Shifting Hues

Forget the image of Earth as the “Blue Planet.” 

In its early days, our planet may have actually been emerald-hued, according to a new study.

Scientists at Nagoya University in Japan suggested that more than 2 billion years ago, Earth’s ancient oceans were green thanks to their unique chemistry and the light-harvesting quirks of early microbial life. 

In their paper, lead author Taro Matsuo and his team conducted a series of simulations that showed that these green oceans emerged sometime during the Archean Eon between 4 and 2.5 billion years ago – a period that saw the emergence of the first lifeforms and continents. 

During this time, Earth’s early waters were heavily saturated with ferrous iron that was released both from rainfall eroding continental rocks and from undersea volcanic vents. 

Then, some 2.4 billion years ago, the planet experienced the Great Oxidation Event when the early cyanobacteria began to perform oxygenic photosynthesis that released oxygen into the oceans and atmosphere. 

But unlike modern photosynthesizers, these ancient microbes didn’t rely solely on chlorophyll. They also used an extra light-harvesting protein called phycoerythrin, which allowed them to soak up green light – then the most abundant light in the oceans, New Atlas reported. 

The team noted that as the oxygen levels rose, it reacted with ferrous iron to form ferric iron, which is insoluble and precipitated as rust-like particles. These particles absorbed blue wavelengths, while water itself absorbed red – leaving green light to dominate the underwater landscape. 

“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.” 

Further supporting this “green Earth” theory was a 2023 field trip to Japan’s Iwo Jima, where the research team observed green-hued waters around the volcanic island. These waters were colored by iron hydroxides, lending real-world support to their model of green Archean oceans. 

But beyond painting a vivid picture of early Earth, the study could also change the way how scientists search for life on other planets.  

If green oceans once signaled the rise of life here, they might do the same elsewhere.  

“This leads us to think that green oceans might be observable from a longer distance, making them easier to detect,” Matsuo added.