Naked Science Forum
General Science => General Science => Topic started by: thedoc on 06/11/2012 17:18:29
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Cows milk is a remarkable food – not only for drinking, but for cheese and butter, yoghurt and cream. So when did we first decide to use cows as walking larders to fulfil our nutritional needs?
Read a transcript of the interview by clicking here (http://www.thenakedscientists.com/HTML/content/interviews/interview/1000014/)
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I find it interesting that there could be such a widespread genetic adaptation to a single food.
I looked up maps on lactose intolerance. There are a few different maps on the internet.
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2F4.bp.blogspot.com%2F_LH_BweOJx_E%2FTTmGnSwPwII%2FAAAAAAAAAGE%2FjNieRVTLbUQ%2Fs1600%2FLactose%252Bintolerance.png&hash=f7d69fc1cefd0c9471b475750607ce00)
http://positivelyblackswan.blogspot.com/2011/01/recent-ancestry-deep-ancestry.html
What I would wonder is whether a large portion of the lactose intolerance is due to a lack of access to milk, and thus no reason to maintain the enzymes.
So, a family that had access to cow's milk, or other milk products may not have the same issues.
Cheese & butter, of course, also gives us a variety of foods (which humans crave), but also a good way to preserve the milk.
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Lactose intolerance after weaning is normal in all mammalian species, perhaps to reduce energy consumption producing unnecessary lactase enzymes (as CliffordK suggests). It is believed that this was the normal condition in all early human populations, prior to the development of agriculture.
Lactose persistence is genetically dominant, and can be enabled by changing as little as a single DNA letter (a SNP: Single Nucleotide Polymorphism) in a promoter region for the Lactase enzyme; several independent mutations are known: http://en.wikipedia.org/wiki/Lactase_persistence#Genetics
From the interview:
within about a thousand years a gene evolves which allows people to tolerate milk so we've become lactose persistent
Re Terminology: You often hear this loosely expressed like "a gene evolves which allows people to digest milk" (or "for bacteria to digest citrate" in the case of the Lenski experiment). Both of these would involve the evolution of a large and complex enzyme (1927 amino acid units/5781 nucleotides, in the case of the lactase protein-coding region: http://en.wikipedia.org/wiki/Lactase#Structure_and_biosynthesis).
However, what we have here is a mutation which disables a regulatory function for an existing enzyme. This disabling can occur due to a point mutation, which is far more likely to occur in a short amount of time, compared to the evolution of a new enzyme.
I guess this confusion is propagated in the general media because when the reporter hears "a new gene", they cannot conceive (and hence cannot convey) the huge difference between "a new enzyme" and "a tweaked promoter for an existing enzyme", so the audience just assumes the former.
It's important for science communicators to be clear about such differences.
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I can imagine the benefits of lifelong milk and calcium consumption for the elderly (post child bearing years).
However, does milk consumption make that much of a difference for children and young adults? How common is calcium deficiency Rickets (http://en.wikipedia.org/wiki/Rickets) (usually caused by Vitamin D deficiency)?
I guess I'm having troubles imagining what could drive the selection process for a single enzyme (or enzyme regulation mechanism). It is easy enough to avoid a "toxic" food.
Could cowpox have helped select for the persistence of the lactase enzyme?
I.E. Those individuals with lactase persistence would have a higher likelihood to be dairy farmers, and could have naturally received resistance to smallpox. And, thus natural selection for dairy farmers.
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Differences in height, weight and fractures were seen when genetics of lactase persistance were considered in a health study: http://en.wikipedia.org/wiki/Lactase_persistence#Evolutionary_advantages
I wonder if another advantage would be in preserving the life of babies whose mothers had died or were too malnourished or thirsty to breastfeed? It's not a big conceptual jump from "I can't feed this baby milk" to "That cow has milk!".
I imagine such children would eventually stop drinking cows milk when lactase production declined, and they started to get an upset stomach.
However, a child with a lactase preservation mutation would happily continue to drink cows/goats/camel milk, even when other people in their tribe were suffering malnutrition or thirst. The fact that even one copy of the gene is very beneficial (ie it is a dominant gene) allows it to spread rapidly in a population.
I think that the biggest jump would be from "That cow has milk!" to "Ah, now I have the milk"!
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Thanks,
I should have known there would be a wikipedia article.
Ashkenazi Jews can keep 20–30 percent of their ability to digest lactose for many years. Of the 10% of the Northern European population that develops lactose intolerance, the development of lactose intolerance is a gradual process spread out over as many as 20 years.
So, this is likely more about preserving the ability of older children and adults to also drink the milk.
It is a good point that humans may have tried to milk goats, cows, camels, and other newly domesticated species to help save the lives of orphans, or children of mothers who had insufficient milk, especially as childbirth was very dangerous in the past.