The shapes the eye makes

Heinrich Klüver, cataloguing what people reported seeing in altered states, found the imagery fell into a handful of recurring geometric types he called form-constants: lattices and honeycombs, cobwebs, tunnels and funnels, and spirals. They are generated by the visual system itself — by the geometry of the mapping between retina and cortex — which is why they show up under mescaline, migraine, pressure on the eyeball, flicker, fasting and near-sleep alike, in people with nothing else in common.

David Lewis-Williams and others have argued that this is why the same abstract motifs recur in Palaeolithic rock art across unconnected continents.

This is the strongest reading on the shelf, and it beats the ancient-astronaut one on its own ground. If the double spiral is a shape the nervous system produces, then unrelated cultures keep drawing it because they keep seeing it — no contact required, no visitors required. That makes the convergence more interesting rather than less, and it puts the shape where it is most surprising to find it: in us. Two plainer readings also deserve their place: a helix is the shape of anything twisted — rope, vine, umbilical cord — and Jung would say the same thing in his own vocabulary.