The arrival of oxygen in Earth’s atmosphere marked a defining moment in the planet’s history, transforming it into a world capable of supporting complex life. This major shift, known as the Great Oxidation Event (GOE), took place approximately 2.1 to 2.4 billion years ago. However, oxygenic photosynthesis – produced by cyanobacteria – had likely evolved hundreds of millions of years before this event. Despite this early ability to generate oxygen, atmospheric levels remained low for a surprisingly long time. Scientists have long debated the cause of this delay, considering explanations such as volcanic emissions, chemical sinks, and biological interactions. Yet no single factor has fully explained why it took so long for oxygen to build up in Earth’s air.

To tackle this enduring question, researchers focused on an often overlooked element of early Earth chemistry: the role of trace compounds such as nickel and urea in cyanobacterial growth.

Lead researcher Dr. Dilan M. Ratnayake from the Institute for Planetary Materials, Okayama University, Japan (current address is Department of Geology, University of Peradeniya, Sri Lanka), explained, “Generating oxygen would be a massive challenge if we are ever to colonizeanother planet. Therefore, we sought to understand how a tiny microbe, cyanobacteria, was capable of altering the Earth’s conditions to make them suitable for the evolution of complex life, including our own. The insights gained from this study will also provide a new framework for the sample analysis strategies for future Mars sample return missions.”

Professors Ryoji Tanaka and Eizo Nakamura from the same institute also collaborated on the work, which was published in the journal in Communications Earth & Environment.

Recreating Early Earth in the Lab

The team conducted a two-part experimental study designed to mimic conditions on the Archean Earth (roughly 4 to 2.5 billion years ago). In the first experiment, mixtures of ammonium, cyanide, and iron compounds were exposed to ultraviolet (UV)-C light, replicating the intense radiation that likely reached Earth’s surface before the ozone layer formed. These tests explored whether urea – an important nitrogen compound for life – could have formed naturally under such conditions.

In the second phase, cultures of cyanobacteria (Synechococcus sp. PCC 7002) were grown under alternating light and dark periods while varying the amounts of nickel and urea in their environment. The researchers monitored growth through optical density and chlorophyll-a levels to measure how these chemical factors affected cyanobacterial productivity.

Based on the results, the team proposed a new model explaining how oxygen gradually accumulated in the atmosphere. During the early Archean, abundant nickel and urea may have restricted cyanobacterial blooms, preventing sustained oxygen release. As Dr. Ratnayake noted, “Nickel has a complex yet fascinating relationship with urea regarding its formation as well as its biological consumption, while the availability of these at lower concentrations can lead to the proliferation of cyanobacteria.” When these levels eventually declined, cyanobacteria were able to thrive more persistently, driving the oxygen increase that culminated in the GOE.

Lessons for Earth and Beyond

The implications of these findings extend far beyond understanding ancient history. “If we can clearly understand the mechanisms for increasing the atmospheric oxygen content, it will shed light upon the biosignature detection in other planets,” said Dr. Ratnayake. He continued, “The findings demonstrate that the interplay among inorganic and organic compounds played crucial roles in Earth’s environmental changes, deepening our understanding of the evolution of Earth’s oxygen and hence the life on it.”

These insights could also inform future planetary exploration, since elements such as nickel and urea may affect the development of oxygen and life on other worlds.

By demonstrating how urea could form naturally under Archean conditions and showing that it acts as both a nutrient and an inhibitor, the researchers revealed how subtle chemical balances shaped Earth’s early biosphere. Their findings suggest that as nickel levels decreased and urea stabilized, cyanobacteria flourished, releasing oxygen in large quantities. This gradual shift ultimately transformed Earth from a lifeless planet into one capable of sustaining complex ecosystems – a profound step in the planet’s long journey toward habitability.