Our planet’s oceans give it a blue color that is noticeable even from space. This is why it has been nicknamed Blue Marble. However, the Earth wasn’t always a symbol of blueness, as new research indicates that the oceans may have previously been a very different color. Scientists from Nagoya University in Japan suggest that more than 2 billion years ago, the planet’s water shimmered with green tones. This discovery not only sheds light on Earth’s past, but could also be the key to finding life outside the Solar System. The results of the study are published in Nature Ecology & Evolution.
The age of iron and first organisms
The Earth was formed 4.5 billion years ago, but the first life forms didn’t appear until 800 million years later. However, even during the lifeless period, the planet was covered with deep oceans rich in divalent iron. This metal, which was released from hydrothermal vents, determined not only the chemical composition of the water, but also its color. Researchers believe: iron absorbed blue and red wavelengths of light, and scattered green wavelengths. According to scientists, this has given the oceans a greener hue. So, cyanobacteria kept producing phycobilins to absorb light through green.
Everything changed with the appearance of cyanobacteria, some of the first organisms capable of photosynthesis. About 4 billion years ago, they began producing oxygen, which reacted with iron. As a result, the divalent iron turned into trivalent iron (insoluble, similar to rust), which gradually settled to the bottom. The process took millions of years until the iron precipitated out. Over time, the oceans lost their greenish hue and became blue.
Genetic key to the mystery
So why did cyanobacteria use phycobilins – pigments that absorb green light – instead of chlorophyll? The answer was found in their genes. The team led by Taro Matsuo found that ancient microorganisms had a phycoerythrin protein specialized for collecting exactly green waves.
“This adaptation allowed them to survive in an environment where iron dominated the water. Phycobilins were their evolutionary weapon,” explains Matsuo.
From skepticism to discovery
Interestingly, Matsuo did not believe in his own hypothesis at first. In 2021, he began research to disprove the idea of green oceans. However, computer models that reproduced light spectra from the Archean era showed: at high iron levels, the water took on a greenish color, not red as previously thought. The latest point was the 2023 survey near Japan’s Iwo Island, where local waters have a unique green coloration similar to ancient oceans due to geological features.
This discovery changes the way we think about how life affects the planet. By producing oxygen, cyanobacteria not only changed the color of the oceans, but also caused the Oxygen Catastrophe 2.4 billion years ago, making Earth suitable for complex life forms. In addition, knowledge of the “color markers” of the oceans will help in the search for exoplanets: by analyzing the spectrum of light reflected from distant worlds, astronomers will be able to identify potential centers of life.
We previously reported on how greenhouse gases would help us detect aliens.
According to popsci.com
