 And so this process called cephalization occurred, where it benefited an organism to have all of its sensory organs at one end of a bilateral body. And then you can imagine how, as those evolved to get a little more complex, as one mutation causes one organism to be a little better at sensing light. So now the genes of that organism live on and take that out thousands of generations. And you have selection for a concentrated collection of sensory organs, which becomes a brain.