The evidence I have mentally cached is brain size. The evolutionary trajectory of brain size is relatively easy to measure just by looking at skulls from archaeological sites, and IIRC it has increased steadily through human evolutionary history and does not seem to be in evolutionary equilibrium.
(Also on priors, even before any evidence, we should strongly expect humans to not be in evolutionary equilibrium. As the saying goes, “humans are the stupidest thing which could take off, otherwise we would have taken off sooner”. I.e. since the timescale of our takeoff is much faster than evolution, the only way we could be at equilibrium is if a maximal-under-constraints intelligence level just happened to be exactly enough for humans to take off.)
There’s probably other kinds of evidence as well; this isn’t a topic I’ve studied much.
If humans are the stupidest thing which could take off, and human civilization arose the moment we became smart enough to build it, there is one set of observations which bothers me:
The Bering land bridge sank around 11,000 BCE, cutting off the Americas from Afroeurasia until the last thousand years.
Around 10,000 BCE, people in the Fertile Crescent started growing wheat, barley, and lentils.
Around 9,000-7,000 BCE, people in Central Mexico started growing corn, beans, and squash.
10-13 separate human groups developed farming on their own with no contact between them. The Sahel region is a clear example, cut off from Eurasia by the Sahara Desert.
Humans had lived in the Fertile Crescent for 40,000-50,000 years before farming started.
Here’s the key point: humans lived all over the world for tens of thousands of years doing basically the same hunter-gatherer thing. Then suddenly, within just a few thousand years starting around 12,000 years ago, many separate groups all invented farming.
I don’t find it plausible that this happened because humans everywhere suddenly evolved to be smarter at the same time across 10+ isolated populations. That’s not how advantageous genetic traits tend to emerge, and if it was the case here, there are some specific bits of genetic evidence I’d expect to see (and I don’t see them).
I like Bellwood’s hypothesis better: a global climate trigger made farming possible in multiple regions at roughly the same time. When the last ice age ended, the climate stabilized, creating reliable growing seasons that allowed early farming to succeed.
If farming is needed for civilization, and farming happened because of climate changes rather than humans reaching some intelligence threshold, I don’t think the “stupidest possible takeoff” hypothesis looks as plausible. Humans had the brains to invent farming long before they actually did it, and it seems unlikely that the evolutionary arms race that made us smarter stopped at exactly the point we became smart enough to develop agriculture and humans just stagnated while waiting for a better climate to take off.
I do agree that the end of the last glacial period was the obvious immediate trigger for agriculture. But the “humans are the stupidest thing which could take off model” still holds, because evolution largely operates on a slower timescale than the glacial cycle.
Specifics: the last glacial period ran from roughly 115k years ago to 12k years ago. Whereas, if you look at a timeline of human evolution, most of the evolution from apes to humans happens on a timescale of 100k − 10M years. So it’s really only the very last little bit where an ice age was blocking takeoff. In particular, if human intelligence has been at evolutionary equilibrium for some time, then we should wonder why humanity didn’t take off 115k years ago, before the last ice age.
In particular, if human intelligence has been at evolutionary equilibrium for some time, then we should wonder why humanity didn’t take off 115k years ago, before the last ice age.
Yes we should wonder that. Specifically, we note
Humans and chimpanzees split about 7M years ago
The transition from archaic to anatomically modern humans was about 200k years ago
Humans didn’t substantially develop agriculture before the last ice age started 115k years ago (we’d expect to see archaeological evidence in the form of e.g. agricultural tools which we don’t see, while we do see stuff like stone axes)
Multiple isolated human populations independently developed agriculture starting about 12k years ago
From this we can conclude that either:
Pre-ice-age humans were on the cusp of being able to develop agriculture, and an extra 100k years of gradual evolution was sufficient to bump them over the relevant threshold
There was some notable period between 115k and 12k years ago where the rate of selective pressure on humans substantially strengthened or changed direction for some reason. Which might correspond to a very tight population bottleneck:
The evidence I have mentally cached is brain size. The evolutionary trajectory of brain size is relatively easy to measure just by looking at skulls from archaeological sites, and IIRC it has increased steadily through human evolutionary history and does not seem to be in evolutionary equilibrium.
