Purple Earth Hypothesis

The hypothesis that ancient photosynthetic organisms on Earth (or on other planets) could have used Retinal based photosynthesis instead of the common Chlorophyll-based photosynthesis today. The Absorption Spectrum of Chlorophyll is bimodal with sensitivity to blue and red regions, and therefore reflects green light, which is why Earth is dominated by green wavelengths.

In humans, Retinal is the Chromophore in all of our Opsins, where it is used for vision, not for energy. However, in some archaea, similar opsins to our Rhodopsins (Bacteriorhodopsin) are used for energy production. They have a Peak Wavelength at about 550nm and absorb the most green and therefore reflect the most purple (green’s Complementary Color). When these archaea were the dominant lifeform on Earth, several billion years ago, Earth could have likely been colored purple.