Purple Earth hypothesis

The Purple Earth Hypothesis (PEH) is an astrobiological hypothesis, first proposed by molecular biologist Shiladitya DasSarma in 2007,[1] 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.

The process is a form of anoxygenic photosynthesis that does not involve carbon fixation, and the haloarchaeal membrane protein pump constitutes one of the simplest known bioenergetic systems for harvesting light energy.

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.

This coincided with a 300 million year-long global ice age at beginning of the Proterozoic known as the Huronian glaciation (which might also have been partly caused by the oxidative depletion of the atmospheric methane — a powerful greenhouse gas — due to the Great Oxygenation) and devastated the anaerobic biota, leaving the niches open for eubacteria that evolved antioxident capabilities (both the aerobic proteobacteria and the photosynthetic cyanobacteria) to exploit and prosper.

Historically, scientists sought out planets reflecting light in the green-yellow range as possible hosts to photosynthetic organisms, due to the implied presence of chlorophyll.