The story of human origins is complicated since our ancestors swapped genes (and probably skills).
The first humans emerged in Africa around two million years ago, long before the modern humans known as Homo sapiens appeared on the same continent.
There’s a lot anthropologists still don’t know about how different groups of humans interacted and mated with each other over this long stretch of prehistory. Thanks to new archaeological and genealogical research, they’re starting to fill in some of the blanks.
The First Humans
Homo habilis individuals chip away at rocks, sharpening them for cutting up game or scraping hides while a woman, with her child, gathers wild berries to eat and branches to make shelters.
First things first: A “human” is anyone who belongs to the genus Homo (Latin for “man”). Scientists still don’t know exactly when or how the first humans evolved, but they’ve identified a few of the oldest ones.
One of the earliest known humans is Homo habilis, or “handy man,” who lived about 2.4 million to 1.4 million years ago in Eastern and Southern Africa. Others include Homo rudolfensis, who lived in Eastern Africa about 1.9 million to 1.8 million years ago (its name comes from its discovery in East Rudolph, Kenya); and Homo erectus, the “upright man” who ranged from Southern Africa all the way to modern-day China and Indonesia from about 1.89 million to 110,000 years ago.
In addition to these early humans, researchers have found evidence of an unknown “superarchaic” group that separated from other humans in Africa around two million years ago. These superarchaic humans mated with the ancestors of Neanderthals and Denisovans, according to a paper published in Science Advances in February 2020. This marks the earliest known instance of human groups mating with each other—something we know happened a lot more later on.
After the superarchaic humans came the archaic ones: Neanderthals, Denisovans and other human groups that no longer exist.
Archaeologists have known about Neanderthals, or Homo neanderthalensis, since the 19th century, but only discovered Denisovans in 2008 (the group is so new it doesn’t have a scientific name yet). Since then, researchers have discovered Neanderthals and Denisovans not only mated with each other, they also mated with modern humans.
“When the Max Plank Institute [for Evolutionary Anthropology] began getting nuclear DNA sequenced data from Neanderthals, then it became very clear very quickly that modern humans carried some Neanderthal DNA,” says Alan R. Rogers, a professor of anthropology and biology at the University of Utah and lead author of the Science Advances paper. “That was a real turning point… It became widely accepted very quickly after that.”
As a more recently-discovered group, we have far less information on Denisovans than Neanderthals. But archaeologists have found evidence that they lived and mated with Neanderthals in Siberia for around 100,000 years. The most direct evidence of this is the recent discovery of a 13-year-old girl who lived in that cave about 90,000 years ago. DNA analysis revealed that her mother was a Neanderthal and her father was a Denisovan.
The human lineage of Australopithecus afarensis, Homo habilis, Homo erectus, Neanderthals and Homo sapiens.
Scientists are still figuring out when all this inter-group mating took place. Modern humans may have mated with Neanderthals after migrating out of Africa and into Europe and Asia around 70,000 years ago. Apparently, this was no one-night stand—research suggests there were multiple encounters between Neanderthals and modern humans.
Less is known about the Denisovans and their movements, but research suggests modern humans mated with them in Asia and Australia between 50,000 and 15,000 years ago.
Until recently, some researchers assumed people of African descent didn’t have Neanderthal ancestry because their predecessors didn’t leave Africa to meet the Neanderthals in Europe and Asia. But in January 2020, a paper in Cell upended that narrative by reporting that modern populations across Africa also carry a significant amount of Neanderthal DNA. Researchers suggest this could be the result of modern humans migrating back into Africa over the past 20,000 years after mating with Neanderthals in Europe and Asia.
Given these types of discoveries, it may be better to think about human evolution as a “braided stream,” rather than a “classical tree of evolution,” says Andrew C. Sorensen, a postdoctoral researcher in archaeology at Leiden University in the Netherlands. Although the majority of modern humans’ DNA still comes from a group that developed in Africa (Neanderthal and Deniosovan DNA accounts for only a small percentage of our genes), new discoveries about inter-group mating have complicated our view of human evolution.
“It seems like the more DNA evidence that we get—every question that gets answered, five more pop up,” he says. “So it’s a bit of an evolutionary wack-a-mole.”
Early Human Ancestors Shared Skills
Human groups that encountered each other probably swapped more than just genes, too. Neanderthals living in modern-day France roughly 50,000 years ago knew how to start a fire, according to a 2018 Nature paper on which Sorensen was the lead author. Fire-starting is a key skill that different human groups could have passed along to each other—possibly even one that Neanderthals taught to some modern humans.
“These early human groups, they really got around,” Sorensen says. “These people just move around so much that it’s very difficult to tease out these relationships.”