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When DNA analysis of ‘Cheddar Man’ uncovered the fact that the first modern Britons were probably black, the news made headlines. But for Professor Mark Thomas, it was just part of a lifetime’s research into the evolutionary genetics that make us what we are.

Words Lucy Jolin Images István Szugyiczky

You may not know the name, but you almost certainly know his work. Earlier this year, Professor Mark Thomas worked on a project studying the transition in Britain from hunter-gathering to farming. This involved Cheddar Man, a 10,000-year-old fossil found in Gough’s Cave in Cheddar Gorge, Somerset. He was a hunter-gatherer, wandering the valleys of the south-west, 4,000 years before farming arrived. Using a combination of modelling and ancient DNA data, Thomas and his team – together with colleagues from the Natural History Museum – concluded that there had been a massive migration of continental farmers into Britain, replacing the hunter-gatherers. Certainly interesting for those in this field, but probably not vastly newsworthy – except for one factor.

As a minor offshoot of the project, the team was able to make some inferences around Cheddar Man’s morphology and pigmentation. A TV company became interested. The Kennis Brothers, who specialise in reconstructions of ancient remains, created a reconstruction of Cheddar Man for the documentary, The First Brit: Secrets of the 10,000 Year Old Man, using the genetic information given to them by the team. Their reconstruction showed a man with bright blue eyes – and dark skin. Every single major newspaper in the UK ran with the story. For some, the interpretation of Cheddar Man’s skin pigmentation was cause for celebration. For others, it was a threat.

“I was absolutely bewildered by how big the news story became,” says Thomas, Professor of Evolutionary Genetics in the research department of Genetics, Evolution and Environment. “And even more bewildered by the reaction from ‘Angry of Tunbridge Wells’ types.

I was getting emails and reading blogs saying that I was part of some liberal elite conspiracy. That I was trying to destroy British identity. To which my response has to be: if your identity is based upon the skin pigmentation of some West Country bloke from 10,000 years ago, you may need to rethink it.

Then there’s his recent contribution to the largest-ever study of ancient DNA, showing that the Beaker people (who created a new style of bell-shaped pottery that spread across western and central Europe around 4,700-4,400 years ago) almost entirely replaced Britain’s Neolithic farmer population.

“The big question now is how could such a massive population replacement happen? Did the incoming Beaker people have better technologies, social organisation or ways of feeding themselves? Or were the early British farmers they mostly replaced already on the slippery slope to population collapse? The sheer scale of population replacement in Britain will surprise many, even though the more we learn from ancient DNA studies, the more we see large-scale migration as the norm in prehistory.”

But challenging established ideas about the past is something Thomas has built a career around, and it’s something he relishes. Originally trained as a molecular biologist – his PhD was in plant molecular biology, and his first postdoc was in molecular medicine – he had always been interested in archaeology, human evolution and linguistics, “the fields that can tell us the most about how humans moved around, and how we came to be what we are today”. He found out about a postdoc position at the University of Cambridge, examining ancient human DNA. “And this was a revelation to me, the idea of combining my day job with my hobby.” Three years later, he came to UCL as a lecturer, intending to continue this trajectory.

Thomas began by studying population histories of particular groups, initially those of Jewish descent and in Africa. He co-authored papers looking at the origins of the priestly caste known as the Kohanim (the anglicised, singular name is the rather more familiar Cohen), and the Lemba, known as the Black Jews of Southern Africa. He began to feel, however, that there had to be better ways of answering his questions than just interpreting patterns of variation in the DNA itself.

“A lot of what we did laid some of the foundations for the ancestry testing industry, and I’ve been very critical of certain aspects of that industry,” he says. “I’d also become very sceptical about many of the methods people were using to make claims about the ancestry of individuals or populations. Telling stories from genetic data is a very popular approach but I think it’s deeply flawed. To do something better than that, I felt that I was going to have to become more of a computer programmer, so that I could actually model the processes rather than just tell stories. Of course, I would also have to learn the stats, but I was lucky to have colleagues like Lounès Chikhi, David Balding, Mike Weale and Ziheng Yang at UCL. I knocked on their doors more than a few time!”

He taught himself computer programming – modelling simulations, statistical analysis and statistical inference – and started to work on new approaches to his most recent interest: natural selection. “And this goes to the very core of what science is: models that we test against empirical data,” he explains. “We don’t just take empirical data and say that it automatically tells us what happens. We always have models in our heads – so let’s make them explicit! When I started seeing things that way, I realised that, actually, we never prove anything in my fields. We just seek the best explanations using statistical approaches. I hung up my pipettes and became primarily a modeller and data analyst, rather than a lab-based person, and this was probably the most important change in my career.”

