Anthropology and Biogeography
The evolution of lactase persistence is thought to have occurred in Middle Eastern dairying populations during the Neolithic era. Prior to this, ancient humans were lactose intolerant like all other mammals. In fact, lactose intolerance persists in most human populations today. However, once the lactase persistence trait evolved, strong selective pressure allowed it spread throughout ancient human populations.
The Neolithic Era
When did humans evolve the ability to digest milk far into adulthood? Through genetic analysis, scientists estimate that this evolutionary step occurred between 5,000 and 7,000 years ago, in the Neolithic Era (Itan, 2009). During this time, humans were making the major transition from hunting and gathering to farming and herding. This offered increased food security and promoted technological innovation as farmers and herders learned new ways to manipulate their environment and resources (Mummert, 2011).
Pastoralism, the cultural practice of milking livestock (such as goats, sheep, cows, and camels), was an innovation of the Neolithic Revolution. The Biocultural Coevolution theory proposes that pastoralism and lactase persistence coevolved. This means that they arose around the same time, and both changes were reinforced by the advantages of the other.
There are significant nutritional benefits of consuming dairy products. Milk contains essential molecules such as water, sugar, fat, protein, and vitamins. Although Neolithic humans could obtain these nutrients from the meat of their livestock, the ability to obtain these nutrients without killing their animals would have afforded them a great increase in efficiency and stainability. Additionally, dairy products could have been a life-saving source of calories during times of famine. Within a pastoralist culture, an individual with lactase persistence would have a distinct advantage in terms of nutrient acquisition.
How did Lactase Persistence spread? Simply stated, it spread with the cultural practice of milking livestock, or Pastoralism. The earliest evidence of pastoralism, 10,500 year old bones of juvenile cattle, was found in the Middle East. It is thought that this practice spread through migration because the remains of the earliest domesticated cattle in Greece and the Balkan States (8,000 years old) were more closely related to the cattle from the Middle East than the wild cattle found in Europe at the time. This indicates that migrants from the Middle East brought their cattle with them. Because Middle Eastern cattle herders had more advanced food technology, they easily out-competed the local hunter-gatherers they encountered in Central and Northern Europe.
Milk vs. Cheese
Although it is estimated that the prevalence of lactase persistence greatly increased in Europe around 5,000 to 7,000 years ago, anthropological evidence, like bones of juvenile livestock and ancient pottery remains, place the cultural adoption of dairying as far back as 12,000 years ago. This suggests that for several thousand years, humans were milking sheep, goats, and cows despite the inability to digest milk.
Shards of ancient ceramic sieves provide some clues as to
how this was possible for early farmers and herders. Many Paleolithic and Neolithic cultures practiced the storage and transport of food and water in animal skins and intestines. When milk was introduced to the Neolithic diet in the Middle East, it was likely stored in an inflated cow stomach, resulting in the separation of the curds and whey. By using ceramic sieves to strain the curds and whey and ferment these diary products into cheese, they were able to greatly reduce the level of lactose in their dairy products, while still retaining the nutritional benefits of milk. (Salque, 2013).
Convergent evolution occurs when separate populations independently evolve the same trait. This is often due to both populations experiencing similar selective pressures that help to fix a new trait in a population once it arises through random mutation. In the case of lactase, the selective force is benefit accrued from milk consumption by human adults.
In Africa, lactase persistence evolved independently from the European lineage. The mutation(s) responsible for lactase persistence are different, but the effect is the same.
Neolithic humans in Africa experienced similar selective pressures to adopt pastoralism and evolve lactase persistence. Through slightly different mutations, G-13915 and G-13907, different peoples in Kenya and Sudan evolved lactase persistence via the same Oct1 transcription factor binding site.
A Special Case
The Somali people of Ethiopia are an example of a modern culture that have adopted a pastoralist way of life, drink milk as adults, and yet have not evolved the lactase persistence trait. How is it possible that adults regularly consume fresh milk from their cows without the negative consequences? The answer is in their large intestines: individuals in this culture have been found to posses unique colonic bacterial populations that metabolize lactose in such a way that does not cause gastrointestinal distress (Ingram, 2008).