by Matthew Cobb
[EDIT: IN JANUARY 2016, KEY POINTS OF THE STUDY DESCRIBED BELOW WERE DISCOVERED TO BE UNTRUE. Read this to see why.]
As regular readers will know, some of the most astonishing discoveries in the whole of science that have occurred over the last few years have been with regard to our understanding of recent human evolution.
In the last five years we have not only sequenced the genome of an extinct form of human, generally known as Neanderthal man, we have also used genomics to prove the existence of another human population, called the Denisovans after the name of the cave in Siberia where one tooth and a little girl’s finger bone – our only physical traces of this type of human – were found.
Studies of these genomes, using the kind of analysis we talked about yesterday, have revealed the amazing fact that our human ancestors mated with both Neanderthals and Denisovans. We know this because we can find traces of the genomes of both these extinct types in modern humans.
For example, it turns out that the gene that helps Tibetans live at high altitude was obtained from the Denisovans! There are only 95 genetic differences between humans and Neanderthals that would produce a difference in an amino acid (the building blocks of proteins), out of the 3 billion genetic bases in our genome.
This exchange of genes between humans and Neanderthals and Denisovans proves that, from a biological point of view, we were all part of the same species, whatever differences there may have been in our morphology (Neanderthals were generally stockier and stronger – their skeletons show many signs of fractures; we know nothing at all about Denisovan morphology beyond that tooth and finger).
Modern reconstructions of Neanderthals tend to look less like a cartoon caveman, and more like this:
These interactions with our extinct cousins may have gone one quite extensively. In spring this year the genome from a human skeleton from 40,000 years ago was analysed, showing that this person – who lived in what is now Romania – had as much Nenaderthal DNA as if he had had a great-great-grandparent who was a Neanderthal…
Up until last week, it was argued that all modern populations outside of Africa contained DNA from the encounters between the humans who left Africa around 60,000 years ago, and then met the Neanderthals in Europe and the Middle East. Those humans who migrated out through Asia towards the far East, Australia, and Oceania, also mated with the Denisovans.
Modern Africans, it was argued, had no Neanderthal DNA, for the simple reason that their African ancestors did not leave the continent, and so did not encounter our Neanderthal cousins. Intriguingly, there are hints that African DNA may contain unique signs of mating with yet another, completely unknown, form of human, in Africa. These DNA sequences are apparently not shared by modern people from outside Africa.
The story has now become even more complicated, following the publication of an article last week studying the DNA from a man who lived in what is now Ethiopia, around 4,500 years ago. This man, who the scientists called Mota, is the first ancient DNA to have been isolated from Africa – most skeletal remains from the continent have been too degraded by bacteria to be useful. So researchers deliberately sought a skeleton in arid mountainous conditions, hoping it would be in a good state to be analysed.
Apart from the technical prowess involved (and the promise of future studies of other, older African skeletons), analysis of Mota’s genome provided a big surprise.
It had long been known from archaeological data that people from Middle East had migrated into Africa around 3,000 years ago, in what is known as ‘back-flow’. So the researchers expected that Mota, who predated this migration, should not have had any genetic connection with Europe. But it turned out that his closest relatives would have come from Sardinia, off the coast of Italy. This suggested that the ‘back-flow’ from West Eurasia into Africa was of far greater duration and extent than archaeologists had previously suspected.
When they compared Mota’s DNA with those of modern African populations, they found that the European sequences he carried were also present deep in the continent, even amongst the Pygmies of the Congo. Even ‘reference’ African genomes, such as those from the Yoruba and Mtubi peoples, which were thought not to have been affected by interbreeding with Europeans, turned out to have around 6% of their DNA from European DNA, like Mota.
This indicates that there was substantial migration into Africa by people from Europe and the Middle East, and that their offspring – and their DNA sequences – mixed deep into the Continent. It should be noted that most of that DNA probably did not code for proteins (only 5% of our DNA does this) but will either have been regulatory DNA that controls gene expression or, more, like, the other 85% of our genome, which apparently does nothing and is ‘junk DNA’.
The final novelty came when the researchers looked at Neanderthal DNA. Mota carried that DNA, just like me, because his ancestors had mated with Neanderthals tens of thousands of years earlier.
And it turned out that some of those Neanderthal sequences could also now be detected in African populations, too. They were very dilute – around 0.5% – but they were clearly there. They do not indicate that there were Neanderthals in Africa, but rather that when the offspring of Mota and others carrying migratory European sequences spread their DNA into Africa, they also spread small amounts of Neanderthal DNA, too.
Those sequences had always been seen, but they had been misinterpreted – because they were present in Neanderthals, Europeans and Africans, researchers had assumed that they were ‘ancestral’, that is, traces of our deep common ancestry in Africa. Now we know that at least some of them were the consequence of distant matings with Neanderthals, carried into Africa by people migrating into the continent.
It is even possible that those rumours of strange DNA sequences in African populations may in fact relate to matings elsewhere in the world, the consequences of which were then carried back into Africa by migrants, leaving their traces in the DNA of today’s populations.
This is a momentous time in the study of human evolution. Sadly, I’m too late in my career to start over, but students should seize the moment and flock into this area. As Wordsworth said with regard to the French Revolution:
Bliss was it in that dawn to be alive, But to be young was very heaven!