Why are radio buttons circles?

I'd think it comes from paper:

check any:

[ ] choice

[ ] choice

[ ] choice

check answer number x:

(1) choice

(2) choice

(3) choice

Squares for checkboxes, circles for arround number choices

So I'd argue circles go better with enumaration. Square is better chan circles for checkboxes, because it's more obvious that it is a frame when something can be placen inside, while a circle can more easily be mistaken for a stand-alone shape.