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:
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.