Scientists uncover the 2.3-billion-year-old secret that helped keep Earth habitable

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

The research, published in Nature Communications, involved scientists including University of California, Riverside geologist Andrey Bekker. Their findings suggest that phosphorus, a nutrient essential for life, played a central role in sustaining the oxygen levels that would eventually help make more complex organisms possible.

How Ancient Oceans Helped Sustain Earth's Oxygen

About 2.3 billion years ago, Earth experienced the Great Oxidation Event, a major turning point when oxygen became a lasting component of the atmosphere. Scientists have understood this transition for decades, but explaining how oxygen remained sufficiently abundant afterward has been much more difficult.

The researchers identified a feedback loop that may help explain what happened. As oxygen spread into the oceans, it altered the chemistry of seawater and increased concentrations of sulfate, a compound containing sulfur and oxygen.

Certain microbes used this sulfate to break down organic material more efficiently. In the process, they released phosphorus back into the water, making this vital nutrient available to other organisms.

With more phosphorus available, biological productivity increased. That meant more organic carbon could eventually become buried in seafloor sediments rather than being broken down. Because the decomposition of organic matter can consume oxygen, burying more carbon helped favor the continued buildup of oxygen in the atmosphere.

This created a reinforcing cycle in which rising oxygen levels encouraged chemical changes that supported biological growth, which in turn helped maintain atmospheric oxygen.

"Living things cannot grow or function properly without phosphorus," Bekker said. "Once more of it became available in the oceans, it allowed more organic carbon to be buried. A side effect of that process is 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 that provides a more detailed picture of phosphorus availability in prehistoric oceans.

Earlier research could determine how much phosphorus was present in ancient rocks, but those measurements revealed little about whether organisms could actually use it. Some phosphorus becomes trapped in minerals, making it effectively inaccessible to living things.

The researchers addressed this limitation by separating phosphorus according to the minerals that contained it. They accomplished this by dissolving different mineral components one at a time, allowing them to distinguish phosphorus that had been available for biological activity from phosphorus locked away in mineral structures.

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

"We can now separate the phosphorus that was available to organisms from phosphorus that was essentially locked away," Bekker said. "That gives us a much clearer picture of nutrient levels in ancient oceans than we had before."

Earth's Oxygen History May Have Been More Turbulent Than Expected

The findings also point to a more complicated history of Earth's atmosphere than scientists previously recognized.

Rather than remaining relatively steady after the Great Oxidation Event, oxygen levels may have risen and fallen much more dramatically. These fluctuations likely continued to influence ocean chemistry and the movement of essential nutrients through marine environments over tens of millions of years.

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

One longstanding question is why complex organisms took so long to appear after oxygen first became a significant part of Earth's atmosphere. Limited oxygen has often been considered a possible explanation for this delay.

However, if oxygen remained relatively abundant during extended periods of Earth's early history, it raises the possibility that other environmental conditions or biological limitations played a greater role in slowing the emergence of complex life.

What Earth's Ancient Oxygen Cycle Means for the Future

Although the research focuses on events billions of years in the past, its implications extend to the oceans we depend on today.

Climate change is already contributing to declining oxygen levels in parts of the world's oceans. If oxygen loss continues, it could disrupt phosphorus recycling and reduce the amount of this essential nutrient available to marine organisms.

Such changes could limit biological productivity and make ocean ecosystems less resilient to environmental stress.

The findings may also help scientists search for signs of life beyond Earth. Worlds with oceans 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 ocean-bearing worlds.

"Earth's history shows that oxygen, nutrients, and life evolved together," Bekker said. "Understanding those connections gives us a more nuanced perspective on our own planet's future and what we might look for on other planets."