Source: NASA Astrobiology
Beginning two billion years of Earth’s history, the amount of oxygen was not sufficient, but rich in carbon dioxide and methane. The early forms have been able to generate metabolic energy from sunlight using the ‘purple’ pigment, predating the evolution of chlorophyll and photosynthesis. The pigment called retinal, if evolved, could absorb green light in the same way the Earth absorbs red and blue light. As reported by Astrobiology at NASA and Earth.com, around 2.4 billion years ago, the Great Oxygenation Event saw the abundance of free oxygen rise in the atmosphere, which may be due to ‘cyanobacteria’ that are said to be able to perform photosynthesis. Researchers now suggest that the purple pigment retinal may have appeared before chlorophyll.
What made Earth look purple in early forms of life
Scientists believe that the earliest life on Earth may have used sunlight for energy in a different way than plants do today. Instead of cholorophyll, these early life-forms may have relied on the pigment called retinal, which absorbs green light while plants absorb mostly red and blue light. After 2.4 billion years, the Earth saw free oxygen rise. However, photosynthetic life existed long before this oxygen increase, and scientists still do not fully understand why oxygen took so long to build up. Some natural processes removed oxygen from the atmosphere, slowing the change. Researchers now suggest that retinal may have appeared before chlorophyll, with both pigments evolving together and absorbing different parts of sunlight.Professor of molecular biology from the University of Maryland, Shiladitya DasSarma, and Dr Edward Schwieterman, an astrobiologist from the University of California, have illustrated that both pigments evolved in absorbing sunlight at different wavelengths. “Retinal-based phototrophic metabolisms are still prevalent throughout the world, especially in the oceans, and represent one of the most important bioenergetic processes on Earth,” says DasSarma.
How ‘purple’ Earth turned green
The green pigment that makes plants look ‘green’ and vibrant is Chlorophyll, and most importantly, it is the powerhouse behind photosynthesis, which lets us turn sunlight into energy. Now, since early forms are believed to have oxygen in scarce amounts and without chlorophyll, it would not have been the same as it exists today. This mainly absorbs light from the red and blue spectra, reflecting leaves to look green. Although modern science on plants relies on chlorophyll, it may not be Earth’s first choice; that might be retinal, which was likely to be present when oxygen was believed to be scarce.During that period of low oxygen, the sunlight was still abundant to power these purple microbes, and points to a different, lush green Earth than what we see now. One of the most prominent examples of a bacterium that absorbs green wavelengths through retinal and reflects red and blue, resulting in a striking purple appearance, is halobacterium producing striking purple appearance, for example, Dead Sea. Over time, since other organisms evolved, the more efficient pigment chlorophyll enable them to harvest sunlight. This did not make retinal-based life disappear, but it is no longer a dominant force shaping the planet’s surface colour. Go to Source
