Researchers (IMAGE) Rice University Caption Geoscientists (from left) James Eguchi, Johnny Seales and Rajdeep Dasgupta published a new theory that attempts to explain the first appearance of significant concentrations of oxygen in Earth's atmosphere about 2.5 billion years ago as well as a puzzling shift in the ratio of carbon isotopes in carbonate minerals that followed. (Photos courtesy of Rice University) Credit Courtesy of Rice University Usage Restrictions Must credit: Rice University 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.