Astrobiology IV: Photosynthesis and energy
Originally I sat down to write about the large-scale history of Earth, and line up the big developments that our biosphere has undergone in the last 4 billion years. But after writing about the reason that Earth is unique in our solar system (that is, photosynthesis being an option here), I guess I needed to explore photosynthesis and other forms of metabolism on Earth in a little more detail and before I knew it I’d written more than 3000 words about it. So, here we are, taking a deep dive into photosynthesis and energy metabolism, and trying to determine if the origin of photosynthesis is a rare event or likely anywhere you get a biosphere with light falling on it. Warning: gets a little technical.
In short, I think it’s clear from the fact that there are multiple origins of it that phototrophy, using light for energy, is likely to show up anywhere there is light and life. I suspect, but cannot rigorously prove, that even though photosynthesis of biomass only emerged once it was an early development in life on Earth emerging very near the root of the Bacterial tree and just produced a very strong first-mover advantage crowding out secondary origins of it, and would probably also show up where there is life and light. As for oxygen-producing photosynthesis, its origin from more mundane other forms of photosynthesis is still being studied. It required a strange chaining together of multiple modes of photosynthesis to make it work, and only ever happened once as well. Its time of emergence, early or late, is pretty unconstrained and I don’t think there’s sufficient evidence to say one way or another if it is likely to happen anywhere there is photosynthesis. It could be subject to the same ‘first mover advantage’ situation that other photosynthesis may have encountered as well. But once it got going, it would naturally take over biomass production and crowd out other forms of photosynthesis due to the inherent chemical advantages it has on any wet planet (that have nothing to do with making oxygen) and its effects on other forms of photosynthesis.
Oxygen in the atmosphere had some important side effects, one which most people care about being allowing big complicated energy-gobbling organisms like animals – all that energy that organisms can get burning biomass in oxygen lets organisms that do so do a lot of interesting stuff. Looking for oxygen in the atmospheres of other terrestrial planets would be an extremely informative experiment, as the presence of this substance would suggest that a process very similar to the process that created our huge diverse and active biosphere were underway.