A single gene has been found to be shared in nearly all living animals—"including sea anemones, worms, insects, marine invertebrates, fish and humans." The finding, by geneticist Eugene Xu from Northwestern University, suggests that the evolution of sperm production happened just once, 600 million years ago, and has not mutated as animals have evolved since then. "People have thought that there was a single common ancestor because we see sperm reproduction in many animals, but previously there was no conclusive evidence that sperm production has a single common origin in all animals," said Xu.