How different would Earth be if anaerobic life were the only life form, and could it have become sentient?
Some anaerobic life is obligate anaerobic – it can live only in oxygen-free environments, such as the human gut or deep mud. Other organisms can live without oxygen, but prefer it, or live anaerobically but aren’t harmed by oxygen.
The biggest problem is that the anaerobic breakdown of sugar is only partial and yields just 5 per cent of the energy that aerobic breakdown does, so anaerobic organisms are generally not multicellular.
Suppose that photosynthetic bacteria never appeared on Earth. If the planet’s atmosphere contained no oxygen, then oxygen-intolerant species that now thrive on Earth only, for example, in deep mud, could live anywhere, including right on the surface.
There are still huge obstacles to this life becoming multicellular and even intelligent. First is the low energy yield of anaerobic respiration, especially in the absence of sugars. Then, any intelligence would be of limited use as there are no predators to avoid and mobility is very limited. These organisms basically just sit there and feed and divide.
Let’s suppose somehow a multicellular, mobile, anaerobic, intelligent organism does evolve. It would probably look like a slime mould and be very slow-moving. It could perceive time slowly – days would seem like minutes to it. Its communication abilities would be limited, and it probably wouldn’t be able to perceive sound. Maybe it gets beyond chemical signals to a form of light signalling (its brain and sensory organs would process things very slowly, too). Maybe it could evolve (very slow-moving) limbs and even develop writing.
But then it faces physical and cultural developmental barriers similar to a marine intelligent organism on a Hycean world (a theoretical exoplanet with a hydrogen-rich atmosphere and oceans of liquid water).
It would have no fire, no metal smelting, no electricity, so its knowledge of chemistry and physics would remain very limited.
Maybe over millions of years it develops in the arts and social sciences, even biology and mathematics, but it would be hard for such an organism to develop space travel, or even build telescopes to study beyond its home planet.
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