Full Idea
The roundworm only has 959 cells, and 302 neurons in a fixed wiring diagram; it eats, mates, approaches and avoids certain smells, and that's about it. This makes it obvious that human 'free' behaviour comes from our complex biological makeup.
Gist of Idea
Roundworms live successfully with 302 neurons, so human freedom comes from our trillions
Source
Steven Pinker (The Blank Slate [2002], Ch.5)
Book Reference
Pinker,Steven: 'The Blank Slate' [Penguin 2003], p.76
A Reaction
I find this a persuasive example. Three hundred trillion neurons cannot possibly produce behaviour which is more than broadly predictable, and then it is the environment and culture that make it predictable, not the biology.