Got milk (alleles)?

As you probably know, everyone should drink milk. Lots and lots and lots of milk. All your life. Or so says the American Dairy Industry, often using those sexy posters of famous people with milk smeared on their faces.

The truly amazing thing about those posters is that the people in them more often than not seem to have an ethnic identity that I, as a trained Biological Anthropologist (and thus keeper of this sort of knowledge) can easily see contraindicates milk consumption. Most of these individuals would likely be unable to break down the lactose in the milk because they have the “wild type” or “normal” allele that facilitates the shutdown of lactase production some time in early life.

Now let’s be clear about this. We humans are mammals, and as mammals, we drink mother’s milk while young. This is facilitated by the production of lactase, an enzyme that breaks down the main energy bearing molecule in milk, the sugar lactose. But your basic well adapted mammals should not bother producing the enzyme lactase after weaning normally occurs … maybe a few years after in a long-lived mammal like humans … because it is inefficient and potentially risky to produce enzymes you don’t need.

Why is it inefficient? Well, there are thousands and thousands of enzymes and if we just produced all of them all the time in all our cells, that would be really costly of raw materials and energy, both of which are required to produce them. So, evolution has shaped, via the brilliant designer of Natural Selection, our multicellular bodies to produce enzymes only in the cells they are needed in (from which they may be exuded on occasion, as is the case with lactase, a digestive enzyme). This is much more efficient. By extension, the system should be (and usually is) selected to produce specific enzymes when they are needed instead of all the time from birth to death. By doing this we save a lot of raw materials and energy.

Why is there a risk of producing an enzyme when it is not needed (beyond the risk that comes with wastefulness)? This is because enzymes are the sharp objects … the scissors, the razor blades, the Jarts … of the cell. They are highly bioactive molecules that are supposed to do a certain thing but when they to fall into the wrong hands … who knows what would happen. You’ll poke your eye out running around with an enzyme.

Lactase has no special privilege. It shouldn’t be operating in adult mammals, and I’d bet it generally is not (though I don’t think very many mammals have been checked for this, frankly.)

So if lactase production in adult mammals is weird and unexpected, then why do adult humans produce it? Well, the simple answer to that is THEY DON’T. The vast majority of people by count (numbers of people), by ethnicity (numbers of ethnic groups) by cultures (i.e., numbers of languages, numbers of traditions, etc. etc.) simply don’t. Only a few groups do.

So why do we consider “lactose intolerance” a disease, rather than thinking of “adult lactase production” or “adult lactose fixation” or even “mature nursing on cow juice” to be strange instead of the other way around?

Well, we don’t do that either, I would guess. The inability to digest the lactose in milk is probably not considered at all strange by the average Chinese person, and there are more average Chinese persons than any other single group. It’s a Western thing, mainly. Just another example of Eurocentrism.

There are two main groups of people with this mutation … the adult lactase production mutation …. many (but not all) Europeans, and some (but not many) Africans.

These two groups are associated with a fairly long history of herding cattle and using dairy products from those cattle. Interestingly, some of the most famous cattle herders, such as the Maasai and similar groups in Africa, do not produce lactase in adulthood. They simply process the milk in a way that breaks down the lactose prior to ingestion.

One of the questions surrounding this observation has always been, which came first, the ability to digest milk, or the need to adapt to digesting milk, in adulthood?

I cannot imagine why such a stupid question has always been asked. Maybe because the people asking it are archaeologists instead of biologists (but that is not entirely true) or because they are anti-adaptationists (or, self styled as anti-panglossianists? Well, thank you Stephen Gould for mucking up our thinking on this one …). This is a dumb question because it is probably very rare indeed that an adaptation to NOTHING arises and then the thing to which this adaptation is adapted to comes along and fits the species. Right… survival of the fittest … ah .. niche. I don’t think so.

But nonetheless this has been a question and it now has a preliminary answer:

Joachim Burger of the University of Mainz, Germany, and colleagues worked with Mark Thomas of University College London, UK, to address this riddle by studying DNA from skeletons scattered throughout Europe. The team examined ten skeletons ranging in age from 3,800 to nearly 6,000 years old. The skeletons were discovered at archaeological sites in Germany, Hungary, Poland and Lithuania.

Cool. They conclude that the adaptation post dates the date they believe cattle were being raised by these people.

Burger and his colleagues say this supports the dominant theory on how milk drinking evolved — that milk-drinking mutations were uncommon before the practice of dairying began. Then, when humans learned to herd cattle, the milk-drinking mutations spread rapidly, because they conferred a huge advantage on those who had them — perhaps due to the extra protein and fats available in cow’s milk, the team speculates.

This is in the current issue (or forthcoming depending on when you read this) of PNAS, and is summarized nicely in Nature News.

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