You may think science fiction writers don’t need to know much about Earth. Sci-Fi is (stereotypically) about exploring space, visiting alien planets, and leaving the homeworld’s cradle behind. So I almost skipped Earth for my Solar System series.
Then I realized that learning how Earth formed, how life evolved here, and why life continues to thrive on this one planet could help me understand what alien worlds might look like.
Studying Earth has left me with a lot to think about. Since this is my final post for Earth month, I thought I’d review some of my still germinating thoughts about life and the environments that might support it.
- Life tends to develop only in chemically active environments. Earth shows plenty of chemical activity, most notably oxygen-based chemistry. As a comparison, the surface of Mercury has virtually no chemical activity, and therefore it’s unlikely life in any form could develop there.
- More energetic chemical reactions allow for more complex organisms to evolve. Chemical reactions involving oxygen can provide far more energy than a single-celled organism needs, which allows for multi-cellular life forms to develop. Other chemical reactions, like those involving sulfur on Venus, might not provide enough excess energy for anything larger than a microbe.
- Microbial life may be absurdly common in the universe, taking advantage of every chemically active niche it can find. Microbes of some kind could exist on Mars or even Venus. They could live on certain moons of Jupiter and Saturn. They might even be able to eek out an existence among asteroids and comets.
- Complex life, on the other hand, may be exceedingly rare. It’s hard to find a chemical that is as profitable, from an energy production standpoint, as oxygen. I’m sure there are viable alternatives, but the list would be short, and this would limit opportunities for the evolution of multi-cellular organisms even in a universe teeming with microbes.
Of course, this is all speculation. Speculation that comes after months of exhausting, headache-inducing research—but still just speculation.
Until scientists can confirm the existence of life on Mars, Venus, or elsewhere, and until they collect more data on the environmental conditions of Earth-like planets orbiting other stars, this is the most realistic picture of life in the universe that I can invent.
So what do you think? Am I on the right track, or is there something I’ve overlooked? Any suggestions on other avenues of research I should pursue? Please leave your thoughts in the comments below. I look forward to getting other people’s perspectives on these questions.
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Today’s post is part of Earth month for the 2015 Mission to the Solar System. Click here for more about this series.
Planet Pailly 
Your view of the probabilities with microscopic and complex life match mine pretty closely. I would add that intelligent life, defined as life that uses sophisticated tools, is probably profoundly rare. The next nearest species might be millions, perhaps even billions, of light years away.
This is borne out by looking at how rare intelligence is in Earth’s history, and by the complete absence of any evidence for us ever having been colonized, despite the fact that the Earth has been sitting here for billions of years, with an oxygen atmosphere that’s detectable across interstellar distances.
Of course, someone might find evidence tomorrow that proves me wrong 🙂
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I really enjoyed your recent posts on the search for Type III civilizations in other galaxies. I have to admit a lot of my thinking on this subject has been influenced by your blog.
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Thanks James! Glad you’ve found them useful.
I’m also enjoying these posts you’ve been doing. Life, space elevators, and thalasocracy (a new term for me): all fascinating stuff.
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That’s an interesting point about the energy production of oxygen. Lots of sci-fi likes to talk about advanced methane-breathing life forms, for example – would that even be possible?
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