The Earth was devoid of atmospheric oxygen two billion years ago. Indeed, free oxygen, as we breathe it today, only appeared in a lasting way at a time called the Great Oxidation Event.
A team from MIT studied the origin of a compound essential for respiration, an enzyme present in the majority of living beings that use oxygen. Their work, published in
Palaeogeography, Palaeoclimatology, Palaeoecology, demonstrates that this enzyme already existed during the Mesoarchean, an era spanning between 3.2 and 2.8 billion years before our era. This period predates by several hundred million years the moment when oxygen accumulated in the air, thus placing the emergence of this biological mechanism much earlier than previously thought.
During this distant era, the main producers of oxygen were cyanobacteria, microbes capable of photosynthesis. Their emergence is estimated to date back to about 2.9 billion years ago, long before the Great Oxidation Event. They were therefore able to release oxygen for a very long period without it accumulating noticeably. For a long time, scientists believed that chemical reactions with rocks absorbed most of this gas. The new study puts forward an additional idea: life itself could have consumed this nascent oxygen.
To reach this conclusion, the researchers analyzed the evolution of the enzyme over time. They examined its genetic sequence in several thousand modern species and placed this information on the evolutionary tree of life. By cross-referencing this data with the known dates of certain fossil species, they were able to estimate the period when the enzyme appeared. Their model indicates that this ability to process oxygen emerged shortly after the rise of cyanobacteria.
Mapping the evolution of the enzyme in thousands of modern species indicates it appeared shortly after the first oxygen producers.
Credit: Fatima Husain According to this scenario, organisms living near cyanobacteria would have rapidly evolved to capture and use the small amounts of oxygen produced. By consuming it, these first breathers would thus have helped delay its accumulation in the atmosphere for hundreds of millions of years. This dynamic would partly explain the long delay between the initial production of oxygen and the moment it became a stable component of the air. The whole story once again shows the rapid adaptability of life in the face of new energy resources.
The researchers from MIT and the University of Oregon behind this study specify that these results change our perception of aerobic respiration. It would no longer be an innovation that occurred only after the atmosphere became rich in oxygen. On the contrary, some living beings had already developed this ability while global conditions were not yet conducive to it.
Ultimately, the history of oxygen on Earth is revealed to be different. Its lasting presence in the air results from a balance between its production by organisms like cyanobacteria and its consumption by other life forms or by rocks. This recent work shows that aerobic respiration is an extremely ancient invention. It likely contributed to the diversification of life well before its accumulation in the atmosphere.