Mesolithic and Neolithic, Of Cheddar and Bread

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.

The arrival of farming to Britain was different

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….

A celebration of Cheddar…Man

It’s been a lot of cheddar the past few weeks. Or should I say Cheddar Man, the 9,150 year old Mesolithic subfossil from the area of Cheddar Gorge in England. This individual is important because it’s the oldest remain of such high quality found in Great Britain. And, in the late 1990s, as reported in Bryan Sykes’ Seven Daughters of Eve and elsewhere, the Cheddar Man subfossil was genotyped for mtDNA, the maternal lineage. There were, and are, lots of controversies about the validity of that result due to contamination being common in those early years of ancient genetics.

But today we have Cheddar Man’s whole genome. The preprint is finally out, and I’m digesting. Additionally, there has been a Channel 4 documentary, and a few weeks of media hype all around the implications of Cheddar Man.

This is an exciting time for genetics, history, and heritage. Since Britain is a major center of interest for these topics, it’s not surprising that Cheddar-mania has taken off. To mark this occasion DNA Geeks commissioned a design of Cheddar Man using Prince as a model. That might seem strange, but it probably is appropriate given Cheddar Man’s other-worldly and ambiguous appearance. You can get t-shirts and framed prints.

I’ll probably be posting about the Cheddar Man preprint, which really transcends Cheddar himself, tonight or tomorrow.

The genome of “Cheddar Man” is about to be published

If you are American you have probably heard about “Cheddar Man” in Bryan Sykes’ Seven Daughters of Eve. If you don’t know, Cheddar Man is a Mesolithic individual from prehistoric Britain, dating to 9,150 years before the present. Sykes’ DNA analysis concluded that he was mtDNA haplogroup U5, which is found in ~10% of modern Europeans, and which ancient DNA has found to be overwhelmingly dominant among European hunter-gatherers. But for years there has been controversy as to whether this result was contamination (after all, if it’s found in ~10% of modern Europeans it wouldn’t be surprising if the DNA was contaminated).

Today that is a moot point. On February 18th Channel 4 in the UK will premier a documentary that seems to indicate genomic analysis of Cheddar Man’s remains have been performed, and he turns out to be exactly what we would have expected. That is, he’s a “Western Hunter-Gatherer” (WHG) with affinities to the remains from Belgium, Spain, and Central Europe. These WHG populations were themselves relatively recent arrivals in Pleistocene Europe, with connections to some populations in the Near East, and with unexplored minor genetic admixture from an East Asian population. Their total contribution to the ancestry of modern Europeans varies, with lower fractions in the south of the continent, and the highest in the northeast.

Overall, the consensus seems to be that in Western Europe the genuine descent from indigenous hunter-gatherers passed down through admixture with Neolithic farmers, and then the Corded Ware and Bell Beaker groups, is around ~10%. This is the number that shows up in the press write-ups. But, there are some researchers who contend it is far less than 10%, and that that fraction is misattribution due to early admixture with relatives of these hunter-gatherers as steppe and farmer peoples were expanding.

Phylogenetics aside, one of the major headline aspects of the Cheddar Man is that reconstructions are now of a very dark-skinned and blue-eyed individual. Some of the more sensationalist press is declaring that the “first Britons were black!” As far as the depiction goes, this is literally true. The reconstruction is of a black-skinned individual in the sense we’d describe black-skinned.

But on one level it is entirely expected that this is what Cheddar Man would look like. The hunter-gatherers of Mesolithic Western Europe were genetically homogenous. They seem to derive from a small founder population. And, on the pigmentation loci which make modern Europeans very distinctive vis-a-vis other populations, SLC24A5, SLC45A2 and HERC2-OCA2, they were quite different from anything we’ve encountered before. First, these peoples seem to have had a frequency for the genetic variants strongly implicated in blue eyes in modern Europeans close to what you find in the Baltic region. The overwhelming majority carried the derived variant, perhaps even in regions such as Spain, which today are mostly brown-eyed because of the frequency of the ancestral variant. Second, these European hunter-gatherers tended to lack the genetic variants at SLC24A5 and SLC45A2 correlated with lighter skin, which today in European is found at frequencies of ~100% and 95% to 80% respectively.

The reason that one of the scientists being interviewed stated that there was a “76 percent probability that Cheddar Man had blue eyes” is that they used something like IrisPlex. They put in the genetic variants and popped out a probability. The problem is that the training set here is modern groups, which may have a very different genetic architecture than ancient populations. Recent work on Africans and East Asians indicate that the focus on European populations when it comes to pigmentation genetics has left huge lacunae in our understanding of common variants which affect variation in outcome.

East Asians, for example, lack both the derived variants of SLC24A5 and SLC45A2 common in Europeans but are often quite light-skinned. A deeper analysis of the pigmentation architecture of WHG might lead us to conclude that they were an olive or light brown-skinned people. This is my suspicion because modern Arctic peoples are neither pale white nor dark brown, but of various shades of olive.

As far as blue eyes go, it is reasonable that these individuals had that eye color because that trait seems somewhat less polygenic than skin color. There are darker complected people with light eyes, from the famous “Afghan girl” to the first black American Miss America, Vanessa Williams. The homozygote of the derived HERC-OCA2 variant seems relatively penetrant. From what I recall the literature indicates many people with blue eyes are not homozygotes on this locus for the derived haplotypes, but those who are homozygotes for the derived haplotypes invariably have blue eyes.

Addendum: It isn’t clear in the press pieces, but it looks like they got a high coverage genome sequence out of Cheddar Man. They refer to sequencing, and, they seem to have hit all the major pigmentation loci. This indicates reasonable coverage of the genome.