A recent study by researchers from Nagoya University in Japan suggests that Earth’s ancient oceans may have once been a vivid shade of green—thanks to their high iron content. Published in Nature Ecology & Evolution, the findings not only offer a fresh perspective on our planet’s distant past but could also reshape the way we search for life on other worlds.
Led by researcher Taro Matsuo, the team proposes that this green hue dominated Earth’s oceans for over 2 billion years—long before they took on the iconic blue we recognize today.
Why Were the Oceans Green?
According to the researchers, this unusual coloration stemmed from the interaction between ferrous iron—released by hydrothermal vents in the early oceans—and the oxygen produced by some of Earth’s first photosynthetic organisms.
This chemical reaction transformed the ferrous iron into ferric iron, an insoluble compound that precipitated out of the water as rust-like particles. These particles absorbed mostly blue and red wavelengths of light, allowing green light to scatter—giving the oceans a distinct green tint.
The Role of Cyanobacteria
The study also highlights the vital role played by cyanobacteria—tiny organisms among the first to perform oxygenic photosynthesis. These microbes, which appeared around 4 billion years ago, used a specialized protein called phycoerythrin to absorb green light, which was dominant in the iron-rich waters of that time.
Matsuo explained that this adaptation gave cyanobacteria a competitive edge in an otherwise hostile environment, allowing them to thrive and multiply.
Their emergence triggered one of Earth’s most pivotal events: the Great Oxidation Event, roughly 2.4 billion years ago. This era marked the gradual accumulation of oxygen in the planet’s atmosphere, laying the foundation for complex life.
Solving a Longstanding Mystery
Why cyanobacteria evolved the ability to absorb green light has puzzled scientists—until now. Genetic analysis conducted by Matsuo’s team suggests that this trait was a direct evolutionary response to the unique optical conditions of Earth’s ancient green oceans.
To support their hypothesis, the researchers carried out advanced computational chemical simulations, recreating ocean conditions from the Archean Eon—a period spanning 4 to 2.5 billion years ago. These simulations helped them analyze how light would have behaved underwater in an iron-rich environment. The results confirmed that iron particles would have significantly altered the way light scattered, making green the most visible wavelength.
Real-World Evidence in Japan
In addition to simulations, the team conducted field studies on Iwo Jima, part of Japan’s Satsunan archipelago southwest of Kyushu. The waters there, tinged green due to iron hydroxides, served as a real-world analogue of ancient oceans.
Observing these waters from a boat, Matsuo said, was a defining moment in the research—it visually confirmed what the models had predicted.
A Shift in Scientific Perspective
Matsuo admitted he was initially skeptical of the green ocean hypothesis. When he began exploring the idea in 2021, he found it hard to believe. But years of research and the fusion of geological and biological data gradually changed his mind.
“My skepticism turned into conviction,” he said, reflecting on how the pieces of the scientific puzzle began to fall into place.
Why This Matters for the Search for Life
This groundbreaking study doesn’t just illuminate Earth’s past—it challenges long-held assumptions about how we detect signs of life on other planets.
By expanding our understanding of the conditions that may have supported early life on Earth, scientists can refine their search for habitable environments elsewhere in the cosmos. The discovery that Earth’s earliest seas may have been green rather than blue opens new doors in the quest to understand life—both here and beyond.
