Steve Olson, Freelance Writer
Seattle, Washington • USA
If ancestors are considered in conventional terms—as a person’s parents, grandparents, great-grandparents, and so on—who was the most recent common ancestor (MCRA) of all living humans? In other words, if everyone knew exactly who their direct ancestors were and could draw a tree of those ancestors extending indefinitely into the past, who is the first person going back in time who is a direct ancestor of everyone living today?
It’s a hard question to answer. It depends on the detailed migration, mating, and survival patterns of humans over hundreds and thousands of years. But in 1999 a professor of statistics at Yale, Joe Chang, published a paper proving that the number of generations back to the MRCA in a randomly mating population is surprisingly small. (Here’s how small, for those who remember a little high school mathematics. Joe Chang proved that the number of generations back to the most recent common ancestor in a randomly mating population is very closely approximated by the base two logarithm of the size of the population. So, for a population of a million people, it takes 20 generations to reach the MCRA—or about 500 years—because 2 to the 20th power is about a million. For a randomly mating population of a billion people, it takes 30 generations—750 years—because 2 to the 30th power is a billion.)
The problem is that human populations don’t mate randomly. Random mating means that anyone has an equal chance of marrying and having children with anyone else in the world. But people are much more likely to have children with people they know, who live near them, who speak the same language, or are from the same social class. The nonrandom nature of human mating is what makes the problem so hard.
The key to the problem lies in what are known as small world networks. A small world network typically consists of clusters of objects (whether points on a paper, networked computers, interacting proteins, or whatever) that share many interconnections. These clusters are in turn linked to other clusters by occasional interconnections that occur more or less at random. The remarkable thing about a small world network is that even when the clusters are loosely connected, the network as a whole can exhibit behavior much like that of a single tightly connected cluster.
It turns out that human mating patterns form excellent small world networks. As a result, the MRCA of the human population did not live that much earlier than the MRCA of a randomly mating population. In the paper “Modelling the Recent Common Ancestry of All Living Humans” Nature 431(2004):562-566, L. T. Rhode, Steve Olson, and Joseph T. Chang prove that conclusion in two independent ways. The first uses the mathematics of small world networks to trace human ancestry through loosely connected networks of randomly mating subgroups. The second uses a computer model to simulate the mating of realistically sized human populations over thousands of years. Both approaches demonstrate that the most recent common ancestor of the entire human population probably lived in the first millennium BC, and possibly in the first millennium AD. Furthermore, just a few thousand years earlier, the entire human population can be divided into two groups: people who are the ancestors of everyone living today, and people who are the ancestors of no one living today.