Here is a statement I find surprising:
The most recent common ancestor of everyone alive today lived only about 3,400 years ago.
Here is one that is even harder to digest:
About 20% of humans living in Europe a millennium ago – in 1018 AD – have no descendants today. The remaining 80% are the ancestor of every person of European decent alive today.
Neither of these statements ring true, but they are. And this is a wonderful example of a finding that was first demonstrated mathematically, and then confirmed empirically.
Start with the math: you have two parents, four grandparents, eight great-grandparents, and so on. But, this geometric progression isn’t borne back ceaselessly into the past. If it were, by the time we get to Charlemagne’s time (750 AD), your family tree would have 137,438,943,472 individuals on it – more people than have ever been alive.
Rather than branching out forever, our family trees fold back on themselves. The same person (your great-great-great grandfather) might hold that one position several times over. So, our family trees aren’t really trees once you get beyond a few generations, but rather meshes (or maybe some sort of vine?).
One early researcher to demonstrate this was Joseph Chang, a statistician from Yale with an interest in ancestry. He constructed a model and found that, given Europe’s current population size, the lines of ascent of every family tree cross about 600 years ago.
Other researchers have since confirmed this through ever-larger DNA sequencing studies. It turns out we’re all related – and a lot more closely and a lot more recently than most of us would guess. And this leads to a remarkable proposition: