Did Earth Used To Be Purple?

What Exactly Does “Purple Earth” Mean?

The Purple Earth hypothesis was first proposed in 2007 by molecular biologist Shiladitya DasSarma of the University of Maryland. DasSarma’s theory takes us all the way back to Earth’s distant past, specifically the Archean Eon between 3.5 and 2.4 billion years ago. (This was before the Great Oxidation Event, which marked the rise of oxygen in the Earth’s atmosphere.) 

According to DasSarma and his colleague Edward W. Schwieterman, early microorganisms may have used retinal-based phototrophy to capture sunlight during that early period. Retinal is a much simpler molecule than chlorophyll, and that biochemical simplicity makes it an attractive candidate for early life. It’s easier to synthesize than the complex porphyrin-based chlorophyll, requiring fewer evolutionary steps and resources. 

And retinal-based organisms still exist today. Halophilic archaea — microorganisms that thrive in extreme environments such as the Dead Sea and the Great Salt Lake — possess adaptive advantages under oxygen-poor, high salt, and high radiation conditions thanks to their retinal proteins. 

The retinal lends those halophiles their vivid purple coloring. Retinal pigments absorb green and yellow light and reflect or transmit red and blue light — making retinal-based life appear magenta or purple in color. 

Chlorophyll, by contrast, absorbs red and blue light, making chlorophyll-based plants appear green, because they reflect green light rather than absorb it. So if our planet was once home to predominantly retinal-based life, the surface biosphere would have appeared purplish rather than its current greenish color. 

So Why Is the Earth Green?

If purple organisms did dominate early Earth, the question becomes: What happened to them? According to DasSarma and Schwieterman, they largely succumbed to their competition, chlorophyll. Multiple evolutionary steps led to chlorophyll pigments with progressively higher efficiency, which eventually led to photosynthetic microorganisms outcompeting retinal-based phototrophic microorganisms in most environments. 

Over time, chlorophyll-based photosynthesizers, including cyanobacteria and eventually algae and plants, ousted their purple predecessors. The ecological success of cyanobacteria was the fundamental driver of the Great Oxidation Event, which began some 2.3 billion years ago, flooding the atmosphere with oxygen and fundamentally altering Earth’s chemistry — along with its color palette. But retinal-based organisms never disappeared entirely — they simply retreated to ecological niches where they continue to thrive today. 

What Does This Imply for Finding Life Beyond Earth?

The Purple Earth hypothesis doesn’t just concern our planet. DasSarma and Schwieterman note that their theory may also have profound implications for astrobiology and the search for extraterrestrial life. 

When scientists look for biosignatures on exoplanets, they often search for chlorophyll’s characteristic “red edge” biosignature — the sharp change in reflected light at red wavelengths caused by vegetation. But if purple organisms were to dominate a planet’s surface, they would create a different spectral signature — a “green edge” in a planet’s spectrum that could potentially be a biosignature for retinal-based life.

So a planet in an earlier stage of biological evolution may appear purple from space, and we could miss it entirely if we’re only looking for Earth-like green biosignatures. What’s more, the simplicity of retinal-based phototrophy may make it even more common throughout the universe than chlorophyll-based life.

So while we can’t say for certain whether Earth truly experienced a purple phase or not, the hypothesis reminds us that the universe may be far quirkier and more colorful than we ever imagined.