The latest genetic evidence is putting an intriguing twist on current thinking about how our species evolved. While an increasing wealth of data supports a recent African origin, new studies suggest that when Homo sapiens left Africa, rather than simply replacing archaic human species such as Neanderthals in other parts of the world, they interbred with some of them.
For the past few decades, scientists have debated 3 main models to explain the origin of modern humans: the Recent African Origin or Out of Africa model, the Multiregional model, and the Assimilation model. Accumulating fossil, archaeological and genetic evidence meant that, by the beginning of this century, the Recent African Origin model had become the dominant view.
The Recent African Origin model was given a huge boost in 1987, when a paper published in the scientific journal Nature, Mitochondrial DNA and Human Evolution, rocked the palaeoanthropology world. It showed that part of our genome, inherited only through mothers and daughters, derived from an African ancestor about 200,000 years ago. This female ancestor became known as Mitochondrial Eve.
Although the paper was contested, the results strongly supported the views that the Natural History Museum’s human origins expert Chris Stringer and others had been developing that we had a recent African origin.
In the following decade, more genetic data both from recent human people and Neanderthal fossils were collected supporting the Recent African Origin model. The idea gained momentum and with it the view that when modern humans began to leave Africa around 60,000 years ago they largely or entirely replaced other archaic human species outside the continent.
The Multiregional model, by contrast, put forward parallel lines of evolution in each inhabited region of Africa, Europe, Asia and Australasia, glued together by interbreeding across the human range.
Modern human (Homo sapiens) skull reconstruction, showing the distinctive chin and high forehead.
Under this model, there was no real ‘origin’ for the modern form of Homo sapiens. A feature like a chin might have evolved in a region such as Africa and spread through interbreeding, followed by selection if it was an advantageous characteristic. Another feature, like our high forehead, might have evolved elsewhere and then spread through interbreeding.
Another group of scientists embraced a third theory – the Assimilation model. Like the recent African origin model, this gave Africa a key role as the place where modern human features evolved, but it imagined a much more gradual spread of those features. Under this view, Neanderthals and archaic people like them were assimilated through widespread interbreeding. This meant that the establishment of modern human features occurred via a blending of populations rather than a rapid replacement.
In recent years, enormous advances in techniques for the recovery and analysis of ancient DNA have unlocked new secrets about our human evolutionary family tree. Two studies in particular have had a dramatic impact on our thinking about where our species evolved.
In 2010, about 60 per cent of the entire genetic code of several Neanderthal fossils was revealed for the first time and led to surprising insights into the evolution of our own species. When the Neanderthal genome was compared with those of modern humans from different continents, it showed that modern populations from Europe, Asia and New Guinea shared more genetic information with Neanderthals than present-day Africans do, with around 2.5 per cent Neanderthal DNA in their genetic make-up.
Now that more of the Neanderthal genome has been sequenced, we know that this figure is more like two per cent.
The most likely explanation for this shared DNA is that a small number of Neanderthals interbred with the ancestors of today’s Europeans, Asians and New Guineans soon after they left Africa around 60,000 years ago.
DNA recovered from this ancient molar tooth found in Denisova Cave, Siberia, revealed a connection to some present-day human populations. © MPI-EVA, Leipzig
In the same year, a fossil finger and molar tooth found in Denisova Cave, Siberia, yielded some remarkable findings. Genomic data revealed that they belong to a previously unrecognised Asian offshoot of the Neanderthal line. However, the data also showed something that was even more startling. It revealed that present-day Melanesians in southeast Asia are related to the Denisovans, as they have become known, sharing about 5 per cent of their genetic code, and this finding has now been extended to native Australians. This provides further evidence of interbreeding.
The Neanderthal and Denisovan genetic studies have given our understanding of our ancient past an exciting twist. Both indicate that modern humans did not completely replace other human species, as had once been suggested. Instead there was some interbreeding. This model has become known as replacement-hybridisation, ‘leaky replacement’, or ‘mostly out of Africa’.