Your grandma married a chimp! And mine too. Well, not exactly our grandmas, but our great- great- great ... great- grandmas. And the ceremony probably wasn't as formal as what we today call marriage. But they were getting it on! And as recently as 5.4 million years ago. This is what the new analysis of the human genome suggests. I find it really exciting (as it no doubt was for them) to think of these early couplings. I love the way the New York Times puts it: "Hybridization [of humans and chimps] could have speeded adapation to this challenging new environment [the drier woodlands opening up between the forests]... But the males in hybrid populations are often sterile. So the females may have had to mate with males in the chimpanzee lineage in order to produce viable descendants." Here, more accessible if you don't have a NYT account, is the BBC report on the findings.
Portrait of Freud, a great-great-great...great grandpa look-alike, courtesy of the Jane Goodall Institute.