Genes and archaeology

Current Biology has released a special issue titled “Global genetic history of Homo sapiens”. There is much of interest in this issue, with seven papers, mostly regionally focused in different parts of the world, but one paper by Jonathan Pritchard and colleagues discussing recent adaptive evolution.

The geneticists to varying extents in this volume depend on archaeological observations, but in many cases read the archaeology very selectively. Speaking as someone who takes archaeology seriously, I find this very frustrating. With more genetic data, we need to demand

An editorial by archaeologist Colin Renfrew leads off the special issue (“Archaeogenetics – towards a ‘new synthesis’?”).

Today, we have an abundance of data about the genetic variation of living people that we did not have ten years ago. In addition to our samples from living populations, we are beginning to find a trove of information about ancient people, from DNA extracted directly from skeletal material. But despite the attempts of geneticists and (rather pitifully few) archaeologists, I don’t see a “new synthesis” emerging.

Reading the first paragraph of his editorial, it seems to me that Colin Renfrew agrees:

It seems a timely moment to review human population historyof the five continents as it emerges from recent archaeogenetic studies, as summarised in the reviews of this special issue of Current Biology. Has the new synthesis between genetics, archaeology and linguistics arrived which I, perhaps incautiously, heralded a few years ago [1]? These highly informative reviews document, it seems to me, both achievement and uncertainty: the achievement relates to the remarkably consistent picture which has now emerged about the out-of-Africa emergence of our own species Homo sapiens and the initial peopling of the Earth. The uncertainty involves the application of archaeogenetics to the more recent, Holocene period, when most of the planet was already peopled except much of Oceania and sedentary, farming-based communities emerged. Here, it appears that much of our current understanding still depends on archaeological or, sometimes, linguistic evidence. And, with a few exceptions, the archaeogenetic evidence has not yet been assimilated into a genuine synthesis; but, let us begin with the good news.

I find it a markedly bad sign that Renfrew thinks the best of “archaeogenetics” is the part with the least archaeological evidence. If the genetics doesn’t seem to work where there is abundant archaeology, why should we believe the genetics in cases where the archaeology is poor?

I write that quite seriously, as someone engaged directly with the genetics. It’s too easy to make stuff up. How can you test a hypothesis that seems consistent with genetic data? The obvious approach is to try to falsify the hypothesis with archaeological observations – but sadly, archaeology is often pitifully silent on the subject of demography and gene flow, or there are many scenarios equally consistent with the same archaeological record.

In the Holocene, archaeology has a lot of power to rule out hypotheses about demography and population movement. So this is where I want to see serious attempts to falsify archaeological models using genetics. And that’s what we’re starting to get! The finding from ancient DNA that early European farmers were neither closely related to earlier hunter-gatherers nor to later agriculturalists has been very surprising. It seems to reject the hypothesis that today’s gene distributions come from an initial dispersal of farmers with their Indo-European languages – the European component of the so-called “language-farming hypothesis”.

Why? Well, because a later massive genetic change suggests that the language transition may well have happened a lot later (as suggested by much of the linguistic evidence itself), and the mtDNA haplotypes carried by the early European farmers have no clear relationship to Near Eastern or central Asian populations.

It’s no surprise that Colin Renfrew would find disagreements with this genetic work; he’s the biggest supporter of the “language-farming hypothesis”.

But I think that the current situation is very healthy. Geneticists are testing hypotheses and showing them to be false. At the same time, they’re proposing models that archaeology can easily show to be false. For example, many recent evaluations of adaptive evolution have looked for genetic outliers against a “neutral” population model that involves very small Holocene population size. From the genetic perspective, this small population size assumption is conservative – it means that some genuine cases of adaptive evolution will look less statistically significant. But archaeology can actually inform us about these cases. Any scenario in which the Holocene population was smaller than millions of individuals must be false. In many cases, a less conservative model is in order.

I think there are tremendous opportunities for integrating adaptive evolution remains to be integrated with our understanding of demography. I don’t put a lot of faith in the current storyline about genetics and the earlier part of prehistory. That story will continue to develop as we deepen our understanding of the demographic and adaptive factors that have shaped human genetic variation within the last 50,000 years.

References:

Renfrew C. 2010. Archaeogenetics -- towards a 'New Synthesis'? Curr Biol 20:R162-R165. doi:10.1016/j.cub.2009.11.056