The hypothesis, that the least recent common ancestor of all living human beings lived 2-5 thousand years ago. This was shown by a simulation of human migration and sexual reproduction behaviour in
http://tedlab.mit.edu/~dr/Papers/Rohde-MRCA-two.pdf
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Um, least recent means longest ago. I propose that the longest-ago common ancestor of all ManKind is also the longest-ago common ancestor of plenty of other living life forms. Far less trivial a problem is the MostRecentCommonAncestorOfMen, and since we can be relatively sure that South American natives and northern Europeans had just about zero contact for tens of thousands of years, then that Human must have been fairly old unless there's been a stupendous amount of homogenizing that I wasn't aware of.
--IanKjos