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Fermi Paradox addendum
A couple things I missed in the previous post.
1. Photosynthesis. File under "never trust Wikipedia": Photosynthesis, in the sense of creating food from sunlight, evolved several times in bacteria. Thanks to bacteria's habit of promiscuously exchanging their genes with one another, there's no clear lineage. But if you break the metabolic pathway down into its component parts, some parts of photosynthesis have several different implementations, which shows they evolved multiple times. Other parts of the apparatus have only one implementation, suggesting they evolved only once. Exactly when anaerobic photosynthesis got started is unclear, but fossils that look like stromatolites have been dated to 3.5 billion years ago (hereafter gya), so only 600 million years after the beginnings of life (currently thought to be around 4.1 gya). And clearly, the Sun is the biggest energy source around, so it makes sense that life would have started using it to make food very quickly.
Clear evidence for free oxygen starting to build up in the atmosphere appears around 2.4 gya. Exactly when aerobic photosynthesis got started, on the other hand, is more equivocal. At 2.7 gya, we have a lake in Australia which lacked any of the minerals necessary for anaerobic photosynthesis, so by process of elimination the abundant stromatolite fossils there must have been aerobic. At 3.2 gya we have shales, also in Australia, where sedimentary deposits hundreds of meters thick and hundreds of kilometers in extent formed with high organic content but very low sulfur or iron content, so the only remaining source of food for the huge amount of biomass would be aerobic photosynthesis. Going further back becomes rather difficult as older sedimentary rocks (the oldest being 3.8gya) are highly metamorphized, so any organic content has been baked and no fossils can be discerned.
In sum, evidence for aerobic photosynthesis goes back almost as far (900 million years after the evolution of life) as the evidence of photosynthesis itself (600 million years after the evolution of life). And our inability to date either further back is quite likely due to the lack of rocks preserving biological evidence from earlier eras.
The only way to extrapolate from Earth to exoplanets is to assume that, first, innovations that evolved multiple times on Earth are pretty inevitable given the right perquisites, and second, things that took a long time to happen after the perquisites were in place are less likely, while those that happened quickly are more likely. So it looks like anaerobic photosynthesis is rather likely, and aerobic photosynthesis is somewhat less likely but not hugely so.
2. Eukaryotes. I totally forgot about this bit. You can't have multicellular organisms without eukaryotes. The oldest fossils with unambiguous eukaryotes in them are only around 2.2gya, but attempts to trace their lineage via genetic analysis yield an ancestry that is probably much older. Furthermore, we know that the basis of eukaryotic evolution - merging of ur-eukaryotes and ur-mitochondria - is symbiosis, and that's so incredibly common that it seems silly to claim that it's an unlikely evolutionary event for two bacteria to decide to hook up.
So, it seems likely that quite a few of those million or so lifebearing planets in the galaxy will have not just life, but multicellular life on them, assuming they have been around about as long as Earth, which needed about 4 billion years to build up enough atmospheric oxygen to support large, complex creatures.
1. Photosynthesis. File under "never trust Wikipedia": Photosynthesis, in the sense of creating food from sunlight, evolved several times in bacteria. Thanks to bacteria's habit of promiscuously exchanging their genes with one another, there's no clear lineage. But if you break the metabolic pathway down into its component parts, some parts of photosynthesis have several different implementations, which shows they evolved multiple times. Other parts of the apparatus have only one implementation, suggesting they evolved only once. Exactly when anaerobic photosynthesis got started is unclear, but fossils that look like stromatolites have been dated to 3.5 billion years ago (hereafter gya), so only 600 million years after the beginnings of life (currently thought to be around 4.1 gya). And clearly, the Sun is the biggest energy source around, so it makes sense that life would have started using it to make food very quickly.
Clear evidence for free oxygen starting to build up in the atmosphere appears around 2.4 gya. Exactly when aerobic photosynthesis got started, on the other hand, is more equivocal. At 2.7 gya, we have a lake in Australia which lacked any of the minerals necessary for anaerobic photosynthesis, so by process of elimination the abundant stromatolite fossils there must have been aerobic. At 3.2 gya we have shales, also in Australia, where sedimentary deposits hundreds of meters thick and hundreds of kilometers in extent formed with high organic content but very low sulfur or iron content, so the only remaining source of food for the huge amount of biomass would be aerobic photosynthesis. Going further back becomes rather difficult as older sedimentary rocks (the oldest being 3.8gya) are highly metamorphized, so any organic content has been baked and no fossils can be discerned.
In sum, evidence for aerobic photosynthesis goes back almost as far (900 million years after the evolution of life) as the evidence of photosynthesis itself (600 million years after the evolution of life). And our inability to date either further back is quite likely due to the lack of rocks preserving biological evidence from earlier eras.
The only way to extrapolate from Earth to exoplanets is to assume that, first, innovations that evolved multiple times on Earth are pretty inevitable given the right perquisites, and second, things that took a long time to happen after the perquisites were in place are less likely, while those that happened quickly are more likely. So it looks like anaerobic photosynthesis is rather likely, and aerobic photosynthesis is somewhat less likely but not hugely so.
2. Eukaryotes. I totally forgot about this bit. You can't have multicellular organisms without eukaryotes. The oldest fossils with unambiguous eukaryotes in them are only around 2.2gya, but attempts to trace their lineage via genetic analysis yield an ancestry that is probably much older. Furthermore, we know that the basis of eukaryotic evolution - merging of ur-eukaryotes and ur-mitochondria - is symbiosis, and that's so incredibly common that it seems silly to claim that it's an unlikely evolutionary event for two bacteria to decide to hook up.
So, it seems likely that quite a few of those million or so lifebearing planets in the galaxy will have not just life, but multicellular life on them, assuming they have been around about as long as Earth, which needed about 4 billion years to build up enough atmospheric oxygen to support large, complex creatures.