— Researchers analysing the DNA in Neolithic human remains claim to have uncovered the first direct evidence that modern humans have evolved changes in response to natural selection.
Just 7000 years ago, Europeans were unable to digest milk, according to a new analysis of fossilised bone samples nowadays more than 90% of this population can.
Europeans must have incurred a rapid change in their genetic make-up because it held an evolutionary advantage for them to be able to digest milk, says Mark Thomas at University College London in the UK, who carried out the study with colleagues.
However, other experts caution that larger studies are needed to support the conclusions of this research.
The majority of humans around the world lose the ability to digest lactose a sugar in milk before reaching adulthood. This is because their gene for the enzyme lactase, which breaks lactose down, is switched off during adolescence. Symptoms of this lactose intolerance include bloating and diarrhoea after drinking milk.
However, over 90% of northern Europeans have a version of the lactase gene that remains active throughout life, enabling them to continue drinking milk as adults.
To determine when this special lactose tolerance evolved in Europe, Thomass team analysed the DNA from 55 bone samples belonging to eight Neolithic Europeans. The skeletons were dated to between 5840 BC and 5000 BC.
After extracting the DNA from the fossils, researchers identified the sequence of the lactase gene for each of the eight Neolithic individuals. Surprisingly, says Thomas, none of the early Europeans had the gene mutation associated with lactose tolerance in modern-day Europeans.
Based on this result, he believes that the mutation for lactose tolerance spontaneously arose in Europe within the past 7000 years and quickly became prevalent through natural selection.
He explains that the ability to digest milk would give a massive survival advantage to people living thousands of years ago: milk from cows is uncontaminated by parasites, making it safer to drink than stream water. It is also available all year long, unlike seasonal crops.
Thomas also notes that the low levels of sunlight in northern Europe during winter mean that people have lowered levels of vitamin D in their bodies, and therefore have difficulty absorbing calcium. Milk solves this problem by providing them with both calcium and some vitamin D.
Prone to error
Thomas says the comparison of the fossil DNA and present-day European genes provides a before-and-after snapshot that provides the first ever direct evidence that humans have changed in response to natural selection.
This is a fascinating and important study, says bio-archaeologist Clark Larsen of Ohio State University in Columbus, Ohio, US. It offers a new window onto past human genetic variation, and is truly an important development in the history of our science and in the study of the past.
But Larsen adds that larger studies of fossils are needed to confirm the absence of the lactose tolerance gene in Neolithic Europeans. He and others stress that the process of extracting and analysing DNA from fossils is very complicated and prone to error.
The ability to decipher genes from fossils is a great feat in and of itself, says Ripan Malhi at the University of Illinois Urbana-Champaign, US. And Malhi says he believes that other examples of direct evidence for relatively recent natural selection acting on humans may be out there.
Journal reference: Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.0607187104)