More than two billion years ago, Earth experienced one of the most important turning points in its history: the rise of oxygen in the atmosphere.
This event, known as the Great Oxidation Event, transformed our planet’s environment and made it possible for complex life to eventually evolve.
But scientists have long puzzled over a mystery.
Cyanobacteria—the tiny microbes that produce oxygen through photosynthesis—are thought to have evolved hundreds of millions of years before oxygen began to accumulate in the atmosphere.
Why, then, was there such a long delay before the oxygen boom?
A new study led by Dr. Dilan M. Ratnayake, now at the University of Peradeniya in Sri Lanka, points to an unexpected answer: the role of two simple chemical players, nickel and urea.
The research, published in Communications Earth & Environment in August 2025, reveals how these compounds acted as both obstacles and enablers in the buildup of oxygen.
To explore early Earth’s conditions, the researchers recreated aspects of the Archean Eon, a time from about 4 to 2.5 billion years ago.
In one set of experiments, they exposed mixtures of ammonium, cyanide, and iron to ultraviolet light—simulating how sunlight might have driven chemical reactions before Earth had an ozone layer.
These tests showed that urea, a key nitrogen compound, could form naturally in such environments.
The second set of experiments tested how cyanobacteria responded to different amounts of nickel and urea in their surroundings.
Using cultures of the microbe Synechococcus, the team found that high concentrations of these substances actually limited cyanobacterial growth, preventing large and lasting blooms.
When levels of nickel and urea dropped, however, cyanobacteria were able to thrive, releasing more oxygen into the atmosphere.
This finding suggests that early in Earth’s history, excess nickel and urea kept oxygen production in check. Over time, as conditions changed and concentrations declined, cyanobacteria expanded, and oxygen began to accumulate on a global scale—ushering in the Great Oxidation Event.
Dr. Ratnayake explains that understanding this ancient process is not just about solving Earth’s past. It also offers clues for the future, including how we might generate oxygen if humans ever colonize other planets, and how we might detect signs of life beyond Earth.
Chemical markers like nickel and urea could influence oxygen levels on exoplanets, shaping whether or not they are habitable.
The study reframes how scientists think about the rise of oxygen. Rather than being driven only by biology or volcanic gases, Earth’s oxygenation was shaped by the delicate balance of trace chemicals in the oceans.
It was the gradual decline of nickel and moderation of urea that finally allowed microbes to reshape the atmosphere—and, ultimately, pave the way for life as we know it.
