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Konkvistador comments on UFAI cannot be the Great Filter - Less Wrong Discussion
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Early or late great filter?
I'm currently leaning strongly towards late filter, because many of the proposed early filters seem to not be such big barriers. We've for example found a bunch of exoplanets in the last decade or so and several of those seem plausibly in the habitable zone. Life on Earth arose very early in its history so if life arising is the hard and rare step I would expect there to be many more hundreds of millions or even billions of years of conditions on Earth being seemingly ripe for it arising and it not doing so.
Obviously maybe the conditions ripe for life being there at all is the tricky part. Another possible hard step is multicellular life.
Correction: Multicelular life seems easy eukaryotic life not so much.
The Cambrian explosion seems to have happened like a billion or more years after it "could" have happened.
Not necessarily - there are those that argue that the cambrian explosion might have had more to do with the increase in atmospheric oxygen over geological time than evolution, and we find vague evidence of multicellular creatures (worm-track type impressions in the seafloor, some strange radially-symmetrical things buried in the sediment) up to a billion and a half years ago. Oxygen lets you have big energy-gobbling multicellular creatures easier, and blocks the destructive ultraviolet radiation that would have previously sterilized the above-water land when it turns to ozone. We also might just see an explosion because that is when hard body parts that fossilize easier appeared.
If the explosion was evolution-driven it could have been due to some kind of runaway arms race between predators and prey, or due to the final establishment of the developmental plans of the various animal phyla that could then be modularly tweaked to enable diversification and rapid evolution.
It's the move to eukaryotic life, or "complex cells" that's unique. Multicellularity given eukaryotic status seems easy, but eukaryote status happened only once, and about halfway through the habitable lifetime of the Earth.
Thank you for correcting me on this, it has been some time since I thought of this.
This argument is faulty if there is more than one hard step (and the update is not that strong, although significant, even with one step). See Robin's paper for details.
The jump to multicellular life seems to be pretty easy, actually. To quote Wikipedia:
It seems to be remarkably easy for eukaryotes, with their excessive number of genes (probably accumulated via drift and non-adaptive processes) which can be co-opted for cell-to-cell communication. There are those that argue that prokaryotes are too heavily optimized for efficient fast reproduction to make huge multicellular complexes- though it turns out that they actually do specialize themselves a bit when they are growing in colonies or biofilms to provide for the colony as a whole.