April 2007

In the US and many other countries, we’ve certainly “got milk,” but not everyone can enjoy it. For around 10% of Americans, 10% of Africa’s Tutsi tribe, 50% of Spanish and French people, and 99% of Chinese, a tall cold glass of milk means an upset stomach and other unpleasant digestive side effects. In fact, most adults in the world are lactose intolerant and cannot digest lactose, the primary sugar in milk. And yet, regardless of our ancestry, most of us began our lives happily drinking milk from a bottle or breast — so what happened in the intervening time? Why do so many babies enjoy lactose and so many adults avoid it? Lactose is broken down by a protein called lactase, which acts as a pair of molecular scissors, snipping the lactose molecule in two. Anyone who drank milk as a baby carries a working version of the gene that codes for lactase. In lactose tolerant individuals, that gene keeps working into adulthood, producing the protein that digests lactose and makes eating ice cream a pleasant experience. But in people who are lactose intolerant, that lactase gene is switched off after weaning. Now, new research reveals that the Stone Age ancestors of European dairy-lovers probably couldn’t digest milk either. So how did they get from bellyaches to milk mustaches? The answer is an evolutionary story that takes us from the milkmaids of the Alps to the Maasai herdsmen of Africa.

Where's the evolution?

Mutations that keep the lactase gene permanently switched on are common among modern Europeans — but not among their ancestors. In March 2007, a team of German and British researchers announced that they went looking for that mutation in the 7000-year-old fossils of ancient Europeans and came up empty-handed. The researchers managed to extract the length of DNA corresponding to the lactose tolerance mutation from eight Neolithic human fossils and one Mesolithic fossil, but those DNA sequences did not carry the telltale mutation. The results suggest that as late as 5000 BC most ancient Europeans could not have digested milk as adults — and that they only later evolved into milk-drinking societies.

Today, the ability to digest milk as an adult seems like a clear benefit, but that wasn’t always the case. Lactose tolerance is only advantageous in environments and cultures where humans have access to domesticated dairy animals. Multiple lines of evidence from human genetics, cattle genetics, and archaeological records suggest that Middle Eastern and North Africans populations domesticated cattle between 7500 and 9000 years ago, and that these animals were later brought into Europe. In that cow-friendly environment, being able to drink milk directly (instead of having to process it into lower-lactose cheese) would have been advantageous, providing additional sustenance and, during droughts, a source of water. The lactose tolerance mutation arose randomly (as all mutations do), but once it arose, it had a distinct advantage in these populations. Natural selection would have favored individuals carrying the lactose tolerance mutation, spreading it through ancient European populations that depended on dairying. Many thousands of years later, we see the indirect (but delicious) effects of this mutation’s success in European cuisines: oozing French cheeses, Swiss milk chocolate, and creamy Italian gelatos.

Surprisingly, with respect to dairying, human populations on separate continents seem to have led parallel lives — or rather, followed parallel evolutionary trajectories. Recent evidence suggests that cattle may have been domesticated independently in several places, including Africa. As African populations began herding cattle, lactose tolerance became an advantageous trait. The stage was set, in Africa too, for the spread of a lactose tolerance mutation. In January 2007, an international team of researchers led by geneticist Sarah Tishkoff announced that they had uncovered the genetic roots of Africans’ lactose tolerance. Just as in Europe, on this continent, mutations (in this case, probably three) randomly arose, and these happened to have the effect of keeping the lactase gene switched on. And just as in Europe, these mutations were favored by natural selection and quickly spread through dairy-dependent populations.

While this discovery answers many questions, it also highlights new mysteries. For example, Tishkoff’s team discovered that in the Hadza population (a group of Tanzanian hunter-gatherers), around 50% are lactose tolerant — a percentage usually indicative of a dairy-dependent society. And yet, as far as is known, the Hadza have never had much to do with cattle or relied on milk in their diets — so what explains their lactose tolerance? Are they the long lost descendents of a group of cattle herders? Has the tribe changed its basic mode of making a living? Or could the lactose tolerance mutation provide some other yet-to-be-discovered advantage, beyond allowing adults to drink milk?

Whatever the answers such spin-off questions, research into the evolutionary origins of lactose tolerance has already clearly illuminated some fascinating aspects of human evolutionary history. Perhaps most intriguingly, the convergent evolution of African and European populations in relation to cattle domestication reveals that shared aspects of human culture across different ethnic groups affects our evolution in similar ways. Regardless of skin color or geography — whether dealing with Stone Age Europeans, Swiss milk maids, Maasai warriors, or modern hunter-gatherers — evolution plays by the same rules.