Very good essay by Carl Zimmer: http://loom.corante.com/archives/2006/05/17/grandma_manimal.php
Human and Chimp Ancestors Might Have Interbred
“The earliest known ancestors of modern humans might have reproduced with early chimpanzees to create a hybrid species, a new genetic analysis suggests.”
Not had a chance to give this a good read yet, so will comment later…
The timeline of human evolution is long and controversial, with significant gaps. Experts do not agree on many of the start and end points of various species – so this chart involves significant estimates.