A recently discovered 550-million-year-old sea sponge is helping solve a long-standing evolutionary mystery about why early sponges seemed to vanish from the fossil record. Credit: Xunlai Yuan At first glance, sea sponges seem almost too simple to be mysterious. They have no brain and no gut, and scientists have long believed they originated around 700 million years ago. Yet clear fossil evidence only dates back to about 540 million years ago, leaving a puzzling 160 million-year gap in the