Ancient Rocks (IMAGE) University of Southern Denmark Caption Sedimentary rocks from Grand Canyon, USA, show evidence for widespread anoxic and sulfidic waters 750 million years ago. Since sulfide is poisonous to animals, it can explain why animals had not evolved on Earth at that time. Credit PD photo (www.pdphoto.org) Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.