An article published in the journal “Nature Geoscience” describes a research on the evolutionary radiation, in the sense of the quick diversification of new species, that happened in the Ordovician period, between 445-485 million years ago. Known as the Great Ordovician Biodiversification Event (GOBE), it led to a huge growth in the number of species and according to a team of researchers it was caused also by an 80% increase in oxygen levels in the air.

The most known and perhaps most important evolutionary radiation event in the history of life on Earth was the Cambrian explosion, which, starting from about 540 million years ago, in a few tens of millions of years led to the evolution of the phyla that exist today. However, at the end of the Cambrian, about 488 million years ago, there was a mass extinction and a subsequent recovery with a new evolutionary radiation event, namely the GOBE.

A series of hypotheses were made to explain the GOBE invoking possible geological causes linked to the continent’s drift and/or a particularly strong volcanic activity and even a possible meteor shower that might have struck the Earth, resulting in very marked climatic changes. This new research focused on an increase of the oxygen availability with the difficulty of finding traces after over 400 million years.

The researchers used two approaches that are almost independent using different geochemical data predicting the same level of oxygenation more or less during the same period of diversification. They also made another link between biodiversity and oxygen levels in the Ordovician, where levels of oxygen close to the current ones were reached about 455 million years ago.

The image shows an area of the formation called Shingle Pass Limestone in Nevada where limestone rocks samples were analyzed to make one of the oxygen estimates. In the chart, the red line represents oxygen levels estimated based on the trends of carbon and sulfur isotopes in the rocks. The blue line represents oxygen levels estimated using carbon isotopic data from rocks and organic matter. The black line represents the levels of global biodiversity during Cambrian, Ordovician and Silurian.

Cole Edwards of Appalachian State, the lead investigator in this research, stressed the fact that probably the rise in oxygen was not the only cause of the GOBE but added to other environmental changes. What’s certain is that the study shows an increase in oxygen from 14% of the atmosphere in the mid-Ordovician, 460-465 million years ago, to 24% in the late Ordovician, 450-455 million years ago.

The assessment of the importance of the various factors in the GOBE is difficult and the researchers are cautious in their conclusions. However, Matthew Saltzman of Ohio State University, one of the authors of the research, stressed the importance of oxygen in understanding the diversification and abundance of animals in the Ordovician, a key period for the evolution of animals.