How Gujjar and Toda Milk Habits Reshaped Human DNA

For certain adults, a glass of milk offers comfort.

For others, it reliably brings on stomach pain. The reason for this difference is a genetic tale far more unusual than it may first appear.

Scientists now suggest that drinking fresh milk has altered human DNA over only a few thousand years, although the process has not unfolded uniformly across the world. Two pastoral communities in South Asia, scarcely marked on many maps, are at the heart of an unexpected new part of this story.

The unusual case of adults who can drink milk

Every human infant can process lactose, milk’s principal sugar. Their intestines make lactase, the enzyme responsible for breaking it down. In most mammals, lactase production declines after weaning, and humans once followed the same pattern. Many adults worldwide still do.

However, vast numbers of people in Northern Europe, parts of East Africa and several other areas can drink fresh, unfermented milk without difficulty. Their bodies continue to produce lactase into adult life. Geneticists describe this as “lactase persistence”. Everyone else experiences lactose intolerance to differing extents, ranging from slight bloating to serious cramps and diarrhoea.

Lactase persistence is not the human default. It is a relatively recent genetic tweak that spread where milk meant survival.

The most familiar genetic switch associated with this characteristic sits near the LCT gene, which encodes lactase. Among Europeans, a mutation commonly known as -13910*T allows the enzyme to remain active well beyond childhood. The frequency of this variant increased sharply alongside the emergence of dairy-farming societies during the past 8,000–9,000 years.

South Asia’s milk paradox

South Asia consumes substantial amounts of milk. Dairy is embedded in everyday life, from chai to yoghurt and ghee. Such widespread consumption could imply that lactase persistence is common throughout the region, but genetic evidence presents another picture.

Research led by geneticist Priya Moorjani, using both ancient and present-day genomes, charts the distribution of the -13910*T variant across the subcontinent. Its pattern is uneven. Northern populations have modest levels of the mutation, while extensive areas in the south and east show almost none.

The study connects the variant with ancestry from “Steppe” pastoralists, communities that travelled from the Eurasian grasslands into South Asia about 3,500 years ago. These migrants seem to have brought the lactase-persistence mutation with them, much as related populations did during their movement into Europe.

Genetic traces of Steppe herders sit like faint fingerprints across South Asia, but the ability to digest milk followed a far more uneven path.

Researchers were especially surprised not by the mutation’s overall rarity, but by two remarkable exceptions.

Two pastoral outliers: Gujjar and Toda

Across most South Asian populations, the European-type lactase-persistence variant is still comparatively uncommon. In two pastoral communities, though-the northern Gujjar and southern Toda-the mutation occurs at levels comparable with those in Scandinavia.

More than 65% of people in these groups carry the -13910*T variant. This proportion is exceptional within the region and cannot be predicted simply by examining a map of dairy consumption.

Milk habits that shape Gujjar and Toda genes

The Gujjar and Toda have an important practice in common: both communities have long consumed fresh, non-fermented milk from their herds. This distinction is significant. Fermented dairy foods, including yoghurt, kefir and many cheeses, contain less lactose because bacteria have already broken down much of the sugar. People with lactose intolerance can often manage these products more easily.

When fresh milk is the main form consumed, people without lactase persistence face a greater challenge. Diarrhoea and intestinal discomfort can cause dehydration, reduced nutrient absorption and increased vulnerability in childhood. These circumstances exert strong evolutionary pressure in favour of variants that enable efficient lactose digestion.

In the Gujjar and Toda, fresh milk acted like a daily genetic filter, pushing the lactase-persistence variant to spread faster than in many European herding societies.

Genetic patterns around the LCT region among the Toda provide clear evidence of powerful selection. DNA stretches close to the variant are unusually alike between individuals, indicating that the mutation became common so rapidly that recombination had little opportunity to rearrange the surrounding genetic landscape. Estimates indicate selection pressures roughly double those inferred for Northern Europe.

Different roads to the same glass of milk

The differences between South Asia, Europe and Africa are prompting a wider reassessment of how lactase persistence developed around the globe. For years, many researchers treated the European account as the standard model: one mutation slowly spreading wherever dairy became a dietary staple.

