Humans started domesticating animals and using their milk around 10,000 years ago, but at that time all humans lacked a genetic variant that allowed them to continue digesting the main sugar in milk – lactose – after they’d been weaned from breast feeding. The enzyme that digests lactose is called lactase, and the genetic variation is referred to as lactase persistence (LP). The gene for LP has rapidly increased in prevalence in some populations over the last 10K years, and in many counties especially in European and with European origin population the vast majority of the population have LP.
It was thought this genetic variant rose rapidly because it allowed people to drink milk, who would otherwise not do so due to symptoms of what is referred to as lactose intolerance – a set of symptoms including cramps, diarrhoea and flatulence following drinking too much milk. LP was thought to provide advantages through being able to consume substantial amounts of milk, which contains useful calories, minerals (including calcium) and many beneficial micronutrients. These advantages led to individuals carrying the LP variant to pass on their genes (including LP) to more surviving offspring than those who didn’t carry the LP variant. This version of the story is indeed a textbook example of natural selection affecting humans.
A new study published in Nature on Wednesday casts doubt on all elements of this story and provides a radically different explanation of the rise of LP. The research combined archaeological chemistry; ancient DNA data analysis; massive contemporary databases and statistical modelling. Involving over 100 collaborators in over 20 countries, the team found that milk consumption was common for thousands of years before that LP gene started increasing dramatically in prevalence. The research also raises fascinating questions about whether some people in the UK who believe they are lactose intolerant might actually be fine if they drank milk.
Prof George Davey Smith, Director of the MRC Integrative Epidemiology Unit, and Professor of Clinical Epidemiology, University of Bristol
Prof Richard Evershed, Professor of Biogeochemistry, University of Bristol, and Fellow of the Royal Society
Prof Mark Thomas, Professor of Evolutionary Genetics, University College London
Dr Melanie Roffet-Salque, Royal Society Research Fellow, University of Bristol