Rocks show the moment when oxygen permanently altered the planet.

Two basic issues are addressed by several events in Earth's distant past: How did we get here? And where are we going? These pivotal moments demonstrate how life adjusts to shifting environmental conditions.

The Great Oxidation Event (GOE) was one particularly noteworthy occurrence. For the first time, oxygen started to build up in the atmosphere more than two billion years ago. Humans and other sophisticated creatures were made feasible by this change.

Before oxygen, the Earth

The Earth's environment was nearly entirely anoxic, or oxygen-deficient, before the GOE. Only anaerobic creatures that depended on fermentation to survive survived; all other life was extinct.

Similar living forms can still be found in harsh environments like deep-sea hydrothermal vents and acidic hot springs. A significant shift was brought about by the GOE. It changed Earth from a planet with little oxygen in the atmosphere to one that could sustain the varied biosphere that we are familiar with.

For a long time, scientists have tried to pinpoint the precise moment and mechanism of this change. In a recent study, scientists from MIT and Syracuse University examined ancient rock cores from South Africa and found new hints.

The study offers a more precise chronology of how biological evolution was driven by increasing oxygen levels, which opened the door for the emergence of the first eukaryotes, or complex cell creatures.

Examining the distant past

The sedimentary strata that the researchers examined range in age from 2.2 to 2.5 billion years. These rocks, which were gathered from carefully selected locations in South Africa, maintain GOE-era chemical evidence.

The researchers discovered evidence of nitrate, a chemical marker that indicates greater oxygen was present in past waters, by examining stable isotopes trapped in these rocks. Benjamin Uveges, who completed the study as a postdoctoral associate at MIT after receiving his Ph.D. from Syracuse University, led the investigation.

Christopher Junium, an associate professor of Earth and environmental sciences at Syracuse University, worked closely with Uveges. Junium's lab specialises in reconstructing historical environmental conditions. Modern equipment kept in Syracuse was used to carry out the chemical analysis.

"The nitrogen concentrations in the rocks we analysed for this study were extremely low, too low to measure with the traditional instrumentation used for this work," Uveges added.

"One of the few instruments in the world that can measure nitrogen isotope ratios in samples that contain 100–1,000 times less nitrogen than the usual minimum is the one that Chris built."

How the study was carried out

The scientists examined the nitrogen isotopes in the lab using an Isotope Ratio Mass Spectrometer (IRMS). To extract particular components, they first ground the rock samples into a powder and then chemically processed them.

After that, the substance was ionised, transformed into gas, and separated according to mass. The ratio of nitrogen isotopes, ¹⁵N to ¹⁴N, was measured to learn more about how bacteria formerly handled nitrogen.

The cryotrapping/capillary-focusing module, a crucial component of the IRMS, enabled accurate measurements even with incredibly low nitrogen levels. The equipment is essential to research like this one and is one of the few of its kind

According to the team's results, aerobic nitrogen cycling may have begun some 100 million years earlier than previously believed. This indicates that long before oxygen accumulated in the atmosphere, oxygen-sensitive activities were already taking place in the oceans.

"This aligns with the growing notion that the GOE was a prolonged struggle in which organisms had to strike a balance between utilising the energy benefits of oxygenic photosynthesis and gradually adapting to cope with its byproduct, oxygen," Junium added.

The increase in oxygen levels on Earth

A series of modifications were triggered by the gradual increase in oxygen levels. Many anaerobic creatures were pushed to extinction when the atmosphere's oxygen content increased. Organisms that could use oxygen to breathe started to flourish in their place. This process, aerobic respiration, provides the energy needed for complex life processes such as muscle movement, brain function, and cell reproduction.

"In the hundreds of millions of years on Earth, there was very little free oxygen in the ocean or atmosphere," Uveges said. "In contrast, today, oxygen forms a fifth of our atmosphere, essentially, as we know it, and as we breathe, essentially, all complex multicellular cells." "In a sense, investigations of the rise in oxygen and its chemical, geological, and biological effects examine how planets and life develop in the current situation."

New chapters in Earth History

These results change as we understand the supply of oxygen to the Earth. They show that the oxygen-rich atmosphere and the journey to complex life that it has made possible are slower and more complicated than before.

According to Uveges, "I hope our findings will inspire more research into this fascinating period." "We can construct an even more comprehensive picture of the GOE and its impact on life on Earth by applying new geochemical techniques to the rock cores we studied.".

We now have a better understanding of how life and the environment on Earth have influenced one another over a very long period, thanks to the research. It serves as a reminder that significant changes take time to occur and that the air we breathe now is the product of gradual, billion-year-long change.