Purple Earth hypothesis

The Purple Earth hypothesis is an astrobiological hypothesis that life forms of early Earth were retinal-based rather than chlorophyll-based, making Earth appear purple rather than green. An example of a retinal-based organism today is the photosynthetic microbe collectively called halobacteria.

The difficulty with this hypothesis is that retinal-based photosynthesis requires either arginine or oxygen in the environment. As a complex amino acid, arginine is not likely to have been available in the levels needed. Oxygen only became available in earth's environment after the development of chlorophyll-based photosynthesis, and given that retinal organisms also require oxygen to synthesize critical substances for life, the possibility that retinal life-forms could have preceded chlorophyll-based ones as the most common photosynthetic organisms is unlikely.