That explanation no longer stands. Several strands of research indicate that:

• Different areas may rely on different mutations near the LCT gene to keep lactase active.
• Selection can intensify in particular places where fresh milk provides major survival advantages, rather than affecting every dairy-consuming community.
• Cultural alternatives, including milk fermentation, may lessen the need for genetic adaptation.

In parts of East Africa, for instance, several separate mutations produce lactase persistence, probably emerging independently as pastoralism became established. By comparison, many South Asians consume large quantities of yoghurt, paneer and dishes simmered for long periods, all of which substantially reduce lactose levels. These culinary practices probably reduced the pressure for a region-wide genetic change.

A patchwork of evolution, not a uniform upgrade

Anthropologists emphasise that lactase persistence is not evidence that one group is “more evolved”. Rather, it represents a particular adaptation to a particular lifestyle. Where herding communities depended on milk as an essential source of calories and water, natural selection favoured genes that maintained lactase production. In places where dairy had a more limited role or was more heavily processed, different approaches were entirely adequate.

The outcome is a worldwide patchwork: areas with extremely high tolerance sit beside neighbouring populations whose adults are largely lactose intolerant, even when cattle or buffalo graze within the same region.

The ability to drink milk in adulthood is not a straight line of progress. It is a series of local solutions to local problems.

What this means for your gut and your plate

For individuals, the evidence on lactase persistence changes the way dietary guidance can be viewed. Someone from a community with low rates of lactase persistence may nevertheless digest milk well because of gut microbiota, gradual adaptation or mixed ancestry. Equally, a person from a highly tolerant population may react poorly as a result of illness, medication or other intestinal problems.

Genetic tests can now examine variants close to the LCT gene directly. They may show whether a person’s DNA matches recognised lactase-persistent profiles. However, these tests do not assess enzyme levels at a given moment. For many people, keeping a food diary and carefully reintroducing dairy, ideally under medical guidance, remains more practical.

Region / group	Typical dairy use	Lactase-persistence pattern
Northern Europe	High fresh milk intake, plus cheese and yoghurt	High frequency of -13910*T variant
East African pastoralists	Fresh milk and fermented drinks	Multiple distinct lactase-persistence mutations
Most South Asian populations	Dairy common, often fermented or cooked	Low to moderate -13910*T levels
Gujjar and Toda (South Asia)	High fresh milk consumption	Very high -13910*T frequency, strong recent selection

Why fast evolution around milk matters for health

Rapid genetic adaptation to diet offers researchers a rare real-time example of human evolution. By following the speed at which the LCT region changed in communities such as the Toda, scientists can gain insight into how other food-related characteristics could alter under modern pressures.

Urbanisation, processed food and changing climate conditions now influence what people eat and drink. Traits that previously supported survival on a pastoral diet may involve different trade-offs in cities shaped by sugary drinks and sedentary ways of life. Lactase persistence itself may be linked with obesity, diabetes and cardiovascular risk, though the available evidence is still mixed.

Scientists also use lactase persistence to help reconstruct earlier population movements. Finding the -13910*T variant in ancient remains can point to contact with Steppe herders or later European populations. This genetic breadcrumb trail assists in refining timelines for migration, language expansion and cultural transformation across Eurasia.

Looking ahead: questions that still bother scientists

A number of questions remain unresolved. Many people report symptoms associated with lactose despite carrying lactase-persistence variants. Others do not have these variants yet tolerate milk fairly well. This inconsistency suggests that the gut microbiome, intestinal health and diet in early life may play a larger part.

Future research seeks to bring together genetics, microbiology and detailed dietary information. One possible approach uses controlled “milk challenges” involving volunteers from different genetic backgrounds. Researchers would track blood sugar, gut bacteria and symptoms over time, before observing how regular exposure influences tolerance. Such studies may show how learnt adaptation and DNA interact in relation to lactose.

The South Asian results also prompt questions about how rapidly these traits might change again if milk-drinking habits shift. Should a pastoral community move into cities and reduce fresh-milk consumption over several generations, would selection for lactase persistence weaken, or has the variant already become neutral baggage? This sort of long-term question could be relevant to numerous other diet-gene interactions appearing in the 21st century.

For the moment, the Gujjar and Toda story demonstrates how an ordinary daily drink can leave deep marks in the genome. Milk is more than food; in certain parts of the world, it has operated as an evolutionary force, quietly determining who flourishes with a glass of it and who chooses plant-based alternatives instead.