Some curiosities to add: Falling dreams. Rock-a-bye baby in the tree top. The sensation of falling that jolts me awake even though I wasn’t quite asleep. The relaxed sleeping state of the body that seems to mitigate injury when falling out of bed (though this is anecdotal, so grain of salt and all that).

As someone who is not afraid of heights, I only ever get the scared feeling of peering off an edge when I don’t have adequate points of contact to something stable (three is usually enough). Of course, I used to climb to the tops of 20 to 30-foot trees when I was little — and that, I think, is the difference. If we slept in trees, or at the very least had to spend time in precarious places, the fear of heights would be overcome; the brain can learn how to handle it.

In fact, because this is such an innate fear — and because it’s a fear, period — I doubt it comes from the human part of the brain, or the primate part, or even the mammal part, but from our lizard brain. I wouldn’t be surprised if a mouse had falling dreams, or an elephant, or a whale, or even birds. Evolution (without agency, mind you) works by pitting contradictory advantages against each other. The curiosity that killed the cat is precisely what enabled the species to survive. Similarly, the overcoming of fear of heights that I believe pervades any brain with a modicum of decision-making was necessary for the survival of many of our ancestors and probably should not be thought of as an innate affinity, but something learned, even if at a very young age.

We’d be more comfortable with heights as a species if we had to be.