Our ability to enjoy dairy delights today could be thanks to the sacrifice of our ancestors who succumbed to disease and famine thousands of years ago, a new study has found.
It was previously thought the genetic variation that allows humans to digest lactose evolved at the same time we started drinking animal milk.
This is because the gene prevented the uncomfortable symptoms of lactose intolerance, such as diarrhoea.
However, a new study by researchers at the University of Bristol and University College London (UCL) suggests that the variation came about later, during times of famine and infectious disease.
During these periods, when people were already weakened due to starvation or illness, drinking milk could have proved fatal for those who were lactose intolerant.
As a result, people who carried the gene for lactose tolerance were more likely to survive and pass the gene on to their offspring, increasing its prevalence within society.
‘When people are severely malnourished, diarrhoea can shift from an inconvenience to a fatal condition,’ explained Mark Thomas, a Professor of Evolutionary Genetics at UCL.
It is known that humans began domesticating animals and consuming their milk around 10,000 years ago. At that time, all humans were unable to digest lactose, the main sugar in milk, after they were weaned from breast feeding. However, since then, a genetic variation that enables the body to digest lactose has grown in prevalence
Symptoms of lactose intolerance could be life-threatening if experienced at the same time as famine or infectious diarrhoeal disease. Individuals without the LP genetic variation were more likely to suffer this combination than those with it, and thus less likely to pass on their genes
It is known that humans began domesticating animals and consuming their milk around 10,000 years ago.
At that time, all humans were unable to digest lactose – the main sugar in milk – after they were weaned off breastfeeding.
The enzyme that digests lactose is called ‘lactase’, and this is produced in the small intestine during fetal development.
After babies had stopped breastfeeding, their body stopped producing lactase, so they could no longer digest lactose in dairy products.
The sugar would therefore be able to travel to the large intestine and cause symptoms of hypolactasia, or lactose intolerance, including cramps, bloating, diarrhoea and flatulence.
However, since then, a genetic variation that continues the production of lactase in the body has grown in prevalence.
This variation is known as lactase persistence (LP), which is today found in about a third of all adults.
Prior to the new research, published today in Nature, it was thought that LP grew in prevalence just as humans started regularly consuming dairy.
Milk and its products contain useful calories – minerals like calcium and many micronutrients – so consuming it gave them a nutritional advantage.
It was thus assumed that the LP variation was passed on through natural selection; where more humans with the LP gene passed on their genes to their offspring than those without.
However, researchers have now found that milk consumption was actually common for thousands of years before the LP gene started increasing in prevalence.
To come to this conclusion, milk fat residues absorbed by shards of unglazed pottery used by ancient farmers were studied to determine when populations began consuming milk.
They showed that European farmers were collecting milk from nearly 9,000 years ago, but this increased and decreased in different regions at different times.
WHY ARE SOME PEOPLE LACTOSE INTOLERANT?
When humans started consuming milk, they were all lactose intolerant.
The enzyme that digest lactose is called ‘lactase’, and this is produced in the small intestine during fetal development.
After babies had stopped breast feeding, their body stopped producing lactase so they could no longer digest lactose after consuming dairy products.
The sugar would therefore be able to travel to the large intestine and cause symptoms of lactose intolerance, like bloating and diarrhoea.
About 5,000 years ago, humans started developing a gene that continued the production of lactase.
This is thought to be because famine and disease because life-threatening when combined with the symptoms of lactose intolerance, so the gene enabled people to survive and pass it on to their offspring.
Now this gene is found in about a third of all adults.
Ancient DNA was also analysed to see when the LP gene arose and increased in frequency, as well as modern DNA databases that link it to health outcomes.
While it revealed the LP variation first came about around 5,000 years ago, the ancient DNA showed no relationship between changes in milk use over time and natural selection for lactase persistence.
The modern DNA data revealed that there were only small differences in the level of milk consumption between those who were lactase tolerant and intolerant.
There were also no huge differences between them in symptoms and health outcomes, like bone density and levels of vitamin D.
It is therefore unlikely that the gene variation came about because it allowed individuals to digest dairy without the symptoms of lactose intolerance, as they were so minor.
The researchers next conducted statistical modelling to establish what environmental factors really drove LP up in ancient populations.
This included using population size as an indicator for threat of famine, as there would be more mouths to feed.
Population density was used an indicator for risk of infectious diseases, as a more packed community meant bacteria could spread more quickly.
It was found that patterns in these factors correlated with the rise of the LP gene.
This is because the symptoms of lactose intolerance could be life-threatening if experienced at the same time as famine or infectious diarrhoeal disease.
Individuals without the LP genetic variation are more likely to suffer this combination than those with it, so those with LP were more likely to survive and pass on the gene.
‘When their crops failed, prehistoric people would have been more likely to consume unfermented high-lactose milk – exactly when they shouldn’t,’ said Professor George Davey Smith, Director of the MRC Integrative Epidemiology Unit at the University of Bristol.
The authors concluded: ‘It seems the same factors that influence human mortality today drove the evolution of this amazing gene through prehistory.’
The fact that the actual symptoms of lactose intolerance are so minor raises questions about whether some people who believe they suffer with it may actually be fine to enjoy milk
The results of the study suggest that it is difficult to tell whether or not a person has the LP variant, as the symptoms of lactose intolerance can be very mild.
The bodily reactions often attributed to lactose intolerance can also easily be confused with those of other functional bowel disorders.
It thus raises questions about whether some people in the UK who believe they are lactose intolerant may actually be able to enjoy milk and cheese.
Professor Thomas said: ‘Often lactose intolerance is blamed when in fact what people have is cows milk allergies.
‘Milk is extremely nutritious so it could be a loss to the diet to avoid milk.’
GREAT BRONZE AGE MIGRATION WAS FUELLED BY MILK
Migration during the Bronze Age was fuelled by milk, a recent study showed.
According to researchers, migrants from Russia brought the genes for lactose tolerance into Europe.
During the most recent Ice Age, milk was essentially a toxin to adults because – unlike children – they could not produce the enzyme required to break down lactose.
But as farming started to replace hunting and gathering in the Middle East around 11,000 years ago, cattle herders learned to reduce lactose to tolerable levels by fermenting milk to make cheese or yogurt.
Read more: The Great Bronze Age migration was fuelled by milk
At the beginning of the Bronze Age, a mass migration of people from the steppes of Russia embarked on a series of journeys that would change history. Now new analysis of dental tartar (pictured) has revealed that the secret to their success was quite simple: they drank milk