Photosynthesis Evolution and Earth's Habitability

from its wavelength according to $E = h \lambda^{-1}$ (16) which, in the case of a red visible wavelength around 680nm, equals an energy per photon around 1.82 eV. Currently, through photosynthesis processes, power on the order of $10^{12}$W is captured. Photosynthesis evolved in the bacterial domain and initially used geochemical compounds, such as ferrous iron or hydrogen, as electron sources without producing oxygen. However, the most relevant for terrestrial evolution is oxygenic photosynthesis that extracts oxygen from water according to the reaction: $$\text{CO}_2 + 2\text{H}_2\text{O} + (h\nu) \rightarrow \text{CH}_2\text{O} + \text{H}_2\text{O} + \text{O}_2$$ This is a distinctive property of cyanobacteria, so it is legitimate to suppose that its appearance as an integrated metabolic pathway marked the appearance of this bacterial lineage probably 3 billion years ago. Equipped with flexible and versatile metabolism, cyanobacteria occupied a new evolutionary landscape without competitors. They spread across the planet, maintaining a position of ecological dominance that ended with the global spread of photosynthetic eukaryotes. Around 2.5 billion years ago, a change in planetary geochemistry allowed the average oxygen concentration to rise above 0.001% with the consequent formation of a thin ozone layer that reduced methane photolysis and triggered the transition to an oxidized atmosphere. The atmospheric oxygen concentration stabilized at a low level for more than a billion years. A second increase in oxygen concentration began about 800 million years ago and then exceeded 10%, thus supporting the evolution of complex life. Through oxygenic photosynthesis, biological life was able to free itself from its dependence on terrestrial thermal sources and emerge onto the Earth's surface. Being a strong oxidant, oxygen is highly toxic to unprepared organisms and for this reason the appearance of oxygen in the atmosphere probably caused the extinction of a large number of life forms. From a threat to life, however, oxygen quickly proved to be a great opportunity. New forms evolved, in fact, that not only could resist the oxidative damage of oxygen, but were able to use this new chemical in high-energy aerobic respiration. The Great Oxidation, described in the previous paragraph, allows us to understand that it would be naive to consider the evolution of Earth without simultaneously considering the evolution of biological life that develops on it. The evolution of intelligent life on a planet requires not only that planetary conditions be favorable to life at the beginning, but also that the planet remains habitable afterwards, without interruptions. This is because there must be enough time to allow life to increase its complexity starting from simple cells, then arriving at more sophisticated single cells, to multicellular life and eventually to intelligent life. Conditions have remained habitable, and at first glance this may not seem surprising. However, observations and deductions have accumulated to demonstrate that such a long duration of habitability is actually a puzzling phenomenon that requires explanation. We note in fact that today's solar warming of Earth is about 30% more intense than that of primitive Earth, so primitive Earth should have been totally frozen, or conversely, since primitive Earth was not frozen, the oceans should now be boiling under today's brighter Sun. Instead, Earth has maintained conditions of habitability compatible with biological life for billions of years, through compensation mechanisms not yet understood.