2 billion years ago, the Earth was devoid of life as we know it. There were no plants, animals or fungi. The first multicellular organisms were still another billion years away. The sun was smaller and cooler. There was not much oxygen in the air. There were only bacteria. They had evolved to tap into the abundant sources of carbon and hydrogen around them. They drained the atmosphere of carbon dioxide, as well as hydrogen and hydrogen sulfide that had been spewed by volcanoes into the atmosphere. Now there was not enough hydrogen. The photosynthetic bacteria began to starve. They hadn't figured out how to extract hydrogen from water (dihydrogen oxide). The bonds between hydrogen and oxygen in water were too strong for them to break. But then some cyanobacteria figured out the impossible. “In an evolutionary innovation unprecedented, as far as we know, in the universe, the blue-green alchemists, using light as energy, had extracted hydrogen from one of the planet’s riches resources, water itself. This single metabolic change in tiny bacteria had major implications for the future history of all life on Earth.” (Margulis and Sagan, Microcosmos) Extracting hydrogen in this way produced a tremendous amount of oxygen as a waste product. As oxygen accumulated in the atmosphere, it began to kill off life in an event referred to as the Oxygen Holocaust. The world holocaust comes from the Greek words ὅλος (whole) and καυτός (burnt), and it means "the complete consumption by fire". “When exposed to oxygen and light, the tissues of these unadapted organisms are instantly destroyed by subtle explosions" (Margulis and Sagan). Meaning, they died from internal combustion. In a moment of sheer desperation, some of the microbes found a metabolic strategy that made the combustion a necessity. That strategy is what we call breathing. “Aerobic respiration, the breathing of oxygen, is an ingeniously efficient way of channeling and exploiting the reactivity of oxygen. It is essentially controlled combustion that breaks down organic molecules and yields carbon dioxide, water, and a great deal of energy into the bargain. Whereas fermentation typically produces two molecules of ATP from every sugar molecule broken down, the respiration of the same sugar molecule utilizing oxygen can produce as many as thirty-six." > With the spontaneous emergence of the controlled combustion that is breathing, life closed a new cycle of participation that had not previously existed. Powered by the sun, some creatures now fed on water and excreted oxygen, while others turned excrements into gifts, using oxygen’s fire in their favor, in turn excreting—gifting back—carbon dioxide and water. The Oxygen Holocaust inaugurated a whole cascade of creativity, leading to untold new food cycles, relationships, mutually beneficial dependencies. Out of a situation of peril and necessity, life spawned gift cycles of participation. This planetary gift exchange needed only the constant gift of solar light to keep everyone nourished. Life was no longer predicated on drawdown, but had learned how to integrate itself more deeply into the body of the Earth. In the process, it had also learned to participate indefinitely with the body of the sun. Life drank sun. It breathed sun. It ate sun. It embodied sun. > – [[Being Salmon, Being Human]] --- Source: [[Being Salmon, Being Human]]