Lactose tolerance gene proves Natural Selection in humans (2/7/2007)

This unassuming gene, may be the strongest genetic evidence that Darwin's natural selection has occurred (and still does occur) in humans.

In a recent paper by Dr. Sarah Tishkoff with the University of Maryland, and others titled "Convergent adaptation of human lactase persistence in Africa and Europe", they describe the results of work collecting and analyzing several gene variants that are the key to the ability for adults to drink milk.

In most people, the ability to digest lactose, the main carbohydrate in milk, declines rapidly after weaning. It is caused by the decrease in production of an enzyme called lactase-phlorizin hydrolase (LPH). LPH breaks down lactose into glucose and galactose, sugars that are easily absorbed in the bloodstream and provide energy.

Descendants from populations that practiced cattle domestication, maintain the ability to digest milk and other dairy products into adulthood. These individuals have the lactase persistence trait.

Previous research had identified the gene in Europeans responsible for the lactase persistence. Dr. Tishkoff's research was based around determining whether Africans with the same lactase persistence trait had the same gene.

She studied several groups of people in Africa by testing their DNA. She also administered a bloodsugar test after having the subject ingest 50 grams of lactose. The intent was to determine how effective the person's body was at absorbing the lactose. She then categorized them based on whether they were lactose persistent, meaning could convert lactose into glucose easily, lactose intermediate persistence, meaning they could convert lactose into glucose somewhat, and lactose non-persistent, meaning they couldn't convert lactose at all.

She then cross referenced the two sets of data, to determine which genes were responsible for the lactose persistence, and to what level. Her study, shows that the lactase persistence gene that has evolved in Africa has evolved independently of the gene variants predominant in Europe. So far she has identified 6 different variants of the gene.

She also discovered that individuals with one of the genes actually converted lactose into sugar much better and faster than the others.

In addition to lactose testing, she also cross referenced the results with whether the people being tested were descended from people who traditionally practiced cattle domestication. In most of the cases, if a person was descended from a family that practiced cattle domestication, they would have some level of lactose persistence.

In one unique case, the Hadza population of Tanzania who are hunters and gatherers can convert about 50% of the lactose they ingest into glucose, suggesting that either the Hadza descend from a pastoralist population or that the lactase persistence trait may be adaptive for something other than milk.

Positive Selection from Lactose Persistence

Dr. Tishkoff's research clearly showed that the lactose tolerance gene is linked to people who have practiced cattle domestication. But why is this important? The ability to digest milk is likely to increase a persons chance at survival because of the increased nutritional benefits of milk (carbohydrates as well as fats, protein and calcium). Milk is also an important source of water in arid regions, devoid of water.

When you consider that the symptoms of lactose intolerance include water loss from diarrhea, people that can digest milk have a big advantage over those that can't. Dr. Tishkoff's research also clearly indicates that because this is such a useful trait, its recent and rapid spread is an excellent example of the ongoing adaptation of humans.

Written By Branden Long, Web Doodle, LLC

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