TL;DR: (1) brain shape has not changed for about 160k years. (2) The framing/facts of the discussion is bad. Mutations are happening all the time, it is 'natural selection' that seemingly made lactose tolerance more prevalent in the last 20k years.

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Long answer:

## Natural Selection / Lactose Tolerance (as an example of a very recent adaptation) / Why the discussion is so far incorrect

In my first reply, I notice now a big flaw. Having an adaptation be present in a population for 6000 years does not mean it took that time to evolve it. (According to [3], the adaptation has become prevalent in 20k years rather 6k)

To frame what we are discussing, I hope we can all agree:
mutations are happening all the time and in aggregate we each individually carry a vast quantity of genetic differences/mutations apart from every other individual.

For lactose tolerance, really what we are talking about is more natural selection. At least I think we are. As an example, a population can change very quickly via natural selection if an event kills off everyone that is missing a mutation. That perhaps 1% mutation suddenly becomes the surviving population. (For completeness, I'll mention that this process can happen more slowly as well over time, but it can depend on single mutations, aggregates of mutations, and environment and random luck [eg: asteriod] are all factors). This is to say, there could have been plenty of lactose tolerant people well before. This NPR piece on the history of lactose tolerance states it well [3] "But now that doesn't happen for most people of Northern and Central European descent and in certain African and Middle Eastern populations. This development of lactose tolerance took only about 20,000 years — the evolutionary equivalent of a hot minute — but it would have required extremely strong selective pressure."

What's more though too, nothing is to say that natural selection always works to favorably select genes. EG: The village idiot might be the only one immune to the plague. It's way complex of course since there's so much variation between every individual, but I just wanted to underscore that natural selection is a function of individual, time & place. Sometimes some mutations are useful, other times they are not and are dumb luck of what is left over from some time before.

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## Brain Size & Human Cognition

With the issue of natural selection out of the way, what I do wonder is how long it took for the brain to get the way it is today. According to this resource, The Smithsonian [1], the answer is approx 7M years, with most of that happening in the last 2M years until 200k years ago. That is in terms of 'size' (does not account for wrinkles).

According to 'newscientist' [2], the shape (now talking wrinkles here) of the human brain today is very similar to what it was 160k years ago, and has not really changed since. The resource mentions that the biggest changes since then have been in how our faces look rather than how our brains are shaped. Those changes are specifically smaller jaws: "Faces in modern humans are far smaller, with subtler indentation, than those of their ancestors. Studies show that this change accelerated when hunter-gatherers became agriculturalists around 12,000 years ago and ate softer foods, probably due to less loading on the skull from chewing."

Summing up, given that brain shape has been about the same for the last 160k years; it's plausible that is how far it goes back for people that felt & thought about exactly the same way we do today. If anyone knows about more research around the rise of human cognition on this topic, I'd certainly like to learn more.

[1] https://humanorigins.si.edu/human-characteristics/brains

[2] https://www.newscientist.com/article/2331652-shape-of-human-...

[3] https://www.npr.org/sections/thesalt/2012/12/27/168144785/an...