It’s been a big week for “Cheddar Man” and the science around him. I already talked about the issue blog-wise for my day job. Additionally, Spencer and I did a podcast on the topic (if you haven’t, please subscribe and leave positive reviews and ratings on iTunes and Stitcher; next we’ll post our conversation with Chris Stringer, don’t miss it!).
So at this point I’ll put some other thoughts here that are “big picture.”
Cheddar Man may have been black but probably wasn’t
Much of the media is focused on the predicted pigmentation of Cheddar Man. That is, dark. Back when the La Brana Western Hunter-Gatherer results came in with the same finding, several population genomics people pointed out that it might not be valid to predict their phenotype based on modern training sets.
Here are some thoughts:
- Cheddar Man and the WHG in general were probably darker than modern Northern Europeans. There is detectable selection in modern Europeans for pigmentation alleles down to the present, and Northern Europeans are the palest people in the world. And, pigmentation is polygenic, but it’s not hyperpolygenic. That’s why GWAS and early selection tests picked up pigmentation loci as hits so often.
- Cheddar Man and the WHG in general were probably not as dark as tropical people. The only people who live(d) at very high latitudes who were very darkly complected were Tasmanian Aboriginals and Australian Aboriginals (Melbourne is at the same latitude south as Lisbon is north). In contrast, we see that Khoisan are brown, sometimes rather lightly so, while the peoples of non-European heritage who live in high latitudes are not dark-skinned, though they are not as light-skinned as Europeans.
We don’t have a time machine, so we won’t know with finality. But, it seems that pigmentation pathways are finite, and eventually we can probably be more confident if Cheddar Man had a genetic architecture that would lead to fewer and smaller melanocytes.
The First Farmers replaced WHG to a great extent in Britain
The preprint that came out with the Cheddar Man documentary really focused mostly on the Neolithic farmers. The data set was large, and it emphasized that the discontinuity between the farmers, who were EEF from Anatolian stock (modern Sardinians are their best proxies), the hunter-gatherers. WHG is genetically homogeneous, so they couldn’t reject the proposition that there was no admixture of British hunter-gatherers into the farmer population Basically, the thesis that Peter Bellwood outlined in First Farmers is well supported by these results. The farmers brought agricullture, and pushed aside or absorbed the hunter-gatherers.
It is notable to me that they found more hunter-gatherer ancestry (possibly) in eastern and northern populations, but not much in farmers from Wales. Additionally, though they couldn’t be definitive about it, the EEF settlers of Britain seem to have more affinities with the Western Mediterranean populations than the Central European ones. This suggests that perhaps the farmers arrived by sea or coast-hugging from the south and west, rather than from the south and east.
Farmers came to Britain later than to the continent. The shift from hunter-gatherer to farming was rapid. One model for why there was lack of admixture is that the farming cultural package was fully adapted to Northern Europe by the time they began settling the island. In contrast, on the mainland farmers were changing a Middle Eastern lifestyle into something that could take root in cold northern climes where there were already local residents.
Sometimes cultural and ecological changes drive rapid expansions of human populations
Today Europe, and much of Western Eurasia, is characterized by isolation by distance dynamics between populations. What you see in the transition from the Mesolithic to the Neolithic, and later with the arrival of metal age populations (Bell Beakers), is that populations can turnover fast, and that rapid expansion and growth can result in homogeneity across huge distances and then sharp continuities across cultural divides. The classical example of this is that hunter-gatherers and farmers in Central Europe did not exchange much in the way of genes for centuries, and their between population variance accounted for ~10% of their pooled variance (this is what you see comparing Han and Europeans). Additionally, WHG and EEF are both relatively homogeneous, at least before the latter began to absorb WHG at different fractions across its range. WHG descends from a late Pleistocene expansion, after the Last Glacial Maximum. Similarly, the EEF expanded rapidly from its Anatolian point of origin.
Britons didin’t become Britons genetically until the Bronze Age
Ten years ago many people thought that Cheddar Man and his people were the ancestors of most of the people who lived in Britain today. At the same time as this preprint came out, the Bell Beaker paper was officially published. We now know that Britain went through two massive demographic transitions in less than 2,000 years, with on the order of a 90% replacement in a few centuries both times.
Why? Was this typical? Those are for a later post….