How did some adults evolve to digest milk?

Most mammals, and many humans, lose the ability to digest lactose after childhood. In some populations, mutations keep the lactase enzyme active throughout adult life, a trait called lactase persistence. It is regarded as an adaptation linked to the domestication of dairy animals and the later consumption of their milk. Natural selection then favoured people carrying these variants, but researchers disagree about what made the advantage so strong. [1] [2]

Five variants are currently known to underlie the trait. In Eurasia it is mostly tied to a single variant, while in Africa it is mostly polygenic. The best known European variant, called -13910 T, is found throughout Europe and also appears in parts of Asia and North Africa, where gene flow likely spread it. [1] [3]

A study of 470 Tanzanians, Kenyans and Sudanese found three different African variants, which arose on different genetic backgrounds from the European one. The authors described this as convergent evolution driven by shared cultural practices, namely animal domestication and adult milk drinking. Their estimates pointed to strong selection within roughly the last 3,000 to 7,000 years, though with wide confidence intervals. Evidence also points to independent variants in Arabia and South Asia. [4] [3]

The long-standing view is gene-culture coevolution: the trait would only help where fresh milk was available, so dairying favoured persistence and persistence favoured dairying. A simulation study using ancient DNA and farming dates inferred that the European variant first came under selection among dairying farmers about 7,500 years ago, between the central Balkans and central Europe. That study also found no support for a higher advantage at northern latitudes through a need for vitamin D. In Africa, researchers have also pointed to milk as a source of nutrition and water in arid regions. [5] [4]

A newer challenge comes from a team led by Richard Evershed, Mark Thomas and colleagues. They found milk-fat residues on about 13,000 pottery fragments from more than 550 European sites, and compared them with genomes of over 1,700 ancient Eurasians. Early farmers drank milk but were largely lactose intolerant, and persistence became common only after the Bronze Age, 5,000 to 4,000 years ago. The team found little overlap between rises in persistence and rises in milk use. They propose that selection was strongest during famine and infection, when diarrhoea from milk could be deadly to people already unwell. Their model linked persistence more strongly to exposure to animal pathogens and famine than to other factors tested. [6]

Not everyone is convinced. Researcher Wilkin, quoted in the coverage, said she did not know whether the team was right, and noted that the idea does not fully explain why the trait was absent or rare in some farming and Bronze Age dairying populations that must also have faced famine and disease. Other researchers have cited the new work as one possible pressure, and suggest selection may have been strongest where crops were hard to establish. [6] [3]

Researchers agree that the persistence variants are recent in evolutionary terms and have been under strong positive selection, and that the trait arose independently several times. The European variant is described as one of the strongest examples of natural selection on the human genome. What remains open is the precise advantage that drove it, whether nutrition, water, or survival through famine and disease. [6] [5]