Another fairly plausible great filter is in the interaction between life and geochemistry- this is a variant on ‘central nervous systems are unlikely’.
There is some reason to think that microbial metabolisms are somewhat destabilizing in the history of the planet, at least on very large scales. There is of course the classic example of photosynthesis producing atmospheric oxygen. Can you imagine what would happen if bacteria injected large amounts of free oxygen in to Titan’s atmosphere? Given the space of all geochemistry for terrestrial planets in the habitable zone, and the plausible metabolisms that could emerge from an RNA world on each of these planets, it may be that only a fraction of a fraction take place within the domain of a chemical ‘stable attractor’ that remains habitable.
This would allow for abiogenesis to be fairly common in the universe, without necessarily hand-waving nervous systems as ‘very hard’ for unspecified reasons. To get there, life is necessarily in a situation where microbes, maximizing their individual energy consumption according to local rules, reproduce exponentially with limited external coordination. The challenge in building a brain may be less about the technical complexity of that organ and more about the capacity of such a system to avoid self-sterilization in the required time scales.
Another fairly plausible great filter is in the interaction between life and geochemistry- this is a variant on ‘central nervous systems are unlikely’.
There is some reason to think that microbial metabolisms are somewhat destabilizing in the history of the planet, at least on very large scales. There is of course the classic example of photosynthesis producing atmospheric oxygen. Can you imagine what would happen if bacteria injected large amounts of free oxygen in to Titan’s atmosphere? Given the space of all geochemistry for terrestrial planets in the habitable zone, and the plausible metabolisms that could emerge from an RNA world on each of these planets, it may be that only a fraction of a fraction take place within the domain of a chemical ‘stable attractor’ that remains habitable.
This would allow for abiogenesis to be fairly common in the universe, without necessarily hand-waving nervous systems as ‘very hard’ for unspecified reasons. To get there, life is necessarily in a situation where microbes, maximizing their individual energy consumption according to local rules, reproduce exponentially with limited external coordination. The challenge in building a brain may be less about the technical complexity of that organ and more about the capacity of such a system to avoid self-sterilization in the required time scales.