Interesting talk from Adam Rutherford, geneticist.

"A Brief History of Everyone Who Ever Lived."  Briefly, his theme was the misleading nature of family trees. I hope I've got this right - must watch it again!  Everyone has 2 parents, 4 grandparents, 8 great-grandparents and so on. If you extend that back far enough you would have many times more ancestors than lived on the planet. Instead of thinking of a neat tree shape, then, he sees it as an interlocking mesh with an element of inbreeding. Not necessarily incest as such, but distant cousins etc.  In extreme cases, of course, it can be disastrous eg the Hapsburg jaw.