Humans and Neanderthals co-existed for thousands of years. During that time we got busy, interbreeding multiple times. Now, most non-Africans have a little bit of Neanderthal inside them. These Neanderthal genetic variants have influenced everything from our skin colour to our immune system in both positive and negative ways.
Now, researchers have found yet another way these Neanderthal "ghosts" have influenced us. It turns out that the shape of the skull is more Neanderthal-like in people with more Neanderthal genes. The underlying brain also changes along these lines and this variation might be tied into schizophrenia.
Stealing skulls from Neanderthals
Neanderthal skulls are famously different to our own, what with their thick brow ridges and all. But some of the more notable differences between our two species were at the back of the skull. Whilst ours is fairly tall and short (creating a rounded) effect, the Neanderthal skull is much more squat and long. This culminates in a relatively large rear to their skull, complete with an expanded "bun". This back bit of the skull part is where the Neanderthal influence comes in.
Researchers gathered a bunch of MRI data on Europeans whose genome has been sequenced (as these will be the ones with some Neandethal ancestry). They created computer models of the skulls based on this data, which allowed them to conduct a very high-resolution analysis of their skull shape. This could then be correlated with their genome, to see if there was a relationship between skull shape and possessing more Neanderthal genes.
And as I gave away at the start, this relationship existed. Those with more Neanderthal genetic variants had a back of the skull which differed from the average human in the sample. This change was very small but still statistically significant, probably impossible to spot just by looking at someone. All of this fancy computer stuff was needed to find it, which probably explains why it's gone undiscovered in the >100 years we've been studying Neanderthal skulls. So we can give early palaeoanthropologists a bit of a break for not finding it sooner.
Neanderthal brains and schizophrenia
This research didn't only find that the skull was linked to Neanderthal genes. That big grey thing inside the skull also shows some variation linked with the Neanderthals. Again, it took place at the back of the skull. There, the size and complexity of the folds of the brain drifted away from the human norm; with the degree of drift correlated with a number of Neanderthal genes. This shift was also small, like the skull, but still statistically significant.
This is where I think things start to get very interesting. See, the researchers in this study didn't want to gather all of this data on skulls and genetics by themselves. So instead they took it from an existing dataset, which just happened to be investigating the heritability of schizophrenia. Now, the people they used in the study didn't have schizophrenia, but they are close relatives of people who do.
This raises the fascinating possibility that there may be some relationship between the Neanderthals (and the variation in the brain they induce in modern humans) and schizophrenia. Perhaps it stems from our two sets of genes not getting along quite right. Or maybe it was a condition that plagued the Neanderthals. Of course, since this study didn't examine people with the condition I am speculating wildly.
However, when we look at the source of the data another problem becomes apparent. Namely, that it might not be the most representative group for people to examine. They were also all Europeans, with some degree of Neanderthal genetic influence. There was no real "control" here.
Finding the Neanderthal genes responsible
At this point, you might be wondering if any particular Neanderthal genes drove this variation. Well, the researchers did you a solid and found some. It turns out most bits of Neanderthal-inherited genetics showed some relationship with this variation. However, there was one bit of chromosome 10 that had an exceptionally strong correlation with these changes.
This 56 thousand base pair segment included part of the gene GPR26. Now, I know not all of us are super cool and know all the great genes. So for reference, this is linked to the bits of the cell membrane that "interact" with the local environment, such as dealing with hormones and neurotransmitters. Like the link to schizophrenia, you could read a lot into this, but it would be wild speculation. And I've already done that this post, so I'll simply leave you with the tl;dr.
tl;dr
People with more Neanderthal genes have a skull that differs from the human "average" producing a more Neanderthal-like skull, although the change is very small. The brain also varies along these lines, although without a Neanderthal brain to compare it to the significance is debatable. Also it might be linked to schizophrenia.
References
Gregory, M.D., Kippenhan, J.S., Eisenberg, D.P., Kohn, P.D., Dickinson, D., Mattay, V.S., Chen, Q., Weinberger, D.R., Saad, Z.S. and Berman, K.F., 2017. Neanderthal-Derived Genetic Variation Shapes Modern Human Cranium and Brain. Scientific Reports, 7.
