Evolution of lactose tolerance probably driven by famine and disease

Milk consumption was widespread thousands of years before people were able to break it down properly, according to the largest study yet on the evolution of lactose tolerance in humans. The ability to break down lactose was probably gained during episodes of acute crisis, not gradually over time, the study found. As babies, all humans produce the enzyme lactase, which breaks down lactose into the more readily absorbed glucose and galactose, but many people have much lower levels of lactase after weaning, meaning they cannot digest milk properly. The spread of lactase persistence – the ability to break down lactose after weaning – is considered one of the best examples of natural selection in humans. One-third of the global population gained this trait in just a couple of thousand years.