When humans are nursing, we all have an enzyme, lactase, that allows us to break down the milk sugar lactose. However, in our early ancestors, the activity of lactase eventually decreased or stopped entirely. Those modern humans that retain this trait are lactose intolerant as adults. However, as we know, some people are able to safely consume milk (and thus lactose) into adulthood. Sarah Tishkoff and her colleagues summarized their recent findings on lactase persistence a couple weeks ago at the American Association for the Advancement of Science meetings in D.C. From one of Tishkoff et al.’s papers on the subject (see reference below):
These individuals have the ‘lactase persistence’ trait. The frequency of lactase persistence is high in northern European populations (>90% in Swedes and Danes), decreases in frequency across southern Europe and the Middle East (~50% in Spanish, French and pastoralist Arab populations) and is low in non-pastoralist Asian and African populations (~1% in Chinese, ~5%–20% in West African agriculturalists). Notably, lactase persistence is common in pastoralist populations from Africa (~90% in Tutsi, ~50% in Fulani)
What do all these populations with high frequencies of the lactase persistence trait have in common? You guessed it….they all have a long history of cattle domestication. What’s cool about this new study is they show that the genetic mutation that gave rise to lactase persistence in modern Europeans is different from that of modern Africans. So, basically, this trait evolved independently at least twice. It also appears as if the evolution and spread of lactase persistence is consistent with a selective sweep (see this post for more info) that began about 7,000 years ago. So, in other words, it’s spread really fast, which means that it conferred a pretty big advantage to those individuals that had it. For more info see this podcast from Scientific American.
Participate in our poll below…are you lactose intolerant? Can you trace your ancestry back to populations that practiced cattle domestication?
References
The 2011 Harriet-Elliott Lecture Series blog 
I’m lactose intolerant…but the majority of my ancestry (so far as family researchers have found) is German, English, and French, in both my maternal and paternal lineages. My father and one of my brothers is lactose intolerant, while my mother and the other two brothers seem able to tolerate lactose well.
Fortunately, though, my family’s lactose intolerance is mild compared to a lot of cases– we get cramps, but have yet to need to go to the emergency room. Maybe we produce a small amount of lactase, but not enough to process the typical amount of dairy products?
I think its really interesting to think about the lactose trait and how it turns off/on. I am lactose intollerant and was always kind of confused why I suddenly was unable to consume milk without problems. It intrigues me that we have traits linked on genes that can turn on and off.