I just curious how you can support your point about squirrels, besides you falling prey of the bias of trying to advance an argument with a sole point.
For example, can these squirrels ancestors had better nutrition than the average squirrel, can we know about that just after seeing they mastering fire or wondering if they could learn calculus? I believe this type of question, the relation between nutrition and intelligence, can be answered much easily than searching for genes and looking for some desirable psychological trait in there, this can be easily tested in animals as well in a large couple of generations.
Either way I am not a biologist or even an psychologist so this is my take as a mathematician/statician. Also I'm not so confident science will ever come as close to explaining this completely.
My point is that humans are much more intelligent than squirrels. Is this because of environment? Not at all, it is because we have the genetic programming to develop huge brains. The difference in intelligence between humans and squirrels is determined almost entirely by heredity.
The reason I point this out is that it is very common to see comments about heredity not being a factor in human intelligence. It may well be that heredity does not explain differences in intelligence between two humans, or it may be that heredity almost completely explains any differences in intelligence among humans. Science will eventually figure out the truth of the matter. However, heredity plays a very dominant role in determining intelligence, the only question is whether humans are all so genetically similar with regards to intelligence that environmental factors dominate genetic factors.
EDIT: The way I phrased the first paragraph sets up a false choice. Intelligence differences among humans could be hereditary, environmental, or most likely a complex combination of the two.
In the comments of this post however I saw exactly the same point you're saying in your first paragraph, maybe in other discussion forums people denying this is more common.
For example, can these squirrels ancestors had better nutrition than the average squirrel, can we know about that just after seeing they mastering fire or wondering if they could learn calculus? I believe this type of question, the relation between nutrition and intelligence, can be answered much easily than searching for genes and looking for some desirable psychological trait in there, this can be easily tested in animals as well in a large couple of generations.
Either way I am not a biologist or even an psychologist so this is my take as a mathematician/statician. Also I'm not so confident science will ever come as close to explaining this completely.