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.