Bernard Wood
Sideways Look - Bernard Wood's blog for the Center for the Advanced Study of Human Paleobiology.
The Leakey Foundation
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Adept Word Management