Imagine Earth as a purple planet. Not a metaphor: purple, in the physical sense of the word. Magenta oceans reflecting red and blue light under a cloudy, oxygen-free sky. This is the hypothesis proposed by Shiladitya DasSarma ofUniversity of MarylandBefore chlorophyll, photosynthesis was based on retinal, a simpler pigment that absorbs green light and colors organisms that use it purple.
The archaea called halobacteria They still do so today, dyeing the salt lakes they inhabit purple. But 3 billion years ago, they dominated the planet, leaving only the crumbs of the solar spectrum for future green plants.
Purple Earth: When Retinal Ruled the Oceans
The Purple Earth Hypothesis, first proposed by DasSarma in 2007, places this phase between 3,5 and 2,4 billion years ago, during the Archean. An era preceding the Great Oxygenation Event, when the atmosphere was still filled with methane and carbon dioxide. Retinal, unlike chlorophyll, which would come later, is a structurally simpler molecule. It doesn't require complex porphyrin structures to function.
It absorbs a single peak of the light spectrum: the green-yellow part, the most energetic. The rest (red and blue) is reflected, producing that characteristic magenta color. It is the exact opposite of chlorophyll, which absorbs red and blue but reflects green. The question arises: why do plants reflect the most energetic portion of the solar spectrum?
The evolutionary trap of green
The answer is simpler than it seems: someone else had already taken that slice. When chlorophyll appeared on the scene, purple retinal-based microorganisms already occupied the most advantageous ecological niche. They devoured yellow-green light, the most abundant in the solar spectrum. Chlorophyll-bearing organisms had to adapt to what was left: red and blue wavelengths.
The coexistence of purple and green organisms is still visible today in microbial carpets, layered colonies where different microbes exploit complementary portions of the light spectrum. It's possible that on early Earth, purple archaea dominated the upper layers of the oceans, forcing bacteria living "in the shade" to evolve using the remaining wavelengths.
And then, chlorophyll created a evolutionary trapIts chemical structure, based on porphyrins (complex rings that coordinate a magnesium atom), is so specialized for absorbing red and blue that it can no longer be modified to capture green light. Chlorophyll-bearing organisms were trapped in this biochemical choice, forever reflecting green even when, after the mass extinction of purple microbes, that portion of the spectrum became available again.
The Great Oxygenation Event
The purple reign ended with the advent of the cyanobacteria, chlorophyll-based photosynthetic organisms that had a lethal characteristic: they produced oxygen as a metabolic waste productAbout 2,4 billion years ago, oxygen began to accumulate, first in the oceans, then in the atmosphere. For the anaerobic organisms that had thrived up until then, it was poisonous.
The process took nearly a billion years. But when the atmosphere became permanently oxidized (the Great Oxygenation Event), it was one of the largest mass extinction events in Earth's historyAnaerobic archaea were forced to take refuge in oxygen-free environments: deep waters, sediments, and areas with minimal oxygen. Or they had to adapt to living in symbiosis with aerobic organisms, paving the way for the endosymbiosis that led to the emergence of eukaryotes.
This event also coincided with the Huronian glaciation, an ice age that lasted 300 million years. Oxygen had destroyed atmospheric methane (a powerful greenhouse gas), cooling the planet. Photosynthesis had just transformed the Earth irreversibly.
The Purple Survivors
The retinal has not disappeared. The Haloarchaea (salt-loving archaea) are organisms still widespread today in extreme environments: from the Dead Sea to the Great Salt Lake in Utah, to the salt lakes of the Andes. When they bloom en masse, they color the water a deep purple. The key protein is bacteriorhodopsin, a derivative of retinal that functions as a light-powered proton pump.
These organisms constitute one of the simplest bioenergetic systems known for capturing light energyThey don't fix carbon, they don't produce oxygen. They pump protons across the cell membrane, generating a gradient that fuels ATP synthesis. This is anoxygenic photosynthesis, a primitive but functional form of solar energy harvesting.
Searching for purple planets
The purple Earth hypothesis has profound astrobiological implications. If retinal is simpler than chlorophyll and may have appeared first on Earth, the same could happen elsewhere. Astrobiologists have traditionally looked for biosignatures Chlorophyll-related: planets that reflect green-yellow light. But if the evolution of retinal is as likely (or perhaps more likely) than that of chlorophyll-bearing systems, we should expand our research.
Planets that reflect red and blue light could host retinal-based biospheres. Purple worlds, biochemically simpler but just as vibrant. As he said, Edward Schwieterman, co-author of the study:
“If the purple Earth hypothesis is correct and there was a dominance of purple organisms on the early Earth, we might find another planet at an earlier evolutionary stage.”.
Earth has changed its wardrobe many times in its 4,54 billion years. Why wouldn't it have been purple? What if out there, on some distant exoplanet, there were still glowing magenta oceans populated by archaea that had never known oxygen?
The discovery of archaeal lipids in ancient sediments supports this hypothesis. These molecules are associated with primitive retinal, not chlorophyll. They suggest that the terrestrial biosphere was fueled by retinal before the advent of chlorophyll-based photosynthesis.
Before green, there was purple. And purple, perhaps, returns every time life begins anew.
