Before the evolution of chlorophyll-based algae turned large swathes of our planet green, young Earth might have sported patches of a lovely purple hue.

This theory, put forward by molecular biology professor Shiladitya DasSarma and astrobiologist Edward Schwieterman, is based on observations that nearly all forms of life, back to most ancient single-celled organisms, produce a pigment that can drive a light-to-energy reaction using photons in the yellow to green spectrum – making it appear purple when light in the blue and red wavelengths are bounced back.  

Outlining the “Purple Earth” argument in the International Journal of Astrobiology, the authors explain that the chromoprotein retinal is excellent at absorbing light in the 490-600 nm range. When bound inside a cell membrane, retinal can use this Sun-harvested energy to build ATP, the cellular fuel molecule. Now, this version of the phototrophic process is not very efficient compared to photosynthesis – and it does not result in the creation of free oxygen or sugars – but retinal is a much simpler molecule than chlorophyll, and is therefore easier for cells to make.

“Retinal-based phototrophic metabolisms are still prevalent throughout the world, especially in the oceans, and represent one of the most important bioenergetic processes on Earth,” DasSarma told Astrobiology Magazine.

Attempts to estimate when early life gained the ability to make the pigment – using a genetic mutation analysis technique called the “molecular clock” – have been hazy thus far, but the evidence suggests it was very soon after life emerged some 4 billion years ago. Similarly, researchers are unsure about the precise timeframe of chlorophyll’s emergence, but we do know that it happened at some point before 2.3 billion years ago – when, due to a combination of poorly understood factors, existing single-celled photosynthetic algae suddenly took over the ecosystem, causing what is called the Great Oxygenation Event.

According to DasSarma and Schwieterman, one significant cause of the switchover was the evolution of increasingly efficient chlorophyll systems, which allowed organisms with them to outcompete the more primitive retinal phototrophs.

“Evolution of anoxygenic photosynthesizers was followed by oxygenic photosynthesizing cyanobacteria and ultimately eukaryotic algae and plants,” they wrote. “The development of eukaryotic algae and complex plants and their spread throughout the terrestrial environment allowed the evolution of land animals and ultimately intelligent life.”

Yet prior to Earth's conquest by complex chlorophyll-based organisms, the first living things may have thrived by evolving the means to harness retinal for metabolism, thus explaining why the genes to produce components of this pathway are present in modern-day organisms whose ancestors branched away from one another very early on. Then, exploiting an unfilled niche, a primordial chlorophyll-based metabolism cropped up, and two existed harmoniously for some time. For chlorophyll, the optimal light energy is around 350 to 500 nm and 680 to 700 nm, the exact bands of the visual spectrum that retinal molecules bounce back entirely.

“This is exactly what got us thinking that the two pigments – retinal and chlorophyll – may have co-evolved,” DasSarma added. To further investigate the “Purple Earth” hypothesis, he and Schwieterman propose future work focusing on natural communities of retinal-based phototrophs in diverse environments, as this may reveal unexpected roles and niches for this metabolism and inform its evolutionary origin.