Field work (IMAGE) Stanford University Caption Researchers from Stanford and Dartmouth College carry rock samples back to camp along the Peel River. Rocks seen at their side and in the background are from around the time of the oxygen change, roughly 410 million to 405 million years ago. Credit Erik Sperling Usage Restrictions Credit required 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.