If there’s a change to human brains that human-evolution could have made, but didn’t, then it is net-neutral or net-negative for inclusive relative genetic fitness. If intelligence is ceteris paribus a fitness advantage, then a change to human brains that increases intelligence must either come with other disadvantages or else be inaccessible to evolution.
You’re assuming a steady state. Firstly, evolution takes time. Secondly, if humans were, for example, in an intelligence arms-race with other humans (for example, if smarter people can reliably con dumber people out of resources often enough to get a selective advantage out of it), then the relative genetic fitness of a specific intelligence level can vary over time, depending on how it compares to the rest of the population. Similarly, if much of the advantage of an IQ of 150 requires being able to find enough IQ 150 coworkers to collaborate with, then the relative genetic fitness of IQ 150 depends on the IQ profile of the rest of the population.
An example I love of a helpful brain adaptation with few downsides that I know of, which hasn’t spread far throughout mammals is one in seal brains. Seals, unlike whales and dolphins, had an evolutionary niche which caused them to not get as good at holding their breathe as would be optimal for them. They had many years of occasionally diving too deep and dying from brain damage related to oxygen deprivation (ROS in neurons). So, some ancient seal had a lucky mutation that gave them a cool trick. The glial cells which support neurons can easily grow back even if their population gets mostly wiped out. Seals have extra mitochondria in their glial cells and none in their neurons, and export the ATP made in the glial cells to the neurons. This means that the reactive oxygen species from oxygen deprivation of the mitochondria all occur in the glia. So, when a seal stays under too long, their glial cells die instead of their neurons. The result is that they suffer some mental deficiencies while the glia grow back over a few days or a couple weeks (depending on the severity), but then they have no lasting damage. Unlike in other mammals, where we lose neurons that can’t grow back.
Given enough time, would humans evolve the same adaptation (if it does turn out to have no downsides)? Maybe, but probably not. There just isn’t enough reproductive loss due to stroke/oxygen-deprivation to give a huge advantage to the rare mutant who lucked into it.
But since we have genetic engineering now… we could just give the ability to someone. People die occasionally competing in deep freediving competitions, and definitely get brain damage. I bet they’d love to have this mod if it were offered.
Also, sometimes there are ‘valleys of failure’ which block off otherwise fruitful directions in evolution. If there’s a later state that would be much better, but to get there would require too many negative mutations before the positive stuff showed up, the species may simply never get lucky enough to make it through the valley of failure.
This means that evolution is heavily limited to things which have mostly clear paths to them. That’s a pretty significant limitation!
You’re assuming a steady state. Firstly, evolution takes time. Secondly, if humans were, for example, in an intelligence arms-race with other humans (for example, if smarter people can reliably con dumber people out of resources often enough to get a selective advantage out of it), then the relative genetic fitness of a specific intelligence level can vary over time, depending on how it compares to the rest of the population. Similarly, if much of the advantage of an IQ of 150 requires being able to find enough IQ 150 coworkers to collaborate with, then the relative genetic fitness of IQ 150 depends on the IQ profile of the rest of the population.
An example I love of a helpful brain adaptation with few downsides that I know of, which hasn’t spread far throughout mammals is one in seal brains. Seals, unlike whales and dolphins, had an evolutionary niche which caused them to not get as good at holding their breathe as would be optimal for them. They had many years of occasionally diving too deep and dying from brain damage related to oxygen deprivation (ROS in neurons). So, some ancient seal had a lucky mutation that gave them a cool trick. The glial cells which support neurons can easily grow back even if their population gets mostly wiped out. Seals have extra mitochondria in their glial cells and none in their neurons, and export the ATP made in the glial cells to the neurons. This means that the reactive oxygen species from oxygen deprivation of the mitochondria all occur in the glia. So, when a seal stays under too long, their glial cells die instead of their neurons. The result is that they suffer some mental deficiencies while the glia grow back over a few days or a couple weeks (depending on the severity), but then they have no lasting damage. Unlike in other mammals, where we lose neurons that can’t grow back.
Given enough time, would humans evolve the same adaptation (if it does turn out to have no downsides)? Maybe, but probably not. There just isn’t enough reproductive loss due to stroke/oxygen-deprivation to give a huge advantage to the rare mutant who lucked into it.
But since we have genetic engineering now… we could just give the ability to someone. People die occasionally competing in deep freediving competitions, and definitely get brain damage. I bet they’d love to have this mod if it were offered.
Also, sometimes there are ‘valleys of failure’ which block off otherwise fruitful directions in evolution. If there’s a later state that would be much better, but to get there would require too many negative mutations before the positive stuff showed up, the species may simply never get lucky enough to make it through the valley of failure.
This means that evolution is heavily limited to things which have mostly clear paths to them. That’s a pretty significant limitation!