New studies suggest multiple migrations over the last 120,000 years, but doubts remain.
By Nathan Collins
Anatomical comparison of skulls of
Homo sapiens
(left) and
Homo neanderthalensis
(right) in the Cleveland Museum of Natural History. (Photo: Wikimedia Commons)
Modern humans emerged in Africa something like 200,000 years ago, but they didn’t leave the continent en masse until much later, giving rise to a great mystery: If they stuck around Africa for so long, why on Earth did they pick up and leave? Four new papers published today in the journal Nature suggest that Homo sapiens’ journey out of Africa was, well, more complicated than previously thought.
Of course, Homo’s expansion was already pretty complicated. Neanderthals (Homo neanderthalensis), for instance, lived in Europe about 300,000 years ago, which is roughly 100,000 years before Homo sapiens even existed. It’s also thought that small groups of Homo sapiens ventured into Asia perhaps as early as 120,000 years ago. But according to one prominent theory, that expansion fizzled out, and our ancestors didn’t actually leave Africa in large numbers until about 60,000 or 70,000 years ago.
The four new studies add to a growing debate about that hypothesis, with some suggesting the expansion 120,000 years ago was the first of several successful excursions into the wider world.
The group that left 120,000 years ago lived on in Asia for some time before intermixing with more recent arrivals.
Three of the projects aimed to expand scientists’ knowledge of modern human genomes, which can be used to construct a kind of giant family tree, including rough dates at which one subpopulation of humans diverged from another.
First, Luca Pagani and about eight dozen colleagues from the United States, Europe, and Asia analyzed 483 genomes covering 148 distinct populations from around the world. Among their more intriguing conclusions: While the genomes of modern inhabitants of Papua New Guinea and the surrounding islands mostly reflect a relatively recent expansion out of Africa, about 2 percent of that genome stems from a much earlier migration — suggesting the group that left 120,000 years ago lived on in Asia for some time before intermixing with more recent arrivals.
Although they disagree with the conclusion about Papuans, Swapan Mallick and several dozen more researchers use a diverse sample of 300 genomes from 142 populations to reach a conceptually similar conclusion: Non-Africans, they argue, separated from some African populations as early as 130,000 years ago.
A third study, however, suggests those conclusions may be the result of mixing with other early human populations. In a paper mostly concerned with the people of Australia, Anna-Sapfo Malaspinas and her colleagues argue that the genomes of Papuans and early Australians reflect intermixing with Denisovans, rather than signs of an early human migration out of Africa.
Assuming for the moment that there was an early, successful migration out of Africa 120,000 years ago, why did humans leave? Alex Timmerman and Tobias Friedrichsuggest the answer lies in climate change — not the human-caused variety, but rather change induced by 21,000-year-long wobbles in the Earth’s axis. Those wobbles mean that, from time to time, northern Africa and the Arabian peninsula would get a bit less sun and a bit more rainfall, creating greener, wetter landscapes — and a series of windows for human expansion off the continent, with the hint that the first migrations may have begun perhaps 100,000 or 125,000 years ago.
