Over the past half-century, our understanding of human evolution has changed dramatically. And now, it seems that one of the outstanding debates about Homo sapiens has become a lot more complicated — because both sides may be right.
Photo via Neanderthal Museum, Germany
Since the first discoveries of early human remains in Africa and beyond, anthropologists have been stumped by a mystery that has only gotten weirder over the years. What happened to all the other kinds of humans who lived on Earth after they met up with Homo sapiens tens of thousands of years ago?
Thirty years ago, anthropologists were divided into two camps: Those who believed that humans had come out of Africa and crushed the Neanderthals (adherents of the "out of Africa" theory), and those who believed that Homo sapiens and early humans in Eurasia were so similar that it was accurate to say that humanity evolved both in and out of Africa over a long period of time (aherents of the "multi-regionalism" theory). Both groups turned out to be right and wrong.
The Great "Out of Africa" Debate
Back in the 1960s, we only knew of one other human species who lived on the planet with our ancestors: Neanderthals, or Homo neanderthalensis. We knew that Homo sapiens started leaving Africa in large numbers about 80 thousand years ago, and we also knew that Neanderthals had been living in Europe for hundreds of thousands of years at that time. And, roughly ten thousand years after encountering their first Homo sapiens, it appeared that the last of the Neanderthals died out, perhaps 35 or 40 thousand years ago.
Today, new discoveries have made that Neanderthals vs. Homo sapiens story a lot more complicated. We know that there were at least two other human groups wandering around at the time humans left Africa: the Denisovans in Eurasia and the Hobbits (Homo floresiensis) in Indonesia. As more fossil discoveries are made in China, we're likely to learn more about the humans who lived there for hundreds of thousands of years before Homo sapiens arrived, too. So we know that there were a lot of different humans living in regions outside Africa before Homo sapiens got there.
Still, that doesn't tell us much about what happened when African early humans met those diverse groups. Why aren't there still Denisovans and Hobbits around today? Were those early Eurasians replaced by Africans or did they all evolve into modern humans?
In 1987, it seemed that we'd gotten our answer. Biochemist Rebecca Cann and her colleagues published the results of an exhaustive study of human genetics and evolution. They had examined small pieces of DNA called mitochondrial DNA (mtDNA), which live outside the cell nucleus, and are passed down directly from mothers to their children unchanged. What they discovered was incredible: All humans on Earth could trace their mtDNA back to one woman in Africa, who lived about 200,000 years ago, long after the ancestors of Neanderthals had left Africa. This seemed to settle the "out of Africa" vs. "multi-regional" debate once and for all. If everybody had this mtDNA, then clearly we all came from this one specific group of Homo sapiens who left Africa around 80 thousand years ago.
This compelling piece of evidence complemented a modern version of the out of Africa theory known as the "recent African replacement theory," which is a nice way of saying that African Homo sapiens destroyed other humans as they took over the world. They "replaced" their extinct human rivals. That's why everybody can trace their genetic heritage back to a person the media dubbed Mitochondrial Eve.
Genetic Evidence for Multi-Regionalism
But in the last ten years, we've gotten a whole new round of genetic evidence that has made the multi-regional theory relevant again — and has put Mitochondrial Eve into a much more complicated relationship with her human counterparts in Europe and Asia. Anthropologist Milford Wolpoff, one of the originators of the multi-regional theory, published a paper in the early 1980s where he argued that "modern populations evolved in different geographic areas from already differentiated ancestral groups of archaic Homo sapiens (or Homo erectus)." But he wouldn't have genetic evidence to bolster this idea until a new millennium had begun.
In the early 2000s, scientists completed a massive project to sequence the human genome, a massive chunk of DNA separate from mtDNA, which represents genetic material inherited from both parents. Then, in the mid-2000s, scientists developed techniques for sequencing genomes taken from fossils up to 40 thousand years old. Within the last few years, we've sequenced the genomes of Neanderthals and Denisovans. And guess what? It turns out that people hailing from regions outside Africa have Neanderthal DNA in their human genomes. Many Asian groups have Denisovan DNA.
Suddenly, that whole African replacement theory got a lot weirder. It seems that Homo sapiens didn't replace other humans — instead, we had children with them. Many humans on Earth are the result of hybridization. Maybe instead of replacing other humans, our ancestors assimilated them. These recent genetic studies lend credence to the multi-regional theory because they suggest that the people in Europe and Asia were already humans when Homo sapiens left Africa. They may not have looked exactly like African early humans, but they could interbreed with them. So humans were evolving outside Africa, after all.
Even Chris Stringer, the paleoanthropologist who popularized the African replacement theory, has said recently that the story is much more complicated than he once imagined. And anthropologist Ian Tattersall, another proponent of the theory, told me last year that of course there was some "Pleistocene hanky panky." That was his joking way of acknowledging the fact that humans today include the progeny of African early humans mating with the Eurasian locals.
Assimilation and Hybridization
Today, the African replacement theory and the multi-regional theory have begun to merge with each other.
After Neanderthals began having children with Homo sapiens, Neanderthal genes entered the Homo sapiens gene pool and spread. This suggests that Neanderthals didn't die out, but were slowly absorbed into the larger, African Homo sapiens population. There were so few Neanderthals compared to Homo sapiens, that with each successive generation, the children of Homo sapiens and Neanderthals looked more like Homo sapiens and less like Neanderthals. In the end, all that was left of Neanderthals were those few genes that were passed along in many people's DNA.
It would appear that the African replacement theory and the multi-regional theory are both right. All humans on Earth can trace their ancestry back to that big population of Homo sapiens who came charging out of Africa 80 thousand years ago. But those Homo sapiens weren't the only humans on Earth. And when they met other humans, they had children with them — which is fairly definitive evidence that they were the same species as Eurasian people in other regions.
I'm not saying the situation was friendly or peaceful. Maybe a lot of those hybrid babies were the unwanted products of war. All we know is that many people on Earth today owe their lives to the offspring of early humans from Africa who mated with early humans from Eurasia. Echoing the multi-regional theory, the genetic evidence suggests that humans were evolving elsewhere in the world while Mitochondrial Eve's people were evolving in Africa.
Put another way, Mitochondrial Eve had children with Denisovans and Neanderthals. We are all progeny of the people who came out of Africa. And most of us are progeny of early cultural mixing in multiple regions, too.
Annalee Newitz is the author of the book, Scatter, Adapt and Remember: How Humans Will Survive a Mass Extinction. Follow her on Twitter.