When that happened, individual cells could turn their reproductive capacity on or off according to the function they served in Volvox's body.

ECONOMIST: How to become multicellular

But Volvox shows the sort of thing to look for: a gene that stops reproduction in single-celled creatures and has been co-opted to do a new job.

ECONOMIST: How to become multicellular

The gene that stops the body cells of Volvox reproducing is called regA. It works by suppressing the production of proteins needed to make new chloroplasts in a cell.

ECONOMIST: How to become multicellular