Genetics interact with environment in ways far more complex and subtle than might be readily apparent. The “unimportant” factor might have ramifications not obviously related to its primary expression. Over long time periods those subtle secondary effects can be overwhelming, and not directly genetic.
An example:
Suppose two notional tribes of humans, separated by continental distances, and having no genetic exchanges over millennial time frames. In one group, there happens to be a suite of genetic factors that strongly favor rhythmic and tonal facility, although humans have not yet developed music. The other tribe has several elements of that suite of genes, but not the entire set, which is necessary for the expression of that innate talent. (Hey, not all God’s children go rhythm.)
In group one, over the first few generations, some individuals begin humming, and expressing musical vocal themes. Pre language, but intentional, and emotional. Among such early Hummers, are women of childbearing years. In the other group, no such phenomenon occurs. The humming moms, like most moms, spend a lot of time with their newborns, only these moms hum to their babies. Their children listen. The non humming moms in the second tribe also make some noises to their infants, but not so often, and not sustained for as long as those which are musical in nature. (an unobvious, but distinctly likely consequence of musical expression.)
Now for a bit of ECD pedantry. Infants exposed to interactive verbal communication over long periods during the first eighteen months of life develop more thoroughly interconnected brain pathways than do those who do not experience such stimulation. (Actually, you can make that infant vertebrates of any species, but for now, let’s concentrate on humans.)
So, now tribe one, let’s call them the Hummers, has had three generations of humming moms, and the same number of generations have come in the Mumers. A thing begins to happen among the Hummers that does not happen among the Mumers. Statistically, highly developed brain functions are more common among the Hummers. These people will become more populous, in comparison because they are led or influenced by more intelligent members. And the members of the tribe will become more highly regarded when they move into nearby groups. That means that suite of genes which started the whole thing will be reproduced more widely, and survive longer.
Then synergy really gets a boost, when one of these smart Hummers starts teaching the tribe to hum together. A bunch of folks humming the same theme is a powerful, and mystical thing, in the earliest days when such things were possible. And, it makes the number of humming moms very high, statistically. It also creates a social pathway from observing infant, to participating tribe member. Lots of good social things happen. Ten generations later, and the Hummers are now thirty tribes, over a subcontinental region, and growing quickly in numbers. The suite of genes which gave us music has joined up with the suite of genes which gave us speech, and one day, someone starts to actually sing.
After that, the Mummers are not safe for long. The Singers now include verbal language in their entire developmental experience, from infancy. Language is given a boost beyond anything that just language itself is able to provide. And then some Singer with a good memory lives a very long life. Now you have Sagas, and in a couple more centuries, you have History. And by the way, that millennium I mentioned earlier is now passed, and guess what?
The Singers have spread over the entire continent, and absorbed the Mummers without even a verse.
Tris