Who are we, and where did we come from? Scientists studying the origin of modern humans, Homo sapiens, keep reaching deeper in time to answer those questions.
Their research is advancing on three fronts. Fossils of skulls and bones expose anatomical changes. Genetics reveals the timing and place of the Eve of modern humans. And archaeology turns up ancient artifacts reflecting abstract and creative thought, and a growing self-awareness. Just last month, researchers made the startling announcement that Stone Age paintings in Spanish caves were much older than previously thought, from a time when Neanderthals were still alive.
To help make sense of this cascade of new information, a leading authority on modern human evolution — British paleoanthropologist Chris Stringer — recently sat for an interview in New York. Stringer, an animated man of 64, is an anthropologist at the Natural History Museum in London and a fellow of the Royal Society. A condensed and edited version of our conversation follows. In it and in a new book, he describes a new wrinkle to the hypothesis of a recent African origin of modern Homo sapiens.
First of all, would you explain the title of your new book?
Yes, the title is Lone Survivors: How We Came to Be the Only Humans on Earth. And this comes from the fact that if we went back 100,000 years, which is very recent, geologically speaking, there might have been as many as six different kinds of humans on the earth. All those other kinds have disappeared, and left us as the sole survivors.
When you started doing research in this field, the origin of modern humans was hardly recognised as a topic worthy of study in science. What has changed since then?
It’s been a fantastic time to be involved in the field, and even when I was writing this book in the last two years, I had to regularly go back and rewrite things I thought I’d finished with, because new developments were coming up all the time. In 1970, for some people, there was no single origin of modern humans: we evolved globally, all over the world. There was a view that in the different regions an earlier species, Homo erectus, evolved relatively seamlessly to modern humans. This idea was known as multiregionalism.
The argument went that we remained one species throughout that evolutionary process, because there was interbreeding among the different populations. It meant that the Neanderthals in Europe, for example, would be the ancestors of modern Europeans; Homo erectus in China would be the ancestor of modern Asians. And Java Man would be a distant ancestor of modern Australian aboriginal populations.
What we have seen since then is a growth in the fossil record, in our ability to date that record and to CT-scan fossils and get minute details out of them. DNA studies have had a huge impact on our field. We now have the genomes of Neanderthals and of these strange people in Siberia called the Denisovans.
What about the African Eve? This established an approximate date for the genetic origin of modern humans, in Africa. As a leading advocate of the recent African origin did you believe this settled the debate?
It’s not been totally resolved, but the Mitochondrial Eve publication of 1987 was a key moment. Up to then, a few of us were arguing for a recent African origin from the fossil and archaeological evidence. But the evidence was pretty skimpy, and the majority opinion was against our view.
When this new genetic technique appeared, it seemed to give clarity to the picture. Here was an independent bit of data, from our mitochondrial DNA, inherited through females, suggesting we originated, all of us, all over the world, from a single ancestral population that lived in Africa maybe 200,000 years ago.
In your book you propose that there was not one place in Africa where modern humans originated.
Earlier, influenced by the mitochondrial DNA data, I felt there was one place in Africa, a sort of garden of Eden, where we evolved, where we changed behaviourally and physically to become modern humans.
But the story is much more complicated. Even the DNA data show that essentially each of our genes has a separate evolutionary history. And so, when you look at the total picture, including the fossil data and archaeological data, there is no single spot in Africa that seems to be the place for our origins genetically.
In my view, different parts of Africa were important at different times, to distinct human species, and this was being controlled by the climate. Populations in different areas would have flourished briefly, developed new ideas, and then maybe could have died out, even — but not before exchanging genes, tools and behavioural strategies. Within the last 100,000 years we start to see the modern pattern behaviourally and physically coalescing from these different regions to become what we call modern humans by about 60,000 years ago.
In your earlier career, you concentrated on Neanderthals. Do you now accept the new evidence of Neanderthal-Homo sapiens interbreeding, which seems to establish that we are more than 2 per cent Neanderthal?
This is one of the remarkable bits of news of the last couple of years. We’ve had the genomes of Neanderthals reconstructed, and yes, indeed, it shows that people outside of Africa have, on average, about 2.5 percent of an input of Neanderthal DNA in them. And, of course, it’s led to a rethinking of our relationship with them; clearly there was viable interbreeding.
We don’t know the circumstances. Maybe a parsimonious view is that there was a single interbreeding period when modern humans came out of Africa. They met some Neanderthals in the Middle East. There was some interbreeding, under circumstances we don’t know yet, and that input of Neanderthal DNA was then transferred as those populations spread to Europe and to China, down to New Guinea, into the Americas; they took that bit of Neanderthal with them.
Who were the Denisovans?
It’s an extraordinary discovery. Two or three years ago I vaguely knew there was an archaeological site in Siberia called Denisova Cave. And then a few teeth, a finger bone have produced a really high-quality genome now that’s posted on the website of the Max Planck Institute for Evolutionary Biology in Leipzig, Germany. The preservation of the DNA is exceptional, and well beyond anything we have from Neanderthals. It seems these Denisovans were related to the Neanderthals, an early branch off the Neanderthal line.
We know a lot about the Denisovans genetically, but physically we know very little. These fossils are so fragmentary. The even more remarkable thing is they are only known from one site in Siberia, and their DNA turns up in people only in really one region today — not in Siberia, or Asia, but down in Australia and New Guinea. That’s extraordinary.
What is the future of human evolution?
That’s a tough one to answer. There’s a lot of data, not my research, mainly geneticists have been working on this, and they’ve showed just how many genetic changes there have been in the last few thousand years in the human genome. We’ve seen, if anything, an acceleration of genetic changes in humans. So, I think human evolution has been going on quite rapidly recently, and it’s going to carry on.
Not everyone agrees. My colleague in London, Steve Jones, has argued essentially that evolution has stopped in humans because we are in control of it. We have medical care. Nearly everyone reaches reproductive age. Everyone has enough food and water. So natural selection has been nullified in humans. I disagree with him because, of course, there are still a lot of people in the world who don’t have the best medical care, who don’t have enough food and water. Think of the impact of AIDS in Africa.
So selection is still operating on many human populations just as much as it ever has done, really. Also, all of us probably have 50 mutations in our DNA compared with our parents. So that’s going on every generation as well. We are still evolving. We will continue to evolve.
Could we ever clone these
The first bit of Neanderthal mitochondrial DNA was recovered in 1997. No one then could have believed that 10 years later we might have most of the genome. And a few years after that, we’d have whole Denisovan and Neanderthal genomes available. So no one would have thought cloning was a possibility. Now, at least theoretically, if someone had enough money, and I’d say stupidity, to do it, you could cut and paste those
Denisovan mutations into a modern human genome, and then implant that into an egg and then grow a Denisovan.
I think it would be completely unethical to do anything like that, but unfortunately someone with enough money, and vanity and arrogance, might attempt it one day.