Biology 101 | Evolution THE DIARRHEA AGE

I wanna tell you about something I call the diarrhea age. And why I think it's one of the greatest examples of human evolution. Baby mammals produce lactase so they can break down the lactose as they're feeding from their mother's milk. With mammals, we are the exception about the fact that we can drink milk into adulthood. And even with humans, those who are the mutants. They are the outliers. Several different human populations evolved this ability convergently, thanks to the fact that their ancestors lived alongside cattle, sheep, goats, animals that produce dairy products. But newer studies have been changing the story. It wasn't a linear progression that we often portrayed. The study analysed thousands of pot shards from across Europe. And they looked at the milk fat residue, so the evidence of milk left behind. They were able to track the spread of milk use across populations going back over 7,000 years ago. This got more interesting when they looked at human DNA and they tried to infer when these alleles that lead to the ability to digest milk into adulthood, when those became more common in the population, there was a period of several thousand years where people were just drinking milk and eating cheese and dealing with the diarrhea. Having milk around was a benefit. It would hydrate you, it's a good source of protein, a good source of healthy fats. And when there wasn't as much food around, like a dry season or something, it's just an extra resource to have available. So even though it upset their tummy, it was worth it to have. The actual strong selection that led to these alleles becoming more common in the human population, which is evolution by natural selection, instead of just gradually occurring over time as they lived alongside these livestock animals, it seems more like during times like the plague and extreme illnesses, being able to digest that milk, you don't have much water, which is likely getting people sick. And also you're not getting sick with stuff like the plague, just increase your odds of surviving tremendously. Essentially all that had to happen, any mutations that would leave that switch open longer, so you continued producing lactose longer, those were beneficial. And then they existed in the population until these times of extreme stress. Wasn't clear, call it the diarrhea age, that period before those alleles became common. But drinking milk was also common. So, like, it was probably a pretty upset tummy era.