Yet another existential mystery

Although humans greatly outnumber our closest living relatives the great apes, for some reason the genetic diversity of modern humans is much lower, posing a puzzle that only gets more puzzling the further geneticists look into our evolutionary past. Not only is this disparity counterintuitive, it contradicts a basic tenet of population genetics theory, that larger populations should display greater genetic diversity.

Luke Premo, an assistant professor of anthropology, has taken a stab, with coauthor Jean-Jacques Hublin, at exploring the conundrum in the Proceedings of the National Academy of Sciences (January 6, 2009).

Premo is an evolutionary anthropologist who studies Pliocene and Pleistocene hominin behavior and demography. The Pleistocene epoch reaches back two million years, and the Pliocene precedes it. “Hominin” refers to humans and extinct members of the human lineage.

The traditional explanation of our low genetic diversity is that sometime between 30,000 and 130,000 years ago a special event, perhaps a volcanic eruption or epidemic, caused a “bottleneck” in our species’ population size. Although our population has rebounded since then, mutation rates, which are relatively slow, have not kept pace with rapid population growth, leaving us with less genetic diversity today than predicted by our current population size.

Such an explanation makes sense, except for the fact that Pleistocene humans, Neanderthals, and their common ancestors are now also known to display an unexpectedly low genetic diversity, suggesting that the cause has a greater antiquity than previously thought.

Also, says Premo, if a volcanic eruption, the more likely cause, decimated the human population, why did it not similarly affect the Pleistocene ancestors of today’s great apes?

Premo and Hublin explore an alternate hypothesis, that “effective population size of human lineage reached its current level more than 500,000 years ago, before the population ancestral to Neanderthals and modern humans split.”

If that takes you aback, you’re paying attention. It is here that theory and reality momentarily diverge in order to explain population history. “Effective population size” refers to the size of an idealized population that would show the same level of diversity as the real-world population of interest.

While at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, Premo and Hublin constructed a computer model to explore their notion that cultural differentiation among spatially defined groups may have played a role in curtailing human genetic diversity.

Culture in this sense is defined as learned information that can lead to variation in behavior. Culture is not exclusive to humans. However, says Premo, the cultural variation among chimpanzee groups—say, the practice of cracking nuts on a rock anvil instead of a log anvil—are unlikely to inhibit individuals from moving between groups and interbreeding.

But with humans, cultural traits, whether they be religion, language, or cuisine, can be more effective in keeping groups reproductively isolated. This is what Premo and Hublin refer to as “culturally mediated migration.” Their computer model accounts for geographic space within which the simulated populations are located. Simulated individuals have simple genomes that are subject, to a small degree, to genetic mutation. They also possess cultural traits that are subject to copying error, or “innovation.”

The model is equipped with an experimental dial of sorts, which Premo and Hublin can vary, thus changing the severity of the cultural difference that mediate migration between the groups.

The result? In populations structured by a high cultural similarity threshold, natural selection could have suppressed genetic diversity “over thousands of generations, even in the absence of bottlenecks or expansions in census population size.”

Ongoing research of the Middle Stone Age archaeological record, says Premo, is likely to provide an empirical test of the provocative conclusion that cultural differentiation may have played an important role in explaining reduced genetic diversity in Pleistocene hominins and us.