The New Story of Humanity's Origins in Africa | Science & Technology |Axisoflogic.com

Several new discoveries suggest that our species didn’t arise from a single point in space. Instead, the entire continent was our cradle.

An ancient human skull from Jebel IrhoudRYAN SOMMA / FLICKR / KATIE MARTIN / THE ATLANTIC

There is a decades-old origin story for our species, in which we descended from a group of hominids who lived somewhere in Africa around 200,000 years ago. Some scientists have placed that origin in East Africa; others championed a southern birthplace. In either case, the narrative always begins in one spot. Those ancestral hominids, probably Homo heidelbergensis, slowly accumulated the characteristic features of our species—the rounded skull, small face, prominent chin, advanced tools, and sophisticated culture. From that early cradle, we then spread throughout Africa, and eventually the world.

But some scientists are now arguing that this textbook narrative is wrong in its simplicity, linearity, and geography. Yes, we evolved from ancestral hominids in Africa, but we did it in a complicated fashion—one that involves the entire continent.

Consider the ancient human fossils from a Moroccan cave called Jebel Irhoud, which were described just last year. These 315,000-year-old bones are the oldest known fossils of Homo sapiens. They not only pushed back the proposed dawn of our species, but they added northwest Africa to the list of possible origin sites. They also had an odd combination of features, combining the flat faces of modern humans with the elongated skulls of ancient species like Homo erectus. From the front, they could have passed for us; from the side, they would have stood out.

Fossils from all over Africa have modern and ancient traits in varied combinations, including the 260,000-year-old Florisbad skull from South Africa; the 195,000-year-old remains from Omo Kibish in Ethiopia; and the 160,000-year-old Herto skull, also from Ethiopia. Some scientists have argued that these remains represent different subspecies of Homo sapiens, or different species altogether.

But perhaps they really were all Homo sapiens, and our species simply used to be far more diverse than we currently are. “If you look at skulls, you’ll see different features of modern humans arising in different locations at different times,” says Eleanor Scerri, an archaeologist at the University of Oxford. And the reason for that, she says, is that “we’re a species with multiple African origins.”

She and others argue that humans originated from several diverse populations that lived across Africa. Separated from each other by geographical barriers, they mostly evolved in isolation, and each group developed some of our hallmark traits, but not others. But their separation wasn’t constant: As a changing climate remodeled the African landscape, greening deserts and drying out forests, those early humans were repeatedly drawn together and pulled apart. Whenever they met, they mated and mingled, exchanging genes and ideas in a continent-wide melting pot that eventually coalesced into the full bingo of features that you or I might recognize.

This theory, known as “African multiregionalism,” is a fundamentally different view of how we came to be. It’s saying that no single place or population gave rise to us. It’s saying that the cradle of humankind was the entirety of Africa.

Scerri recently convened with 22 other anthropologists, archaeologists, geneticists, and climatologists in London to review the evidence for African multiregionalism. Their discussions are described in a paper that is published today, and that Mark Thomas, a co-author, describes as a call to arms. “We’re saying that it’s extremely unlikely that humans evolved in one location and then spread throughout the world,” he says. “Our ancestry will have reached to many, many corners of Africa.”

“It’s a good paper and I definitely agree,” says Louise Leakey, who has long studied hominid fossils in East Africa. “The numerous finds that have emerged from different sites in Africa [suggest] a patchwork of highly structured populations living across the continent.”

This can be a tricky concept to grasp, because we’re so used to thinking about ancestry in terms of trees, whether it’s a family tree that unites members of a clan or an evolutionary tree that charts the relationships between species. Trees have single trunks that splay out into neatly dividing branches. They shift our thoughts toward single origins. Even if humans were widespread throughout Africa 300,000 years ago, surely we must have started somewhere.

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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