Very Early Earth and the Origin of Life
Earth is our one and only known example of a planet that has supported life and continues to do so. Long may life here continue…
We don’t know for certain where life started. It may have begun at hydrothermal vents deep in the ocean, in a small warm pond at the ocean’s edge, or in rocks deep below earth’s crust.
The vast majority of earth’s ecosystems are based directly off sunlight. The amount of energy received from the sun has gradually been increasing but the temperature of the planet does not appear to have done so in sync with the amount of solar energy received. Carbon dioxide levels and oxygen levels have changed enormously.
It is perfectly possible to have entire ecosystems where the energy source is not the sun. Certain chemical reactions are used as an energy source by some micro-organisms. These reactions can occur in the absence of sunlight. For example, hydrogen sulfide is used as an energy source by certain micro-organisms, most famously those living in hydrothermal vents in the deep ocean.
The Arrival of Oxygen on Early Earth
There was very little free oxygen in the atmosphere of early Earth. Life was an anaerobic activity which means it happened without using oxygen. Oxygen is actually a potent toxin to most organisms evolved to live without it. While some organisms at this time used photosynthesis, the method they used did not produce oxygen as a byproduct. This meant oxygen did not build up in the atmosphere.
Then cyanobacteria evolved, flourished, and caused the release of large amounts of oxygen into the earth’s atmosphere. Many of the life forms on Earth at the time were poisoned by the oxygen. These went extinct or became restricted to environments in which oxygen is scarce or nonexistent such as the black mud in mud flats or on the bottom of some lakes.
Evolution of Multicellular Life
With all of this oxygen available, organisms that required oxygen evolved. These are called aerobic organisms. These organisms not only could survive with oxygen present but died without it. It is believed that aerobic metabolisms were necessary for complex animal life to evolve because aerobic metabolism is more efficient and produces more energy for the organism. Scientists believe that it is this that allowed the first multicellular organisms to evolve.
Implications for the Science Fiction Writer
Of course, it’s possible that they could be wrong. With only the one planet to study, it’s very hard to make generalizations. Perhaps if oxygen-producing photosynthesis had not evolved on Earth, multicellular organisms based on some other chemical pathway would have evolved and survived. Perhaps there are others out there that never evolved on Earth that are even more efficient. However, it does suggest that for complex alien life oxygen-based metabolisms and oxygen-producing photosynthesis is a good place to start.
It may well be that on many planets oxygen-producing photosynthesis never evolved and nor did complex multicellular life, so microbe-only ecosystems on a planet with an oxygen-poor atmosphere are an excellent and likely setting. You can also use the widest number of energy sources and the harshest conditions if you are only dealing with microbes.
It seems the more complex life gets, the more delicate it is. This is a bit frustrating when writing science fiction, as most of the memorable aliens imagined are not microbes. Of course, science fiction is fiction and the balance you strike between fantasy and fact will be different for each writer, and probably for each story you write.
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