Got milk?

I hope you readers have a good 2007.

While at dinner with family over the holidays, I observed a little something. Now, if you take a pack of wild wolves (or gorillas or something else), if they obtain a meal, there is a fight for the tastiest/best morsels of food, and the least desirable food is left for last. If a solitary animal finds food, it too gobbles up the best part of the meal, saving or leaving the less desirable rest.

It probably was the same for the hunter-gatherer humans of old. Find food, eat the best, and leave the rest for later. Probably, as food security increased, humans became less worried about not getting the tastiest morsels, and so were less worried about gobbling food up. Well, I’ve been domesticated to such an extent that I usually save the tastiest portions of the meal for last, so that I can relish it at leisure, savoring the smell and taste, taking in small bites, and allowing it to linger.

I wouldn’t last a day in the wild.

Anyway, coming to probably a more interesting result of natural selection; agriculture and domestication of animals dramatically changed human life, and certain traits have rapidly evolved in humans due to this. One of them is a tolerance for lactose, and the ability to drink milk as an adult. Most mammals drink milk as infants, but are then weaned away from milk. However, many of us drink milk with out any problems as an adult. The reason for this is that lactase phlorizin hydrolase (LPH), the enzyme that breaks down lactose (the major sugar in milk) in to glucose and galactose, is expressed in us as adults. In most mammals, the expression of this enzyme is shut-down or repressed as adults.

But many, many people in the world are lactose intolerant, and cannot handle milk. Lactose intolerance is high among Africans, Native Americans and East Asians. Lots of researchers have tried to find out the genetic reasons for lactose intolerance, and varying degrees of lactose intolerance in populations. The gene that codes for LPH is long known, and researchers surprisingly did not find too many differences or mutations in the coding region of the gene (the part that actually becomes a functional protein) across populations. About four years ago, a group found a single specific mutation (a single nucleotide polymorphism, where a cytosine in the DNA was mutated to a thymine) in a completely different gene, MCM6, in Finnish populations that surprisingly had an effect on the expression of LPH, and therefore lactose tolerance. Since then, it is widely believed that this mutation is what gives Europeans lactose tolerance.

However, this mutation was not as prevalent in southern European or Middle Eastern populations, and very rare in African populations, which were also lactose tolerant. So, could there be other mutations that also cause tolerance to lactose? A group of researchers examined over forty African ethnic groups, and for starters searched in the same general region as the earlier mutation, and found three new variants all of which were associated with lactose tolerance. Additionally, their study also showed different mutations spread across different populations, with more pastoral populations having much stronger lactose tolerance, and a clear positive selection for one of the mutations (that provided lactose tolerance).

This study is a really nice example of convergent evolution . One can imagine a tremendous benefit to humans who acquired lactose tolerance. Once animals were domesticated, the meat was an obvious source of food, but the milk was a ready and highly nutritious (and sustainable) source of food as well. So, it clearly benefits pastoral societies to develop tolerance for milk at adulthood. In order to obtain the same outcome (lactose tolerance) nature came up with different was to achieve it. Different mutations all lead to lactose tolerance.

(This reminds me of my school days, where we had to prove theorems in class, in physics or mathematics. We knew the answer (g=9.8 m/s2), but some of us came up with extremely innovative methods to obtain that solution. Beautiful convergent evolution).

This study is really quite nicely done, and suggests how much variation is likely for what were likely “important” needs under strong natural selection. There’s one large population of people closer to home that I’m betting is going to have a lot of variation for SNPs determining lactose tolerance. South Asians. Dairy products are an essential part of every meal (as ghee, yogurt, milk, and every dessert), and India has more cows and produces more milk than any other country. Given the sheer diversity of populations in India, with differences in dietary habits, I’m very curious to know if a majority of the population carries a single mutation conferring lactose tolerance, or if (just like the country itself) there is tremendous diversity in mutations that provide tolerance.

(Read the original research paper here (Nature Genetics - 39, 31 - 40 (2006) )