The Great Oxidation Event occured around 2.3 billion years ago, when it was no longer possible for newly created oxygen to be captured in chemical compounds. Instead, it started to accumulate as oxygen in the oceans and in the atmosphere. Before this event, in the Earth's early atmosphere, there were only traces of free oxygen. All life was based exclusively on anaerobic processes - chemical reactions that did not require oxygen. With the emergence of cyanobacteria that oxidized water with the help of light and produced oxygen as a by-product, the conditions for life on Earth gradually began to transform.
New research by scientists at the University of Bristol and Boston University suggests that the evolution of multicellularity coincided with increased diversification of cyanobacteria and the Great Oxidation Event. Cyanobacteria are among the most diverse prokaryotic phyla, with morphotypes ranging from unicellular to multicellular filamentous forms, including those able to irreversibly differentiate in form and function. It has been suggested that cyanobacteria raised oxygen levels in the atmosphere around 2.45–2.32 billion years ago during the Great Oxidation Event and dramatically changing life on the planet.
However, little is known about the possible interplay between the origin of multicellularity, diversification of cyanobacteria, and the rise of atmospheric oxygen. The team tested whether the evolution of multicellularity overlapped with the Great Oxidation, and whether multicellularity is associated with significant shifts in diversification rates in cyanobacteria.
The results indicate an origin of cyanobacteria before the rise of atmospheric oxygen. The evolution of multicellular forms coincided with the onset of the Great Oxidation Event and an increase in diversification rates, suggesting that multicellularity could have played a key role in triggering cyanobacterial evolution. In prior studies, geochemists challenged the simple notion of an up-only trend for early oxygen and provided the first compelling direct evidence for a major drop in oxygen after The Great Oxidation event some, which was critical for the origin and evolution of the first forms of eukaryotic life. The second big step in the up-only hypothesis occurred almost two billion years later, coinciding with the first appearances and earliest diversification of animals.