The Ancestors We All Share

Every stranger you pass is your distant cousin — and the math behind that promise is stranger than it sounds. Pick any two people on Earth. Go back far enough and you will find a single person who is the direct ancestor of both of them. Not a metaphor. An actual human being who lived, breathed, had a name no one remembers. Geneticists have worked out roughly when that person lived: somewhere around three thousand years ago. Not the deep past. Not the age of dinosaurs. Three thousand years ago, people were building the Parthenon. The person who is the ancestor of every living human was alive while Homer was composing the Iliad.

But here is the part that quietly unsettles everything. Beneath all our visible differences — the languages, the distances, the ten thousand years of separate histories — human DNA is nearly identical. Eerily so. Two chimpanzees living in the same forest are more genetically different from each other than you are from a stranger born on the other side of the planet. We are one of the least genetically diverse large mammals on Earth. Scientists have a name for why: a bottleneck. At some point in the not-so-distant past, something reduced us — the entire human species — to a terrifyingly small group. A handful of survivors. And every person alive today descended from them.

What nearly erased us is still not entirely agreed upon. The answer, when it comes, will be the darkest chapter in the story of being human.