Purple Earth hypothesis

The Purple Earth hypothesis 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 its current greenish color. The time would date somewhere between 3.5 to 2.4 billion years ago, prior to the Great Oxygenation Event and Huronian glaciation.
Retinal-containing cell membrane exhibits a single light absorption peak centered in the energy-rich green-yellow region of the visible spectrum, but transmit and reflects red and blue light, resulting in a magenta color. Chlorophyll pigments, in contrast, absorb red and blue light, but little or no green light, which results in the characteristic green color of plants, green algae, cyanobacteria 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 membrane prior to the turquoise of the Canfield ocean and later green photosynthetic organisms.[citation needed]
Category:Scientific theories
Category:Fermi Paradox
Category:First Contact