Contributed by Seth Appiah-Opoku, Judy Chen, and Steven Sun
Do you know anybody who is lactose intolerant? Today, about one-third of people in the world are unable to digest lactose, milk sugar. It is likely that before the agricultural revolution, most people were lactose intolerant. After populations began to raise cattle rather than search for their food on a daily basis, people had more access to milk, a luxury that was previously only consumed when they were infants. On the chance that food became rare, individuals who were able to digest lactose—lactose persistent individuals—were more likely to survive because they could consume milk instead. Because they were more likely to survive, these people were also more likely to have children, passing on their ability to digest lactose, and therefore increasing the presence of this ability in the population.
According to current research on the topic, the genetic mutation that causes lactose persistence appeared first in the Arabian Peninsula and Middle East around 6,000 to 2,000 years ago before spreading to northern Africa. Around that similar time, the domestication of cattle was already established in northern Africa. Within Africa, cattle domestication further spread from the Sahara to Sudan and northern Kenya about 4,500 years ago. Then, about 3,300 years ago, cattle domestication continued to spread into southern Kenya and northern Tanzania. Following a similar time scheme, the Arab expansion led to an increased mixing of different populations, and the lactose persistence gene was introduced into eastern Africa within the last 1,400 years. Ultimately, lactose persistence spread to southern Africa within the last 1,000 years.
Further research is being conducted; however there is strong evidence that because lactose persistence and the domestication of cattle arose around the same time in similar areas, it is likely that the development of lactose persistence was a result of cattle domestication.
To read the paper behind this information, check out:
Ranciaro, A., Campbell, M. C., Hirbo, J. B., Ko, W., Froment, A., Anagnostou, P., … Tishkoff, S. A. (2013). Genetic origins of lactase persistence and the spread of pastoralism in africa. American Journal of Human Genetics, 94, 1-15.