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29 Oct—Oxygen Catastrophe
A catastrophe had been in the works ever since the discovery of oxygenic photosynthesis. The organisms who converted sunlight, water, and carbon dioxide into food and oxygen were slowly poisoning their planet. About 2,400 million years ago—October 29th on the Cosmic Calendar—the buildup of oxygen in the environment led to a crisis for life on Earth.
Oxygen was a deadly poison to early life. It reacts strongly with organic chemicals, burning them in a kind of slow fire.
At first, the iron in rocks absorbed the oxygen created by photosynthesis. The rocks sequestered the oxygen and kept life safe from the toxic waste, but after millions of years, the rocks couldn't store any more oxygen. Organisms were also being buried and taking their carbon with them where it couldn't react with free oxygen to become carbon dioxide. Over time, oxygen began to find its way into the environment, and the Oxygen Catastrophe began.
Life couldn't stay the same. It had to adapt to the environment it had created. It had to find a way to live on or face extinction. Many lifeforms died out. They had evolved in an environment without free-roaming oxygen and were unprepared to face this change. The Oxygen Catastrophe was the Earth's first mass extinction.
Other forms of life found a way to cope with their toxic planet. They adapted showing life's amazing talent for innovation. It not only survived, life turned this new environment to its favor and learned to thrive.
Oxygen remade the surface of the Earth and made complex life possible. It's hard to overstate the difference that oxygen has made in the history of life on Earth.
The biggest difference it made was in making more energy available. Large organisms could eat small organisms and get more out of their meal. Earlier forms of biochemistry were terribly inefficient. At each level of the food chain, 90% of the energy stored in an organism's body would be lost when eaten by another. Oxygenic biochemistry lost only 40% at each level. This allowed much more complex food chains, and much larger, more complex lifeforms. Life may never have become multicellular if cyanobacteria hadn't discovered oxygenic photosynthesis.
Oxygen also created a protective layer in the upper atmosphere, the Ozone Layer, which blocked harmful ultraviolet radiation from reaching the Earth's surface. This paved the way for the later colonization of the land which had been to dangerous for life up to that point.
Oxygen even prevented the oceans from evaporating. Without oxygen, ultraviolet radiation would occasionally split ocean water into oxygen and hydrogen. The oxygen might react with something else leaving the hydrogen (even lighter than helium) to float up through the atmosphere and escape into space. The oceans would have evaporated into space over the course of eons.
With free oxygen in the atmosphere courtesy of photosynthesizers, any hydrogen that escapes the ocean today will bump into an oxygen molecule and become water again. Instead of escaping the Earth, it falls back to the surface as rain.
Mars shows us what may have happened to Earth. Mars probably had oceans long ago. Without life that knew how to photosynthesize, Mars probably lost its oceans and became a rusty desert as the oceans escaped into space.1
As if all that weren't enough, oxygen is essential in the formation of lignin and collagen which give structure to plants and animals respectively. Without lignin to surround their cell walls, plants would be too weak to stand on their own. Collagen surrounds animal cells and is the primary component of cartilage, ligaments, tendons, bone, and skin. Without collagen, animals would have all looked like amoebas lacking any structure.
To top it all off, oxygen makes the sky blue.
Throughout the Oxygen Catastrophe, life showed resilience and innovation. What looked like an utter disaster turned out to also be an opportunity for life to grow in new and exciting ways.
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1. Mars may have lost its oceans even if it had oxygenic photosynthesizers, because of its small size and consequently lower gravity.