Why Early Earth May Have Been Mostly Purple
Anyone who has seen an image of Earth from space can instantly recall our planet's fiercely blue oceans, resplendent green forests, and splotchy brown deserts. This work of art is our wondrous home.
But between 3 and 3.8 billion years ago, Earth may have been unrecognizable, its modern medley of colors instead dominated by one: purple.
University of Maryland-Baltimore Professor Shiladitya DasSarma originally painted this picture at 2007's American Astronomical Society Meeting. His 'Purple Earth' hypothesis has remained a speculative delight ever since and now has growing evidence to support it.
The hypothesis is that Earth's earliest microbes were phototrophs, capturing photons from sunlight to produce energy for themselves. This isn't too surprising – the plants that dominate Earth today are also phototrophs. However, unlike modern plants, which utilize the pigment chlorophyll to capture light, these ancient microbes might have used a pigment called retinal. Chlorophyll absorbs red and blue light while reflecting green. Retinal does the opposite, meaning microbes making use of it would appear purple.
The San Francisco salt ponds pictured at the top of the article tease what Earth's oceans might have looked like billions of years ago. Back then, the planet was far hotter and bathed in copious ultraviolet light, with much higher concentrations of sulphur and methane. Modern retinal-using microbes call haloarchaea love these sorts of environments, and it could have been their descendants that dominated a purple planet.
Why might retinal have arisen before chlorophyll? First off, it is very easy to produce. Second, the Sun sends out a lot of photons in the green light spectrum, far more than in blue or red actually, so it makes sense that phototrophs capable of feasting on this light would dominate early on. Moreover, as DasSarma wrote, "A retinal-based phototrophic system clearly represents one of the simplest bioenergetic mechanisms conceivable." Chlorophyll-based phototrophy, on the other hand, is far more complex.
But it's also more efficient. DasSarma hypothesizes that green, chlorophyll-based microbes might have evolved after purple, retinal-based microbes and slotted themselves below the retinal microbes in the water column, where they absorbed the photons in the blue and red spectrum of light not reflected by the retinal microbes. Eventually, the green microbes outcompeted their purple overlords, gradually changing Earth's colors as they evolved and diversified into algae and plants.
DasSarma's tantalizing vision is highly speculative, but it makes sense. Earth has gone through numerous wardrobe changes in its 4.54 billion-year history before arriving at its present "Blue Marble" motif, why couldn't it have been purple at one time?
More than anything, the "Purple Earth" hypothesis grants astrobiologists cause to search for life on more exoplanets that don't look like ours. Life, after all, comes in all sizes, shapes, and colors.