New discovery of human ancestor; 3.2 million yrs old

A discovery of what is considered to be a human ancestor dating back 3.2 million years has been released; a good article at National Geagraphic. While this is news, and I haven't seen the data myself, I'm always curious how they know it is a human ancestor and not a cousin. But the discovery is important as it is more of a missing link than the previous find of Lucy since it is much older and shares more traits with tree apes, but is less similar to chimps than was previously thought.

An evolutional map is also shown in NG site fits the new finding into our evolutional history.