Editor's Note:

An international team of scientists has uncovered new evidence linking early animal evolution to extreme climate change. A dramatic rise in atmospheric oxygen levels has long been speculated as the trigger for early animal evolution, but direct evidence has proven elusive to scientists. In the Sept. 27 issue of the journal Nature, a UNLV-led research team for the first time offers evidence of a causal link between trends in early biological diversity and shifts in Earth system processes. The fossil record shows a marked increase in animal and algae fossils roughly 635 million years ago. An analysis of organic-rich rocks from South China points to a sudden spike in oceanic oxygen levels at this time – in the wake of severe glaciation – allowing animal life to flourish. The new evidence pre-dates previous estimates of a life-sustaining oxygenation event by more than 50 million years. “For more than three quarters of the Earth’s history, the oxygen level in the atmosphere and ocean was insufficient to support animal life,” said Swapan Sahoo, lead author and Ph.D. student in UNLV’s Geoscience Department. “Our findings support a link between glaciation, oxygenation of surface environments and the diversification of animals. Knowing the environment where the first animals lived is critical for understanding the evolutionary stress of ecosystems.” An analysis of iron and trace metal concentrations in shale collected from the Doushantuo Formation in South China revealed spikes in concentrations of metals that denote higher oxygen levels in seawater. These elevated levels of molybdenum, vanadium and uranium slightly predated the earliest oxygen-demanding animal fossils, supporting the link between ocean oxygenation and animal evolution. High element concentrations found in the South China rocks are comparable to modern ocean sediments and point to a substantial oxygen increase in the ocean-atmosphere system. Researchers say the oxygen rise is likely due to increased organic carbon burial, a result of more nutrient availability following Earth’s extreme cold climate. “Photosynthesis is the most efficient process to generate oxygen,” said Ganqing Jiang, UNLV associate professor of geoscience and principle investigator on the project. “Fast burial of a large quantity of photosynthetic organic carbon in sediments would leave free oxygen in the ocean-atmosphere system, leading to significant oxygen rise.” The large variability of iron content and trace metal concentrations in the South China rocks may cause scientists to rethink existing geological interpretations about ancient oceans and could lead to accompanying investigations of similar-aged rocks in other continents. The joint research was supported by grants from the National Science Foundation, the NASA Exobiology Program, and National Natural Science Foundation of China. The research team includes Swapan K. Sahoo and Ganqing Jiang of the UNLV Department of Geoscience; Noah J. Planavsky and Timothy W. Lyons of University of California, Riverside; Brian Kendall and Ariel D. Anbar of Arizona State University; Xinqiang Wang and Xiaoying Shi of the China University of Geosciences (Beijing); and Clint Scott of McGill University, Canada. The study, “Ocean oxygenation in the wake of the Marinoan glaciation,” appears in the Sept. 27 edition of the journal Nature.