Evolutionary biologists have found that the California sea hare, a mollusk that goes by the scientific name of Aplysia californica, has a protein similar to proteins in people that respond to estrogen and other steroid hormones. The surprising finding suggests that estrogen was the first such hormone to evolve and that the estrogen-signaling system dates back more than 600 million years. Contrary to past thinking, the estrogen system apparently evolved before the divergence of invertebrates, such as mollusks and insects, and vertebrates, such as fish and mammals. The hormone-binding proteins known as steroid receptors, "had never been found outside the vertebrates," says Joseph Thornton of the University of Oregon in Eugene, who led the work. "Everyone assumed they emerged somewhere deep in the vertebrate lineage."