One of the greatest mysteries about the evolution of complex life on Earth is why it emerged so late, more than 3.5 billion years after the formation of the planet. Most scientists believe that it was perhaps the lack of oxygen in the atmosphere that prevented the proliferation of large organisms. Research over the last decade has shown that atmospheric oxygen reached modern levels only about 800-500 million years ago, roughly coinciding with the first appearance of complex fossils.
However, new analyses of rocks that formed between 2.0 and 2.4 billion years ago indicate that oxygen levels may have risen and dropped again during this time interval, long before the appearance of plants and animals. This work – recently published on Science Daily – was carried out by an international research group including Dr Eva Stüeken from the University of St Andrews, and it was based on measurements of selenium isotopes.
Selenium only becomes soluble in the ocean when it reacts with oxygen. Oxygen-rich seawater therefore contains more selenium than oxygen-poor waters. Once the selenium is dissolved, it can react with biomass and minerals and get incorporated into sedimentary rocks. These rocks thus archive the presence of selenium over billion-year timescales.
The 2.0-2.4 billion-year-old rocks analysed in this study show unusually high selenium concentrations compared to rocks that are slightly older and younger. This pattern is evidence for a rise and drop of oxygen in the surface ocean during that time. Recent work on the nitrogen cycle by Dr Aubrey Zerkle and co-workers at the University of St Andrews confirms this conclusion. In their study they found evidence of nitrate in seawater during the same time interval. Nitrate requires oxygen to be produced.
The six stable isotopes of selenium provide additional insights into the availability of oxygen at this time. The isotopes behave slightly differently when the selenium gets incorporated into sediments. By measuring their ratios, Dr Eva Stüeken and her colleagues could infer that the most oxygenated conditions existed in the surface ocean, whereas the deep ocean probably remained anoxic. Perhaps it was this strong gradient that prevented complex life from emerging 2 billion years ago. Or perhaps complex organisms did exist in the surface ocean but have not yet been found. It is also possible that oxygen is not the only necessary ingredient to spur biological evolution.
Dr Eva Stüeken is setting up facilities for measuring selenium isotopes at the University of St Andrews to address these questions and to learn more about one of the most fascinating intervals in Earth’s history.