Purple Earth hypothesis

The Purple Earth hypothesis (PEH) is an astrobiological hypothesis, first proposed by molecular biologist Shiladitya DasSarma in 2007, that the earliest photosynthetic life forms of Early Earth were based on the simpler molecule retinal rather than the more complex porphyrin-based chlorophyll, making the surface biosphere appear purplish rather than its current greenish color. It is estimated to have occurred between 3.5 and 2.4 billion years ago during the Archean eon, prior to the Great Oxygenation Event and Huronian glaciation.

Retinal-containing cell membranes exhibit a single light absorption peak centered in the energy-rich green-yellow region of the visible spectrum, but transmit and reflect red and blue light, resulting in a purple color. Chlorophyll pigments, in contrast, absorb red and blue light, but little or no green light, which results in the characteristic green reflection of plants, cyanobacteria, green algae, and other organisms with chlorophyllic organelles. The simplicity of retinal pigments in comparison to the more complex chlorophyll, their association with isoprenoid lipids in the cell membrane, as well as the discovery of archaeal membrane components in ancient sediments on the Early Earth are consistent with an early appearance of life forms with purple membranes prior to the turquoise of the Canfield ocean and later green photosynthetic organisms.

Evidence

The discovery of archaeal membrane components in ancient sediments on the Early Earth supports the PEH because these lipids are linked to archaeal retinal, a primitive light-harvesting pigment instead of chlorophyll, suggesting that early Earth's biosphere was purple and powered by retinal before the advent of chlorophyll-based photosynthesis which was introduced during the Great Oxygenation Event.

Modern examples of retinal-based photosynthesis

An example of retinal-based organisms that exist today are photosynthetic microbes collectively called Haloarchaea. Many Haloarchaea contain the retinal derivative protein bacteriorhodopsin in their cell membrane, which carries out photon-driven proton pumping, generating a proton-motive gradient across the membrane and driving ATP synthesis. The process is a form of anoxygenic photosynthesis that does not involve carbon fixation, and the haloarchaeal membrane proton pump constitutes one of the simplest known bioenergetic systems for harvesting light energy.

Evolutionary history

Microorganisms with purple and green photopigments frequently co-exist in stratified colonies known as microbial mats, where they may utilize complementary regions of the solar spectrum.