Starting around 100,000 years ago—when genetic evidence suggests that Homo sapiens permanently departed Africa—these three forms of ancient human inhabited much of the same territory, interacting and occasionally mating with each other. Today, only Homo sapiens survives, though some modern humans retain Neanderthal or Denisovan DNA.
But this new find suggests that some early humans ranged much of the Earth well before that late date. “This is way earlier than Neanderthals—way, way earlier,” Petraglia told me.
The finding suggests that ancient humans migrated out of Africa many times, though these migrations were not always successful, he argued. “Some populations got all the way over to eastern Asia, but we have to imagine that these were small, sort of hunting-and-gathering populations. And while they may have mated across East Asia, it doesn’t mean they survived for a long period of time. Some populations might have become isolated, and some might have become extinct.”
These migrations may even have predated the worst of the modern Ice Ages. While the Earth endured some large swings in temperature about 2 million years ago, the mighty, continent-churning glaciations of the past million years had yet to parade down from the North Pole.
The Loess Plateau likely alternated between arid steppe and moist grassland every 40,000 years. Tools also appear to grow more scarce in the site during these cold and dry intervals, suggesting that ancient humans could only adapt so much to life out of the tropics. And it’s worth noting that the new study does not claim that hominins have continuously inhabited Asia for the past 2 million years.
Kappelman, the University of Texas professor, took a more robust view of this ancient-human community. He noted that different types of australopithecus—a hominin that briefly shared Africa with Homo habilis for 500,000 years—lived for millions of years without ever appearing to leave Africa. But 2.1 million years ago, as the first fossils from our genus, Homo, appear—scientists suddenly find evidence of an ancient-human presence across much of the Old World.
“What this paper suggests is: Boom! You get this dispersal, all the way across what was then the known Earth. The pieces were being filled in there very early on,” Kappelman told me. “It’s the kind of thing where, if we saw this for some other species, it would be remarkable.”
He could recall only one other mammal that moved into a new territory and immediately dispersed across it: ancient horses. “They originated in North America, then migrated into the Old World about 11 million years ago. And then, boom, it’s like gangbusters. They’re everywhere,” he said.
Ancient humans might have gone through a similar expansion, even before they gained all the traits that define a modern Homo sapiens. We just have less evidence of this explosion: Since ancient humans were carnivores, they never would have been as abundant as horses, which are lower on the food chain.
“The world is overrun with people today,” Kappelman said. “There was a period in time when it wasn’t.” The challenge is trying to piece together how humans went from scarce to ubiquitous. “What was it about their behavior? Because we don’t see australopithecus going out, even though they are around at the same time. We just don’t find other hominins in Eurasia until the rise of Homo.”