The Neanderthals were our closest living relatives. At least, they were until they went extinct ~30,000 years ago. But a little piece of them lives on in us because our ancestors interbred with them.
This interspecies hook-up was more important than a one-night stand. We inherited some of the genes we needed to flourish out of Africa from the Neanderthals ( and a few bad habits too).
But it seems like the Neanderthals also felt the same way. New data suggests they kept coming back for more; with there being at least three different periods of Neanderthal/human hanky-panky.
Excavating the Neanderthal genome
We've known for about a decade that our ancestors interbred with the Neanderthals. But when, where, why, and how often have remained a mystery.
Ok, that's a bit of an exaggeration. We actually know a fair bit about this interbreeding. Genetic studies show it happened relatively soon after we left Africa ( perhaps ~60,000 years ago). As a result the first non-Africans all got a healthy dose of Neanderthal genes; which they still have (with 3 - 4% of their DNA coming from Neanderthals). So scientists have been delving deeper, trying to find out more about this early period of inter-species love.
But it turns out palaeoanthropologists were thinking too small. New research has found evidence of two additional times during which humans and Neanderthals interbred.
This discovery was made possible by the 1,000 genome project. Which, as the name suggests, is a database of 1,000 human genomes (because scientists are bad at coming up with creative names for things). This allowed researchers to look at the Neanderthal DNA present in all of these people. This revealed some interesting facts.
First, is that a lot of the Neanderthal genome lives on. Whilst each individual only has 3 - 4% Neanderthal DNA, different groups have a different 3 - 4%. So when you put it all together it adds up to about half of the entire Neanderthal genome continuing to exist.
The second - and perhaps more interesting point I've already spoiled - is that they found two extra periods of Neanderthal/human interbreeding. The way each population had a different bit of Neanderthal genome didn't match the pattern expected if there was only one period we interbred. Instead, it looked like there was at least two extra periods of mating between us.
Our new (incesty) family tree
The graph bellow (labelled C because I'm bad at cropping) shows the new family tree of humans, Neanderthals and Denisovans produced by this research. And there's a lot of crossover.
- The top arrow represents the case of interbreeding we already knew about. This happened just after our species left Africa. Hence why the African branch didn't get any of these genes. But all the non-Africans did. The fact that they all got it confirms it was an early case of interbreeding, before regional populations had emerged outside of Africa. It probably happened just as we left home.
- The second period of interbreeding was something that had been speculated about. It happened after some regional populations had emerged outside of Africa, but we hadn't fully split up into the populations we know and love today. As such it could have happened in Central Eurasia, allowing for all the interbreeding babies to split off in different directions and eventually split into the regional populations.
- The final case of interbreeding was between Neanderthals and the branch which ultimately became East Asian populations.
What makes this new family tree extra interesting is that these events are spread out in time and place. Humans and Neanderthals were having a bit of fun all over the globe. Including areas that seem to be outside the traditional Neanderthal heartland of Europe.
As an interesting aside, they also found some other reasons we don't all have the same segments of Neanderthal DNA. Natural selection had resulted in some bits being stripped out of our genome. It seems that a lot of Neanderthal DNA was harmful so was soon selected against. Different bits were more or less harmful in different places; hence why different populations wound up with a slightly different pattern of Neanderthal DNA.
This is something we already sort of knew; with previous studies finding many "harmful" genes we inherited from the Neanderthals. But this research helps find the genes that were so bad they were killed off. Although they didn't look at this area in much detail, this research could form the basis for a new study that finds some of the really nasty stuff the Neanderthals gave us.
Maybe as punishment for driving them extinct?
Don't forget the Denisovans
Neanderthals get all the attention. And since they were so special to our ancestors, I think that's justified. However, our ancestors also had a fling with another member of the human family.
The Denisovans get their name from Denisova cave in Siberia ( confirming once again scientists are awful at naming things). A finger bone was found there which at first appeared normal; but actually contained the DNA of a whole other species of hominin. As if that wasn't odd enough, they also discovered that the ancient Siberians also interbred with modern humans.
However, the Denisovans had left before we woke up in the morning. Initial surveys revealed that the interbreeding between our two species was a lot more limited than with the Neanderthals. Denisovan genes were only found in Melanesian populations. So it looks like the ancestors of those modern populations had a brief run in with the Denisovans, whilst we avoided it.
This most recent study confirms that picture. Evidence of only period of interbreeding was found; and it only appears to have involved the ancestors of Melanesians and the Denisovans.
But whilst we might not have spent too long with the Denisovans, we still inherited a few interesting traits from them. The researchers found that Melanesian populations had several Denisovan genes linked to blood glucose and metabolism (although they have no idea precisely what they do). Previous research also revealed that Tibetan populations have some Denisovan genes that help them deal with the high altitude.
Based on all this, I'm now in favour bringing the Neanderthals and Denisovans back from extinction. Only to get them interbreeding again to create the perfect hominin hybrid that WILL RULE THE GALAXY.
tl;dr
Humans and Neanderthals interbred. This happened during at least 3 distinct periods as humans spread around the world. Clearly we couldn't keep our hands to ourself.
References
Hackinger, S., Kraaijenbrink, T., Xue, Y., Mezzavilla, M., van Driem, G., Jobling, M.A., de Knijff, P., Tyler-Smith, C. and Ayub, Q., 2016. Wide distribution and altitude correlation of an archaic high-altitude-adaptive EPAS1 haplotype in the Himalayas. Human genetics, pp.1-10.
Meyer, M., Kircher, M., Gansauge, M.T., Li, H., Racimo, F., Mallick, S., Schraiber, J.G., Jay, F., Prüfer, K., de Filippo, C. and Sudmant, P.H., 2012. A high-coverage genome sequence from an archaic Denisovan individual. Science, 338(6104), pp.222-226.
Reich, D., Green, R.E., Kircher, M., Krause, J., Patterson, N., Durand, E.Y., Viola, B., Briggs, A.W., Stenzel, U., Johnson, P.L. and Maricic, T., 2010. Genetic history of an archaic hominin group from Denisova Cave in Siberia. Nature, 468(7327), pp.1053-1060.
Vernot, B., Tucci, S., Kelso, J., Schraiber, J.G., Wolf, A.B., Gittelman, R.M., Dannemann, M., Grote, S., McCoy, R.C., Norton, H. and Scheinfeldt, L.B., 2016. Excavating Neandertal and Denisovan DNA from the genomes of Melanesian individuals. Science, p.aad9416.
