Thread by Adam Rutherford

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  • Jul 27, 2022
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MILK TIME đŸ„›! Have you ever considered that it’s a bit weird that you drink the milk of another animal, and it’s totally normal? Milk is a billion-dollar industry.

A *HUGE* new human evolution paper just dropped, upending one of the great modern evolutionary ideas. đŸ§” 1/n
This paper is possibly the biggest crossover event since Avengers Infinity War, from m’colleagues @mt_genes, @mendel_random @ogu_bristol (follow them), Richard Evershed Melanie Roffet-Salque

đŸ§” by me as a proxy for the authors 2/n
The story until today: many people around the world drink milk from other animals throughout their lives. Though seemingly normal, they are unusual in that regard, because most people in history didn’t drink milk after weaning without tummy troubles. 3/n
We started using milk from other animals around 10k years ago, before the emergence of this ability. It all comes down to a mutation in a gene that enables the digestion of a sugar called LACTOSE 4/n
People who have this mutation are Lactase Persistent (LP). Without it, you are LNP, and could be Lactose Intolerant. 5/n
Everyone can digest lactose when they’re a nipper, but the gene typically switches off by the age of 5. The LP mutation has increased in frequency very rapidly became extremely common, to the extent that most Europeans are LP 6/n
[Sidenote 1: 
as indeed are many populations around the world who have dairy pastoralism in their ancestors. Something that Neo-Nazis failed to recognise when they started chugging milk to prove their racial purity.] 7/n
[🧬 Sidenote 2: the typical mutation is actually in an enhancer for the gene LCT, 13,000 base pairs up-stream from the gene itself, the enzyme that breaks down lactase is unaltered, but its expression is maintained into adulthood in LP.] 8/n
LP is a textbook (and *ahem* popular science book) example of GENE-CULTURE CO-EVOLUTION. We began farming milk, and a novel genetic mutation allowed us to drink it into adulthood, so rapidly was +ve selected – AND THAT’S WHY YOU CAN HAVE DIARRHOEA-FREE MILK IN YOUR CORNFLAKES 9/n
BUT! The new study in @nature shows that this story is not correct. It’s incredible, 360 work, combining archaeology, chemistry, genetics, epidemiology, and more than 500,000 people living and dead, to reveal the wild complexity of human evolution. 10/n
go.nature.com/3S85w31
METHODS
* Detection of 6899 animal fats on ancient 13,181 potshards from 550 sites
* DNA from 1,786 long-dead people on when/where the LP gene was in the past

#TOTALBIOLOGY
METHODS
* Epidemiology to relate LP to milk consumption/health outcomes
* Modelling on environmental factors driving LP evolution / natural selection
12/n
RESULTS
* It turns out that milk consumption was common for thousands of years *before* the LP gene spread significantly: so selection of this gene is unlikely to be due to its positive effects because it allowed individuals to consume more milk.
13/n
* Epidemiology of a cohort of 500,000 contemporary Europeans in UK Biobank reveals that LP people don’t drink much more than the lactase non-persistent, and there are no major differences in symptoms and health outcomes between LP and LNP individuals
* It wasn’t milk use that drove the spread of LP. Instead, it looks like threats of famine, and infectious agents statistically explain where and when the LP gene took off.
It wasn’t that dairy emerged and spread as a result of the LP mutation bestowing benefits to the drinkers. Milk drinking was common in LNP people, and had little detrimental effects *until* periods of famine or increased pathogen exposure 16/n
In malnourished people, lactose-induced diarrhoea can shift from not being evident or at most an inconvenience to a fatal condition rapidly, what the authors call a ‘crisis mechanism’. The same goes for pathogen exposure as a ‘chronic mechanism’ 17/n
So, those who teach human evolution - or are merely interested in the greatest story ever told - have to rewrite your lectures. 18/n
I fucking LOVE this paper. If anything, it shows that even our well evidenced theories of human adaptation and selection are often reductively naĂŻve. 19/n
LP is one of the areas of human evolution that we know best, and yet our best explanations so far turned out to be hugely simplistic, if not wrong.

Stunning work by @mt_genes, @mendel_random (who are very responsive here), Richard Evershed, Melanie Roffet-Salque et al.
20/20
21/20, non-paywalled link
go.nature.com/3vHVEn5
22/20 A very useful News and Views by @ShevanWilkin
go.nature.com/3oAVREA
23/20
The Telegraph decided to go for the work angle: Fine, apart from all the times they shat themselves to death so hard hard that LP became the norm.
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