Reddit reviews The Puzzle of Left-handedness

>handedness isn't uniquely human

Why does that pose a problem to the theory u/cannibalismo references? That theory doesn't purported to explain the origin of handedness in humans, only the origin of the proportional bias specifically favoring right-handedness.

"Handedness" (or "footedness") is prevalent in other animals, yes, but the sources I've encountered (here's one, for example) suggest that, at the species level, there usually isn't a clear statistical bias favoring one side of the body or the other—i.e. the proportion of 'righties' vs. 'lefties' tends to be even mixed across the population as a whole.