Very good essay by Carl Zimmer:

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…

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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.