Organic-rich Shale Samples (IMAGE) University of California - Riverside Caption Organic-rich shale samples, such as these from the 2.5-billion-year-old Mount McRae Shale from Western Australia, were analyzed for their zinc contents. The results confirm that the early ocean was not Zn-lean and that other controls must be invoked to explain the protracted appearance and proliferation of eukaryotic life. Credit Arizona State University 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.