I think of myself as someone who keeps those who are interested in Indo- Europeans and Ancient DNA up-to-date on developments, and up until a few years ago that was true-ish. But it is all happening and changing so quickly these days that I would have to devote much more of my life to it than I choose to. Yet I still listen and hear interesting things, so I will link to those and bring up some points that intrigued me.
Dwarkesh Patel has a lengthy interview with David Reich about what has been happening the last few years, in particular a paper headed by Avi Akbari that has been discussed in preprint for three years and now out as a paper in Nature. Why the Bronze Age was an Inflection Point in Human Evolution. I am not symmarising that paper but am drawing from it. Razib also makes extensive reference to it in his 10,000 years of selection in Western Asia monologue and his interview with Greg Cochran. I'm reading the new JP Mallory, just read Proto, and I'm no summarising any of those either. For openers, I'm not sure I understand any of them well enough to get them right. But here are some interesting bits.
You will hear it asserted that there has been very little selection over the last 50,000 years in the human genome, and the differences between us then and now are small. You will also hear that selection has been intense and recent, creating significant group changes. Both are true in their way. 98% of the genome isn't moving much. 2% is under intense evolutionary pressure.
Immunity tends to be the area of greatest change. When groups interbreed, the variants that keep people alive have the biggest effect, withnlittle explanation needed. The genes tend to be fewer, or even individual, such as genes for Down Syndrome, Tay-Sachs, or Cystic Fibrosis. Africans have an enormous number of variants offering partial resistance to malaria and other insect-borne diseases. They would have fullresistance, but the diseases change their chemistry to live off humans, and it is an ongoing competition. The studies that show the various blood types or those with more Denisovan ancestry having more covid resistance are remnants of those same battles a quarter-million years ago.
Behavioral and psychiatric traits are more polygenic, so while they are also under heavy selection compared to the 98% of the genome, it is less strong than immune selection. Cognitive ability, height, and BMI are more polygenic still. There are a thousand places to be a little taller or smarter, and a hundred versions of slight improvement at each place. There is no overall tall gene except for diseases like acromegaly, which kills you early, like Andre the Giant.
Oddly, the selection for the cognitive traits has not been strongest in the last 2000 years, but the 2000 years before that. The genes which correlate with educational attainment now became more concentrated long before anyone had formal education. But those genes also correlate with age of first birth for women and value of grave goods for men, which one can see might be associated with better planning and better self-control in both social and financial situations. The biggest changes were 2-4000 y/a, the second biggest 4-6,000 y/a, but our last 2000, which one would think held the big ticket items for cognitive traits, are not as dramatic. There is a lot more than in most of human history, but not as dramatic as the move from the Bronze Age to the Iron Age. The researchers expected to see the biggest changes with the beginning of agriculture, but the genetic changes did not happen until well after. There are signals that it was not the beginning of agriculture but the final end of foraging in a society that put the pressure on. The genetic patterns needed for foraging held on in selection long after they disappeared in the skeletal and burial evidence. As long as they were still needed occasionally, they persisted.
We picture tribes moving into a colder place because they have slightly better adaptation to cold, but that is cart before horse. They move first, then adaptations slowly come in. Tribes are likely driven by necessity to keep on milk longer, eat riskier food, or move into the cold before they are ready. Desperation is the driver, not slight advantage. At that point, a hundred tiny improvements might keep one person alive a little longer, and those accumulate over time. It's not a single big improvement, but a slight one, then a slight improvement in that.
A lot of extra material that has nothing to do with cold or dairy comes along as well. If someone has a mutation of slight advantage, when the DNA helix folds on itself, it doesn't do it only at that one spot. a bunch of other genes on either side of it get pulled in as well. It can be tough to tell which of the hundred SNPs in a chunk are the advantageous ones and while are only along for the ride, or even deleterious for some other trait. From a single ancestor, each child and each grandchild will have that helical twist at a different place, but it will still take generations to isolate the one with 1% cold tolerance advantage, and six in the area that increase the likelihood of curly hair will be associated accidentally.
Eh, I'm done for now. That's only one small point and I've taken forever. I have just demonstrated by bad example why teachers who can package such things clearly and compactly onto a greenboard or into a single analogy are so valuable.
Whatever isn't clear, ask. I might be able to answer rather than sending you to the pros to do your own research.
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