Teeth and Jawbone of <I>Sifrhippus</I> (IMAGE) U.S. National Science Foundation Caption From fossils, researchers determined oxygen levels on Earth some 56 million years ago. Credit Kristen Grace, UFL 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.