[tsimionescu]

While I agree it makes it very hard to test some of these hypotheses, "use it or lose it" is very clearly true of at least some bodily and cognitive capacities. So it's not unfair to posit it could apply to these as well.

If you think quadruped walking is only a bunch of reflexes and human walking isn't, that still doesn't completely negate my point. Primate infants also display significantly more agility than human babies right afer birth, and "learn" to walk significantly faster.

Here [0] is a study for example that finds that the age of walking is 94% predictable for any mammalian species that walks on the ground by a particular ratio between the mass of the infant brain and the adult brain (that is, an infant animal essentially learns to walk when its brain reaches a certain percentage of its final brain size).

Besides this type of animal comparison, another signal we can use to distinguish between built-in capacities and things we learn from experience is to see whether a capacity can be developed later in life. For example, if you lack visual stimuli in the early part of brain development, even if you get it later, you will never develop vision (we know this from studies on kittens...).

[0] https://www.pnas.org/doi/full/10.1073/pnas.0905777106