Geologists have uncovered evidence in the oil fields of Oman that explains how Earth could suddenly have changed 540 million years ago to favour the evolution of the single-celled life forms to the multi cellular forms we know today. Reporting in the December 7 issue of the journal „Nature“, researchers from MIT, the California Institute of Technology, and Indiana University show that there was a sudden change in the oxygenation of the world’s oceans at the time just before the „Cambrian explosion“, one of the most significant adaptive radiations in the history of life. With an increased availability of oxygen, single-celled life forms that had dominated the planet for the previous three billion years were able to evolve into the diverse metazoan phyla that still characterize life on Earth. The key insight was derived when core samples and drillings taken at a depth of about three kilometers from oil wells in Oman were analysed, which are known to have the oldest commercially viable oil on the planet. The results of carbon and sulfur isotopic analyses from the material led the team to the conclusion that the oceanic conditions that laid down the deposits in Oman were quite different from conditions of today. When organic matter falls into an ocean that doesn’t stir, it becomes deprived of sufficient oxygen and cannot survive as multi cellular forms. For this reason, with a limited amount of oxygen, life continued in its single-celled form for the first three billion years. But about 550 million years ago, the deep oxygen began mixing its contents with the shallow ocean, resulting for the first time in a fully oxidized deep ocean.

http://www.caltech.edu/article/12931