(Also on priors, even before any evidence, we should strongly expect humans to not be in evolutionary equilibrium. As the saying goes, “humans are the stupidest thing which could take off, otherwise we would have taken off sooner”. I.e. since the timescale of our takeoff is much faster than evolution, the only way we could be at equilibrium is if a maximal-under-constraints intelligence level just happened to be exactly enough for humans to take off.)
There’s probably other kinds of evidence as well; this isn’t a topic I’ve studied much.
If humans are the stupidest thing which could take off, and human civilization arose the moment we became smart enough to build it, there is one set of observations which bothers me:
The Bering land bridge sank around 11,000 BCE, cutting off the Americas from Afroeurasia until the last thousand years.
Around 10,000 BCE, people in the Fertile Crescent started growing wheat, barley, and lentils.
Around 9,000-7,000 BCE, people in Central Mexico started growing corn, beans, and squash.
10-13 separate human groups developed farming on their own with no contact between them. The Sahel region is a clear example, cut off from Eurasia by the Sahara Desert.
Humans had lived in the Fertile Crescent for 40,000-50,000 years before farming started.
Here’s the key point: humans lived all over the world for tens of thousands of years doing basically the same hunter-gatherer thing. Then suddenly, within just a few thousand years starting around 12,000 years ago, many separate groups all invented farming.
I don’t find it plausible that this happened because humans everywhere suddenly evolved to be smarter at the same time across 10+ isolated populations. That’s not how advantageous genetic traits tend to emerge, and if it was the case here, there are some specific bits of genetic evidence I’d expect to see (and I don’t see them).
I like Bellwood’s hypothesis better: a global climate trigger made farming possible in multiple regions at roughly the same time. When the last ice age ended, the climate stabilized, creating reliable growing seasons that allowed early farming to succeed.
If farming is needed for civilization, and farming happened because of climate changes rather than humans reaching some intelligence threshold, I don’t think the “stupidest possible takeoff” hypothesis looks as plausible. Humans had the brains to invent farming long before they actually did it, and it seems unlikely that the evolutionary arms race that made us smarter stopped at exactly the point we became smart enough to develop agriculture and humans just stagnated while waiting for a better climate to take off.
I do agree that the end of the last glacial period was the obvious immediate trigger for agriculture. But the “humans are the stupidest thing which could take off model” still holds, because evolution largely operates on a slower timescale than the glacial cycle.
Specifics: the last glacial period ran from roughly 115k years ago to 12k years ago. Whereas, if you look at a timeline of human evolution, most of the evolution from apes to humans happens on a timescale of 100k − 10M years. So it’s really only the very last little bit where an ice age was blocking takeoff. In particular, if human intelligence has been at evolutionary equilibrium for some time, then we should wonder why humanity didn’t take off 115k years ago, before the last ice age.
Yes we should wonder that. Specifically, we note
Humans and chimpanzees split about 7M years ago
The transition from archaic to anatomically modern humans was about 200k years ago
Humans didn’t substantially develop agriculture before the last ice age started 115k years ago (we’d expect to see archaeological evidence in the form of e.g. agricultural tools which we don’t see, while we do see stuff like stone axes)
Multiple isolated human populations independently developed agriculture starting about 12k years ago
From this we can conclude that either:
Pre-ice-age humans were on the cusp of being able to develop agriculture, and an extra 100k years of gradual evolution was sufficient to bump them over the relevant threshold
There was some notable period between 115k and 12k years ago where the rate of selective pressure on humans substantially strengthened or changed direction for some reason. Which might correspond to a very tight population bottleneck:
source: Robust and scalable inference of population history from hundreds of unphased whole-genomes
Note that “bigger brains” might also not have been the adaptation that enabled agriculture.