Researchers say the answer may lie in an ancient recycling system within Earth’s oceans. By studying rocks from South Africa, they found evidence that changes in ocean chemistry helped create a natural cycle in which biological activity supported the continued accumulation of oxygen in the atmosphere.

The study, published in Nature Communications, involved scientists including geologist Andrei Becker of the University of California, Riverside. Its findings suggest that phosphorus—an essential nutrient for life—played a central role in maintaining oxygen levels and may have helped create conditions that eventually supported more complex organisms.

How ancient oceans helped maintain Earth’s oxygen

About 2.3 billion years ago, Earth experienced a major oxidation event. This turning point made oxygen a permanent component of the atmosphere. Scientists have understood this transition for decades, but explaining how oxygen remained sufficiently abundant has been more difficult.

The researchers identified a feedback loop that may help explain what happened. As oxygen spread through the oceans, chemical changes in seawater increased the concentration of sulfates, compounds containing sulfur and oxygen.

Certain microorganisms used these sulfates to break down organic material more efficiently. During this process, they released phosphorus into the water, making the essential nutrient more available to other organisms.

As more phosphorus became available, biological productivity increased. This allowed more organic carbon to become buried in seafloor sediments rather than decomposing. Because decomposition can consume oxygen, the burial of additional carbon helped oxygen continue accumulating in the atmosphere.

This created a reinforcing cycle: rising oxygen levels promoted chemical changes in the oceans, those changes supported biological growth, and biological activity helped preserve oxygen in the atmosphere.

“Without phosphorus, living things cannot grow or function properly,” Becker says. “As more oxygen became available in the ocean, more organic carbon was able to become buried. A side effect of that process was that more oxygen continued to be released into the atmosphere.”

Ancient rocks reveal a hidden phosphorus cycle

To investigate this process, the scientists examined ancient rock samples from South Africa using a technique designed to reveal how available phosphorus may have been in prehistoric oceans.

Previous research had measured the total amount of phosphorus in ancient rocks, but those measurements provided limited information about whether organisms could actually use it. Some phosphorus becomes trapped in minerals, making it nearly inaccessible to living organisms.

The researchers addressed this problem by separating phosphorus according to the minerals that contained it. They dissolved different mineral components one at a time, allowing them to distinguish between phosphorus available for biological activity and phosphorus locked within mineral structures.

This distinction matters because the total amount of phosphorus in an ocean does not necessarily show how much of the nutrient is available to support life. The new approach offers a clearer way to reconstruct ancient nutrient cycles and understand how they influenced atmospheric oxygen.

“We can now separate the bioavailable phosphorus from the phosphorus that was essentially locked up,” Becker said. “This gives us a much clearer picture of nutrient levels in ancient oceans than before.”

Earth’s oxygen history may have been more turbulent than expected

The discovery suggests that the history of Earth’s atmosphere was more complex than scientists previously realized.

Following a major oxidative event, oxygen levels may have risen and fallen much more dramatically instead of remaining relatively stable. These fluctuations likely continued to influence ocean chemistry and the movement of essential nutrients through marine environments for tens of millions of years.

The findings could also change how scientists interpret the evolution of early life.

One long-standing question is why complex life took so long to emerge after oxygen became a significant part of Earth’s atmosphere. Scientists have often considered oxygen limitation a possible reason for the delay.

However, if oxygen was relatively abundant for extended periods of Earth’s early history, other environmental conditions and biological limitations may have played a larger role in delaying the emergence of complex life.

What Earth’s ancient oxygen cycle means for the future

Although the research focuses on events that occurred billions of years ago, its implications extend to the oceans people depend on today.

Climate change is already contributing to declining oxygen levels in parts of the world’s oceans. Continued oxygen loss can interfere with phosphorus recycling and reduce the amount of this essential nutrient available to marine life.

These changes may limit biological productivity and reduce the resilience of marine ecosystems facing environmental stress.

The discovery could also help scientists search for signs of life beyond Earth. Oceanic worlds are considered promising places to investigate potential habitability, but the presence of water alone does not guarantee that conditions can support life over long periods.

Understanding how oxygen, nutrients and biological processes interacted throughout Earth’s history could help researchers identify similar chemical relationships on distant planets and other oceanic worlds.

“Earth’s history shows that oxygen, nutrients and life evolved together,” Becker says. “Understanding those connections gives us a more nuanced perspective on the future of our own planet and what to expect for other planets.”