BRISTOL, United Kingdom — Do modern humans owe their taste for milk and dairy products to diseases and famines thousands of years ago? Researchers say our ancestors were drinking milk long before a gene mutation that allows humans to tolerate lactose.
Their study adds that ancient humans would have been more likely to drink unfermented, high-lactose milk when their crops failed — exactly when they shouldn’t. Study authors found that genes that give people lactose tolerance were under more pressure from natural selection when there was more famine and disease.
They say drinking milk at a time of poor sanitation, disease, and famine led to death. Those without lactose tolerance would have been especially vulnerable. Eventually, many people who did not carry the gene mutation would have died before having children, pushing up the percentage of people with the gene mutation.
Many scientists thought that people are able to tolerate lactose in order to consume more milk and dairy, but the new findings dispute that. Most European adults can drink milk without any problems, but the same is not true for two-thirds of the world’s adults today — and almost everyone living 5,000 years ago.
People have been lactose intolerant for centuries
If people don’t digest lactose — which is a sugar — properly, it can cause cramps and diarrhea. The research team found a gene mutation which enables people to tolerate milk and dairy was not widespread until around 1000 B.C., although it was first detected between 4700 and 4600 B.C.
“To digest lactose we need to produce the enzyme lactase in our gut,” says Professor George Davey Smith from the University of Bristol in a media release.
“Almost all babies produce lactase, but in the majority of people globally that production declines rapidly between weaning and adolescence. However, a genetic trait called lactase persistence has evolved multiple times over the last 10,000 years and spread in various milk-drinking populations in Europe, central and southern Asia, the Middle East and Africa,” Smith continues.
“Today, around one third of adults in the world are lactase persistent.”
Ancient pottery sheds light on the history of milk
The team, led by University of Bristol and University College London researchers, says milk and dairy tolerance became so widespread because of natural selection. However, the process that caused this to happen has been unclear.
For the study, the team mapped patterns of milk use over the last 9,000 years, probed UK Biobank data about living Brits, and combined ancient DNA, radiocarbon, and archaeological data using new computer models. They assembled a huge database of almost 7,000 organic animal fat residues from more than 13,000 pieces of pottery from 554 archaeological sites to work out where and when people started drinking milk.
Researchers say milk was a common ingredient in prehistoric Europe, but its use rose and fell at different times.
Study authors assembled a database focusing on the presence or absence of the gene mutation that allows people to tolerate lactose. To do this, they used published ancient DNA sequences from more than 1,700 prehistoric European and Asian people. The mutation first appeared around 5,000 years ago. By 3,000 years ago, it was more widespread. Today, it is very common to have the gene mutation.
Next, they used statistics to work out how well changes in milk use through time explain the fact people became tolerant of milk and dairy. They found no relationship, even though they were able to show they could detect the link, if it existed.
The surprising finding challenged the long-held view that people built up a tolerance as they ate and drank more dairy and milk. Facts and figures about 300,000 living Brits showed almost no difference in the amount of milk consumed by people with genes which suggest they are tolerant to the drink and those who are not.
Evolution kept people from dying from drinking milk
Drinking milk caused some discomfort but did not have short or long-term negative health effects on people who are lactose intolerant, according to the team.
“Our findings show milk use was widespread in Europe for at least 9,000 years, and healthy humans, even those who are not lactase persistent, could happily consume milk without getting ill,” Prof. Smith says.
“However, drinking milk in lactase non-persistent individuals does lead to a high concentration of lactose in the intestine, which can draw fluid into the colon, and dehydration can result when this is combined with diarrheal disease.”
“If you are healthy and lactase non-persistent, and you drink lots of milk, you may experience some discomfort, but you not going to die of it. However, if you are severely malnourished and have diarrhea, then you’ve got life-threatening problems. When their crops failed, prehistoric people would have been more likely to consume unfermented high-lactose milk – exactly when they shouldn’t.”
“Our study demonstrates how, in later prehistory, as populations and settlement sizes grew, human health would have been increasingly impacted by poor sanitation and increasing diarrheal diseases, especially those of animal origin,” the research team adds.
“Under these conditions consuming milk would have resulted in increasing death rates, with individuals lacking lactase persistence being especially vulnerable. This situation would have been further exacerbated under famine conditions, when disease and malnutrition rates are increased. This would lead to individuals who did not carry a copy of the lactase persistence gene variant being more likely to die before or during their reproductive years, which would push the population prevalence of lactase persistence up.”
“It seems the same factors that influence human mortality today drove the evolution of this amazing gene through prehistory,” study authors conclude.
The findings are published in the journal Nature.
South West News Service writer Gwyn Wright contributed to this report.