Earth was once a 'Green Marble' before its oceans turned blue

When astronauts gaze at our planet from space, they call it the “Blue Marble.” But that iconic color, which reflects the vast oceans and sunlit sky, may be a relatively new look. Dive deeper into Earth’s past – about 2.5 to 4 billion years ago – and you might see a very different hue.

Scientists now argue that Earth’s oceans once shimmered green, not blue.

These emerald waters weren’t just a geological oddity – they were a cradle for evolution, especially for one of life’s most influential architects: the cyanobacteria.

A recent study by Taro Matsuo and his team at Nagoya University, which is published in Nature Ecology & Evolution, reimagines Earth’s early biosphere.

By exploring how underwater light filtered through iron-rich seas, the researchers offer a bold hypothesis: the evolution of cyanobacteria’s photosynthetic systems was deeply shaped by the presence of green light.

This green ocean world could change how we recognize life on other planets – and how we view our own beginnings.

Early oceans were iron-rich and green

Four and a half billion years ago, Earth formed from cosmic debris. Life took its time to appear, only arriving around 3.7 billion years ago.

Before the first cells emerged, oceans covered the planet’s surface, but their appearance differed greatly from what we see today. Hydrothermal vents, erupting across ocean floors, pumped reduced iron – Fe(II) – into the water, filling the seas with ferrous iron.

This chemistry defined the early ocean’s interaction with sunlight. With no oxygen in the atmosphere and an abundance of Fe(II), the oceans lacked the reflective quality of today’s blue waters.

There was no ozone to block UV rays, and no land plants to shift the atmosphere. What these iron-saturated oceans had instead was a strange relationship with light – one that bent toward green.

With the arrival of cyanobacteria, oxygen started appearing in the water. That oxygen transformed ferrous iron into ferric iron, which is insoluble and formed rust-like particles.

These particles sank slowly but didn’t disappear quickly. Instead, they lingered and affected the light that filtered into the ocean depths.

Green light shapes evolutionary paths

The ferric iron, suspended in water as fine iron hydroxide, had a powerful optical effect. It absorbed red and blue wavelengths but let green light pass.

As a result, the oceans themselves took on a green hue. Had anyone been there to witness it, they would have seen Earth’s waters glowing emerald.

This green-light environment had significant consequences for the organisms living in it. Cyanobacteria, the early masters of photosynthesis, began to adapt.

They already used chlorophyll a, a pigment that absorbs red and blue light. But under green light, chlorophyll a becomes far less effective.

To survive, cyanobacteria evolved phycobilisomes – large antenna-like structures that are packed with accessory pigments. Among these pigments, phycoerythrobilin (PEB) proved especially vital.

It absorbed green light and passed that energy to chlorophyll a for photosynthesis. These structures enabled cyanobacteria to dominate the early oceans.

“Genetic analysis revealed that cyanobacteria had a specialized phycobilin protein called phycoerythrin that efficiently absorbed green light,” Matsuo said in a statement. “We believe that this adaptation allowed them to thrive in the iron-rich, green oceans.”

Simulating Earth’s early green oceans

To test their theory, Matsuo and his colleagues ran detailed numerical simulations, recreating the underwater light environment from the Archean eon. These simulations accounted for ocean chemistry, light diffusion, and pigment absorption.

They discovered that at depths of 5 to 20 meters (16 to 66 feet), iron hydroxide particles created a persistent green-light window. Even tenfold variations in particle concentration did not shift the spectrum significantly.

Under these conditions, green light dominated, matching perfectly with the absorption range of PEB. Phycobilisomes equipped with PEB gave cyanobacteria an evolutionary edge.

In addition to models, the team performed genetic experiments. They engineered cyanobacteria strains to produce PEB.

When exposed to green light, the modified strains outgrew the wild-type strains. The energy transfer was so efficient that PEB didn’t even need its usual protein partner, phycoerythrin – it could work by attaching directly to phycocyanin instead.

Water bodies match ancient green oceans

Still, simulations and lab results weren’t enough for Matsuo. In 2023, he journeyed to Iwo Island, part of Japan’s Satsunan archipelago.

There, nature offered a rare analog to Earth’s ancient oceans. Hydrothermal vents beneath the island release Fe(II), which oxidizes into iron hydroxide – just as it did in the early oceans, billions of years ago.

“From the boat, we could see that the surrounding waters had a distinct green shimmer due to iron hydroxides, exactly like how I imagined the Earth used to look,” said Matsuo.

Measurements at 5.5 meters depth (18 feet) confirmed the dominance of green light. Fluorescence analysis showed that cyanobacteria at this depth contained more PEB than those at the surface. It was a perfect modern echo of the ancient world.

Phycobilisomes are tools born from light

Phycobilisomes remain a defining trait of the cyanobacteria. Made up of three major proteins – phycoerythrin (PE), phycocyanin (PC), and allophycocyanin (APC) – these structures evolved to absorb light in a range of wavelengths.

In the Archean oceans, PE’s ability to absorb green light and transfer it to chlorophyll a was unmatched.

Phylogenetic analyses suggest that even the earliest cyanobacteria had the complete PE–PC–APC system. Over time, some cyanobacteria lost PE, especially those that adapted to brighter or different light conditions. But for the early dwellers of green oceans, PE was crucial.

Further analysis showed that energy transfer from PEB was more efficient than from carotenoids like β-carotene. In fact, the distance over which PEB could transfer energy to chlorophyll was about seven times greater.

This meant that even in low-light conditions, PEB-packed phycobilisomes kept photosynthesis humming.

Why Earth’s green oceans matter

The implications of green oceans stretch beyond Earth’s history. They reshape how we might detect life on alien worlds.

Traditionally, astronomers have searched for blue planets, assuming that water reflects that hue. But as Matsuo’s team showed, oceans rich in iron hydroxide appear brighter and greener from a distance.

“Remote-sensing data show that waters rich in iron hydroxide, such as those around Iwo Island in the Satsunan archipelago, appear noticeably brighter than typical blue oceans,” he explained. “This leads us to think that green oceans might be observable from a longer distance, making them easier to detect.”

In the search for extraterrestrial life, scientists might do well to widen their palette. A green tint could signal oceans teeming with microbial activity, and shaped by chemistry just as Earth’s once were.

What we’ve learned from this study

The light window on Earth has not always remained the same. Before the Great Oxidation Event (GOE), the planet’s light environment favored green.

After the GOE, as oxygen levels rose and iron dropped, oceans cleared, and the light window shifted toward white. Land plants evolved under this white light, and cyanobacteria on land lost their need for PE.

Still, in dim underwater habitats, PE remained essential. The evolution of phycobilisomes thus mirrors Earth’s atmospheric and oceanic transformations. Each stage of light drove new adaptations, and carved paths for life to expand and diversify.

Matsuo wasn’t always convinced. “When I first started pondering the idea in 2021, I was more skeptical than anything else,” he said. “But now, after years of research, as geological and biological insights gradually came together like pieces of a puzzle, my skepticism has turned into conviction.”

That conviction now paints a different portrait of early Earth as a pale green sphere where light sculpted life in unexpected ways.

The study was published in the journal Nature Ecology and Evolution.

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