We Are All Cousins – October 17, 2010

Recent studies in population genetics (see, for example, the Genographic Project) indicate that every human being alive on the planet can trace his or her ancestry back to a band of approximately 100 people living in east Africa approximately 60,000 years ago. From this one group, our species spread west into Africa and north and east into the rest of the world.

We weren’t the only members of genus Homo to migrate around the planet, of course. Waves of erectus, and neanderthalensis preceded us, creating the fossils found in Germany, Java, and elsewhere. But we were the last and most successful: the most adaptable, the best hunters, the most efficient killers.1

If we allow 20 years to be the average length of a human breeding cycle, then 60,000 years is just about 3,000 generations ago. Fathers and mothers begetting sons and daughters across desert and tundra, through the last stages of the most recent Ice Age. Three thousand generations is a large but not an uncountable number.

Today, we all interbreed as one species. And the variation among human populations that we can notice—the genetic and phenotypic artifacts that we call “race”—are largely ornamental: shape of nose or lips, color and texture of hair, amount of skin pigmentation. Almost none are survival factors, except the amount of melanin, which modulates the effects of sunlight, and a few isolated genetic traits like sickle cell, which give protection against local diseases. In every way, we are one big family.

If you doubt this, consider a simple thought experiment.

There are six to seven billion humans alive on the planet today. Each person has two parents, four grandparents, eight great-grandparents, sixteen great-great-grandparents, and so on exponentially. If every person today claimed a unique heritage—“These are my ancestors and no one else’s!”—then by the time you get back to the Elizabethan Age and the flowering of Shakespeare about 400 years ago, you would need a world population of about 53 quadrillion people to satisfy that claim.

Of course, my brother and I have the same unique set of ancestors, and my first cousins and I share at least half of our ancestors. So, taking into account the large families of pre-industrial societies, the actual requirement is probably something more like 20 or 30 billion people alive around the world in the year 1600.

And that’s still absurd. World populations were much smaller than today. There have never been more humans alive and breeding on the planet all at once than there are today.

So, unless I’ve done the math wrong—always a possibility!—then my great-great … um, great … grandfather is probably your g’g’g’g’grandfather as well.

Or, as Robert A. Heinlein put it in Stranger in a Strange Land: “Casual bastardy is far in excess of that ever admitted.”

Hello, Cuz!

1. Please contain your outrage. Really, if there had been better killers out there, they would be holding this discussion, and we would be the stony bones in the shallow graves.