Prehistoric people acquired a taste for milk thousands of years before humans evolved the genetic trait that allows us to digest it without sore stomachs and gastrointestinal upsets, according to a new paper published in the journal Nature.
It was previously assumed that humans evolved a tolerance to lactose, also known as lactase persistence, because they started to drink milk more and more as agriculture become more prolific across Eurasia. However, to the researchers’ surprise, this was found to be not true.
The study authors analyzed thousands of animal fat residues found on over 13,000 fragments of pottery from 554 archaeological sites across Europe. Microscopic traces of milk on the pottery shards suggests that human consumption of milk was, in fact, high in Neolithic Europe from around 7,000 BCE onwards.
This was a time before the overwhelming majority of the population was able to digest it. The sugar in milk – lactose – is turned into glucose and galactose by the enzyme lactase. Without the enzyme, or with insufficient amounts of it, lactose isn’t broken down and fuels bacterial fermentation in the gut, leading to gas, diarrhoea, and stomach pain.
Unexpectedly, genetic evidence from prehistoric European and Asian people showed that the gene that codes for the production of lactase was not common until around 1,000 BCE, nearly 4,000 years after it was first detected around 4,700 BCE. It then spread across the continent like wildfire within just a few thousand years.
"This is really quite shocking," Professor Mark Thomas, an expert in Evolutionary Genetics and Ancient DNA at University College London, said in an online press conference.
"The frequency of the genetic variant that causes lactase persistence has increased incredibly rapidly. Ridiculously rapidly. Inexplicably, almost rapidly," he added.
"It's probably the most selected single gene trait to have evolved in Europeans, African, Middle Eastern, and South Asian populations over the last 10,000 years," continued Thomas.
It was previously assumed that increasing milk consumption was a key driver for lactase tolerance, but this clearly doesn’t seem to be the case. So, how and why did the gene for lactose tolerance quickly emerge in a significant portion of the population in Europe, southern Asia, the Middle East, and Africa?
Further probing by the researchers showed that drinking milk with lactose intolerance may be unpleasant, but it most likely won’t kill you.
Genetic and medical data on more than 300,000 people in the UK found that people with the lactase gene and people with lactose intolerance almost drink the same amount of milk. Furthermore, the large majority of people who were genetically lactase intolerant only experienced very mild negative health effects when they drank milk.
This changes, however, if a population is under stress from famine or disease as this is when a stomach upset could mean life or death. The team applied indicators of past famine and pathogen exposure to their statistical models, which clearly showed that the lactase tolerance gene variant was under stronger natural selection when there was famine and more pathogens.
“If you're lactase non-persistent and you drink a lot of milk, you might get a bit of diarrhoea. You may fart a lot. You may get cramps. It might be uncomfortable and embarrassing, but you’re not going to die of it,” explained Profesor Thomas.
“But, if you give yourself diarrhoea when you're exposed to other pathogens, then that can turn from an inconvenience to a fatal condition.”
In other words, if you’re part of the minority of humans currently on the planet who can digest milk happily, then you can probably thank the decades of famine and diseases that your ancestors managed to survive though.