The starting point for this approach, says Thomas, is not the DNA available, but an idea. “You start with what you think happened and then you simulate that. And then you see if the theory fits what’s out there. That seems to be a scientifically and statistically more robust, explicit, clear and transparent way of trying to infer what happened in the past. Learning those skills was just so empowering. I suddenly realised that if I wanted to know what had happened, I had to come up with ideas about what happened and see if they could explain the data. I couldn’t just look at the data and say that automatically tells you what happened, because the world isn’t like that at all.”

He’s used this approach in numerous high-profile studies that challenge established thinking, such as his work on lactose persistence (our ability to digest the main sugar in milk). “Influenced by my colleague Dallas Swallow, we modelled the spread of a genetic variant known as the -13,910*T allele, which enables people to digest lactose as adults. We inferred that this trait gave our ancestors a massive survival advantage, more than any other that has evolved over the last 7,000 or so years.”

His work has also helped to answer the questions of how farming spread into and throughout Europe. “As with the spread of other cultural features, archaeologists generally favoured the view that farming spread as a package of ideas, rather than by large movements of people. I think that’s now wholeheartedly been rejected,” he says. “With a few notable exceptions, the genetic data now tends to favour farming and other technologies being spread by people migrating and diffusing.” In 2009, he co-authored a paper published in Science with Dr Adam Powell and Professor Stephen Shennan, where they set out the case for population density – rather than increased brainpower – being responsible for the explosion in technology over the last 100,000 years. “Just like today,” he points out, “success is about who you’re connected with, rather than how intelligent you are.”

Modelling has, he says, enabled him to creep into areas that weren’t traditionally his own – like cultural evolution – and have a lot of fun there. He’s currently working on new thinking around dietary changes. “There’s a big industry out there about the Palaeolithic diet, which is basically nonsense, as we don’t know much about what those diets were. We are trying to quantify the nutrients in ancient and modern hunter-gatherer diets, and compare them with modern developed world diets. And rather than making some arm-waving, qualitative statements about the changes, we aim to work out quantitatively what is changing, how we might be adapting and how we, maybe, have failed to adapt to those changes.”

Whichever avenue his work leads him down next, Thomas says he’s looking forward to the challenges to his thinking that will inevitably follow. “If my work undermines outdated notions of things like race, ancestry and origins – these sometimes pernicious and unpleasant views – I am very proud of that,” he says. “But I also find these debates very intriguing among people who are interested in the past, like me. When you work in a very interdisciplinary area, you get all the benefits of learning from people who have deep knowledge about something you don’t know about. That’s great fun. But there are also often fundamental clashes of philosophy, and I enjoy that side of things too. I think it’s important to understand where we come from, as a species. It’s absolutely central to understanding what we are.”


Around 50,000 to 100,000 years ago, humanity suddenly got smarter. We picked up the stones, tusks, bones and antlers that littered our plains, and realised we could use them for all sorts of things: creating beautiful images on our cave walls, making strangely pleasing noises and helping us to sew warm clothes. “It’s when we became the thinking creatures that we pride ourselves to be,” says Professor Mark Thomas.

Sharing skills

But what caused this advance? In his paper, Late Pleistocene demography and the appearance of modern human behaviour, Thomas and his collaborators Adam Powell and Stephen Shennan argue that the technology explosion can be explained not by how smart we are in terms of cognitive capacity, but by how we share and hold on to ideas and skills. In other words, it’s not what we knew, but who we knew: as true 50,000 years ago as it is today.

Other theories to explain this technology explosion have been put forward in the past, Thomas points out: most notably the idea that our brains suddenly got bigger.

“But that’s just not true. If anything, our brains have got slightly smaller over that particular timeframe. Our brains actually started getting bigger two and a half million years ago. Over that period, when our brains almost tripled in size, our technologies barely changed. Our brain size has been static over this period where, nevertheless, we have a massive increase in technology and art.”

Mark Thomas is Professor of Evolutionary Genetics in the Research Department of Genetics, Evolution and Environment.

Thomas, Powell and Shennan theorised that the technological revolution might have been caused by larger populations, which are more efficient at holding on to skills and technologies than smaller populations. They wrote a computer simulation to test the idea – and it worked. It showed that areas with high population densities and lots of migration maintained more complex skills and technologies than those with low population densities.

Then, they used genetic information to estimate population densities around the world at the time of the technology explosion. “It turned out that where we could estimate the population sizes reliably, they all had the same population densities when these technologies started exploding,” he says. “This suggests is that it is population density and migratory activity, and not brain biology, that drives this explosion. In other words, it’s not how smart you are but how well connected you are and how many connections you have with different people that makes a difference.”

Support systems

Indeed, while plenty of species have culture, humans are the only ones to accumulate culture over time. That means when we move and meet, we share even more. “We know things that were worked out hundreds or thousands of years ago. We didn’t learn them ourselves and we didn’t learn them from somebody who learned them themselves,” he says. “I’m now convinced that it is our support system that is the real story of our success.”

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Portico Issue 5. 2018/19