Given the task definition and input, reply with output. Given a scientific question and its correct answer, generate supporting facts for the answer. This supporting fact can be an explanation for the answer to the given question.

Question: Flatworms have a concentration of nerve tissue in the head end, which was a major step in the evolution of what organ? Answer: brain
Flatworms have a concentration of nerve tissue in the head end. This was a major step in the evolution of a brain. It was also needed for bilateral symmetry.