Not so, according to the African-multiregionalism advocates. They’re arguing that Homo sapiens emerged from an ancestral hominid that was itself widespread through Africa, and had already separated into lots of isolated populations. We evolved within these groups, which occasionally mated with each other, and perhaps with other contemporaneous hominids like Homo naledi.

The best metaphor for this isn’t a tree. It’s a braided river—a group of streams that are all part of the same system, but that weave into and out of each other.

These streams eventually merge into the same big channel, but it takes time—hundreds of thousands of years. For most of our history, any one group of Homo sapiens had just some of the full constellation of features that we use to define ourselves. “People back then looked more different to each other than any populations do today," says Scerri, “and it’s very hard to answer what an early Homo sapiens looked like. But there was then a continent-wide trend to the modern human form.” Indeed, the first people who had the complete set probably appeared between 40,000 and 100,000 years ago.

Our behavior likely evolved in the same patchwork way. For a few million years, hominids made the same style of large stone handaxes from one millennium to the next. But that technological stagnation ended around 300,000 years ago—the same age as the earliest Homo sapiens fossils. From that time period, archeologists have recovered new kinds of specialized and sophisticated stone tools, like awls and spear tips.

These tools of the so-called Middle Stone Age show that the modern human mind developed at roughly the same time as the modern human body. And they hint that this transition happened at a continental scale, for such tools have been found at Jebel Irhoud in Morocco, at Olorgesaillie in Kenya, and at Florisbad in South Africa, with regional differences at each site.

There’s one large potential problem with the African multiregionalism story. Genetic studies of today’s African populations suggest that they diverged from one another between 100,000 and 150,000 years ago—far later than the early, continent-wide origin suggested by the bones and tools. That deep and broad origin might be right, “but, it’s not something that we geneticists have formally tested,” says Brenna Henn from UC Davis, who is an author on the new paper. “We have discussed ways of doing that, but there’s no published paper yet saying that there is deep population structure in Africa.”

But the DNA of today’s Africans has been shaped by more recent population upheavals that have obscured the goings-on of 300,000 years ago. What’s more, the studies that analyzed this modern DNA have largely relied on tree-like population models in which a single lineage grows from a single place—exactly the scenario that proponents of African multiregionalism say is wrong. “In science, we use simple models for good reasons, because often we don’t have sufficient data to inform more complex models,” says Thomas, who is a geneticist himself. “But there’s a difference between using simple models and believing in them.”

“We’re just at the beginning of trying to figure out how to refine this new theory,” says Scerri. “To know more about what happened, we need to get more data from many of the gaps in Africa. The earliest Homo sapiens fossils we have come from 10 percent of Africa, and we’re extrapolating to 90 percent of the continent. Most of it remains unexplored. We’re effectively saying those places aren’t worth looking at because we have the answer from 10 percent. How can we possibly know that?”

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