A sweeping analysis of over 8,000 genomes from India, Bangladesh and Pakistan has demonstrated that South Asians’ ability to digest milk has roots in ancient
migrations.

Most South Asians inherited the capacity to digest milk in adulthood from ancient Steppe herders, except for the Todas in south India and Gujjars in northwest India and Pakistan whose pastoralist lifestyles appear to have favoured unusually high milk tolerance, the study has shown. The ability to digest lactose — a natural sugar found in milk — after weaning is unusual in humans and depends on the continued production of an enzyme called lactase in adulthood, a gene-linked capacity called lactase persistence.

Scientists have known since the late 1960s that lactase persistence is not uniformly distributed worldwide but geographically restricted, most common in northwestern Europe, decreasing east and south and reaching near-absence levels east of the Indian subcontinent.

Now, a study led by Priya Moorjani at the University of California, Berkeley, and her collaborators in India
and France has revealed that a genetic variant that facilitates lactase persistence
first appeared in South Asia during historical and medieval periods through Steppe pastoralists.

Their study, released as a preprint, has shown that unlike in other worldwide populations, the prevalence of lactase persistence is almost entirely explained through Steppe ancestry. Multiple earlier studies have suggested that Steppe pastoralists from the Caspian Sea region arrived in the Indian subcontinent between 3,000 and 4,000 years ago.

Lactase persistence is much more common across north India and Pakistan — around 45 per cent, but declines towards the east and south, dropping to 14 per cent in the east and 7 per cent in the south. However, the study showed that two pastoral communities — the Todas in south India and the Gujjars in northwest India and Pakistan — stood out, with lactase persistence as high as 89 per cent.

Moorjani and her colleagues analysed 8,173 South Asian genome sequences, including 181 ancient genomes dating back to around 3,000 BC, many from Pakistan’s Swat Valley.

“In most South Asian populations, present-day differences in milk tolerance can be explained almost entirely by how much Steppe ancestry they inherited,” Kumarasamy Thangaraj, a population geneticist at the Centre for Cellular and Molecular Biology, Hyderabad, and a study team member, told The Telegraph.

Despite South Asia’s long association with dairy, many communities appear to have relied on what scientists call “cultural solutions” — such as fermenting milk into curd, paneer, or cheese — to consume milk products even without adequate levels of lactase persistence. The fermentation process breaks down lactose and friendly bacteria called probiotics in curd help digest residual lactose.

Scientists believe the exceptionally high levels of milk tolerance in the traditionally pastoral Todas and Gujjars reflect how their reliance on fresh milk over generations created strong evolutionary pressure that increased the trait over time.

Amid evidence of independent cattle domestication in South Asia around 7,000 years ago, Thangaraj and his colleagues had used a genetic study published over a decade ago to determine whether lactase persistence had evolved independently in South Asia as in Europe and Africa. Their earlier study published in 2012 had found that a gene variant for lactase persistence that originated in Europe accounted for nearly all lactase persistence variation in South Asia.