File this under sometimes the obvious turns out to be true. Or maybe, you can get a research grant for anything. Researchers have shown that the Himba, a seminomadic people living in a remote region of northwestern Namibia can play one of these things is not like the others with objects having either curved or straight edges.
The Himba have little exposure to manufactured regularly shaped objects, and the researchers wanted to see if they perceive what they call "non-accidental properties"* of an object the same way as westerners. If so, the ability to distinguish straight lines and curved edges is built-in to the brain.
They do, so it is.
*They say non-accidental properties are those that don't change when the object rotates in space, such as whether the edges are curved or straight. This is distinguished from what they call metric qualities, such as the degree of curvature, which can apparently change when an object is rotated in space.