We saw a paper years ago by a bod in a neuorology journal where he claimed to have identified some of the "visual primitives" for the human vision system. Seems our vision system has pattern recgonition of some simple shapes hardwired into the firmware/hardware, the claim was that hypnogogic displays and certain types of hallucinatory states could stimulate direct perception of these shapes. He was actually pushing the idea that the shapes turned up in "primitive" art (of the paint-on variety), but, if memory serves, many of the primitives would be familiar to rock art afficionardos.

It's not too hard to believe, humans definitely have face recognition hardwired in (some brain injuries can remove this, with serious consequences) and that is a much more complex task than simple geometrics.
If you are still not convinced, you may like consider why we can seemingly identify familiar shapes in clouds, of flames in the fire.

Mind you, the guy that carved the pudding basin with squid must have had some serious chemical assistance.......

Maggie & Keith