Earth is “the blue planet”, or “the blue dot” if you prefer a definition that dates back to 1990, when the Voyager probe took one of the most famous photos in the history of astronomy. An idea that, however, has been questioned several times in recent years: in April this year, for example, we told you about a study that suggested that the Earth’s oceans were once green, not blue.
There is another even more extreme hypothesis in this regard, first proposed by Shiladitya DasSarma, a molecular biologist, and Edward Schwieterman, an astrobiologist: the idea that the Earth was once neither green nor blue, but purple.
Blue, green or purple oceans? The purple Earth hypothesis looks even further back than the green oceans hypothesis. According to this last theory, the first photosynthetic bacteria coexisted with others that, instead of using sea water as a catalyst, used iron: these bacteria thrived in green waters, and the large concentrations of iron created with the birth of photosynthesis suggests that the Earth’s oceans were, in fact, greener than blue.
Purple Earth, on the other hand, is based on the fact that photosynthesis arrived “late”, and that before the birth of chlorophyll as a “light-absorbing” molecule, the first life forms used another one, called retinal.
Chromatic issues. Chlorophyll absorbs all light radiation except green: this is the reason why modern plants are more or less all this colour. The retinal, on the other hand, absorbs green and reflects violet; Furthermore, it is a simpler molecule than chlorophyll, so according to those who formulated the hypothesis it appeared first on Earth.
Another “trick” of retinal is that it does not need oxygen to carry out photosynthesis: at the time (we are talking about 3 billion years ago), oxygen was present in very low concentrations in the atmosphere, and bacteria that used retinal could thrive.
All the colors of life. According to those who formulated the purple Earth hypothesis, the situation changed radically 2.4 billion years ago, with the start of the Great Oxidative Event, also known by the more incisive name of the oxygen catastrophe. It is the moment in which the cyanobacteria that used chlorophyll began to dominate: their photosynthesis was not limited to producing energy, but also oxygen, in fact, as waste material. The gas began to accumulate in the atmosphere to levels that allowed “classical” photosynthetics to thrive, to the detriment of the purple ones.
Today. Since then, and excluding the period mentioned above during which the oceans were more green than blue, the color of the Earth has changed; today there are very few bacteria that use retinal instead of chlorophyll: most are in the class of Halobacteriawhich survive in environments poor in oxygen but rich in salt.
Life on Earth today depends largely on chlorophyll, photosynthesis and the color green, but these hypotheses show us that there are also other ways (and other colors) to create it.