STUDYING recent human evolution can be frustrating. Take lactose tolerance—the ability of some people, particularly of European descent, to digest this sugar, found in milk, even into adulthood. In the evolutionary past the gene for lactase, the enzyme that does the digesting, normally got switched off in adults. But one variant (known as an allele) of the gene that does the switching off instead leaves the system running, conferring adult lactose tolerance. That lets this allele’s bearers consume a diet rich in dairy products, a useful trick for a species beginning to domesticate milk-producing animals. Genetic analysis shows that lactose tolerance emerged between 5,000 and 10,000 years ago. In evolutionary terms, it is thus extremely recent. In human terms, though, that timescale means how it spread and how this process intertwined with the rise of dairy farming are lost in the mists of prehistory.

It would be nice, therefore, to have a technique that could look at human evolution on a scale of centuries, rather than millennia. And on May 7th a team led by Jonathan Pritchard, a biologist at Stanford University, announced they had come up with one. In a paper posted to bioRxiv, they describe it, and also how they have used it to track ways the inhabitants of Britain have altered over the past 2,000 years.

The team’s technique looks for changes not in alleles themselves, but in the DNA that surrounds those alleles. If a particular allele is more beneficial than other variants of a gene, it will tend, as lactose tolerance did, to spread through the population. As it does so, it will carry with it neighbouring DNA which is not strictly part of the gene and does not affect its function. This DNA can thus mutate without damaging the allele. And it is the amount of mutation this peripheral DNA has undergone which is the giveaway. DNA neighbouring an allele that has recently spread quickly will have had less time to accumulate mutations than that near one which evolution has been ignoring. By looking for evidence of mutations around particular alleles, Dr Pritchard and his team can reconstruct their history. Apply the method to lots of people, and it is possible to discern what evolution has been up to.

To test their method, the researchers turned to 3,195 genomes taken from the UK10K project, which has collected the genetic data of 10,000 Britons. The largest signal they found did indeed come from the allele for lactose tolerance, suggesting that evolution has continued to press the case for milk digestion well into recorded human history. They also found evidence of strong selection for blond hair and blue eyes. This, they speculate, may be a consequence of the sexual preferences of ancient Britons rather than any environmental influence.

Lactose tolerance and hair- and eye-colour are traits strongly influenced by small numbers of genes. But Dr Pritchard’s method can also detect “polygenic” selection, in which the accumulated effects of thousands of small variants conspire to produce a big change in a particular trait. One example is height, which is influenced by the combined actions of thousands of different genes.

Dr Pritchard and his colleagues found that in Britain evolution has, of late, been extremely keen on taller people. Selection for increased height appears to have taken place across nearly the entire human genome. They also found evidence that other ancient evolutionary pressures still operate in comparatively modern populations. Human babies have big heads to contain their outsized brains. Women, in turn, need wide hips to give safe birth to their big-headed offspring. Genetic variations associated with head circumference in infants, and wider hips in women, seem to have become noticeably more common even over the past couple of millennia.

Dr Pritchard’s technique thus adds evolutionary biology to the armoury of archaeological anthropology. His results confirm that modern Britons are subtly but definitely different from those who tried (unsuccessfully) to fight off the Romans. The effects of two thousand years of evolution, covering a hundred or so generations, are such that if ancient Britons were given all the benefits of a modern diet and modern medicine, they would still end up shorter than their modern counterparts, have narrower hips, and give birth to babies with slightly smaller heads.