How do you differentiate an event that is very improbable and so takes a billion years from an event that is inevitable, but requires many prerequisites, and so takes a billion years?
The article seems to simply assume the former model ("We assume that once an evolutionary transition is possible (i.e., once the previous transition has occurred), it occurs at a constant average rate λi, so that each ti is exponentially distributed with an expected transition time of βi = 1/λi"), but that feels unjustified to me. Is it really the case that the transition from prokaryote to eukaryote was equally likely at any time, and things simply sat stagnant for a billion years until one day life got lucky?
> an event that is inevitable, but requires many prerequisites, and so takes a billion years?
In support of this idea, the Cambrian explosion of large (large relative to microbial life) multicellular organisms happened immediately after the 2 billion years of work by blue-green algae had produced enough oxygen into the atmosphere to make larger organisms possible.
https://en.wikipedia.org/wiki/Great_Oxidation_Event
https://www.sciencedaily.com/releases/2010/12/101217145647.h...
I guess it all depends on how you balance occam's razor against abusing the anthropic principle.
How do you differentiate an event that is very improbable and so takes a billion years from an event that is inevitable, but requires many prerequisites, and so takes a billion years?
I suspect that' gnawing for any research like this. But I could suggest that if we can show that a lot of fundamental changes have come from external shock rather than internal processes, then we might be closer. Further, if the theory of "puncture equilibrium" is true[1], that this would tend to be the case. If we can show that many evolutionary steps involved such external shocks, then we might be closer to this also.
The question would be whether external shocks inevitably produce advances or whether such advances are indeed lucky.
This doesn't contradict the conclusion of the paper, it makes no difference as to whether we're going to find anybody we can have a conversation with, ("hey, how annoying is light speed?") but I think our definition for simple and complex life is off.
eg "more likely to find simple life on Mars."
That ape and octopus exist, says that earth didn't win the intelligence lottery, we are watching the long slow grind of compound interest.
When you look at our world we see that the lipid bilayer is preserved, that rna is preserved in every living thing, but eyes have evolved independently many times. That the last common ancestor of octopus and human was a little wriggly thing, leads me to believe that forming a boundary between self and non self is hard (the bilayer), that forming a system of heredity is hard (first rna then dna). That the accumulation of processes that use energy, resources and information from the environment in new ways is a long relentless grind. The basic body form, multicellularity and a gut were, big difficult changes, but maybe intelligence is just paint on a hard won foundation.
It also seems to assume that there is only one "path" to "observer" status. For example, could there be an alternative to Eukaryote cells that would allow equivalent cognition to our own? We don't know because we only have one instance of "observers" coming around.
Sure, all of this is speculative, but the eukaryotic jump is notorious for getting casually dismissed on account of being misunderstood. I think it's one of the better candidates for a filter, so let me advocate it for a second and make sure that if you still want to dismiss it you (and the bystanders) are not doing so by accident.
The problem has nothing to do with having a nucleus. That's the solution. As you note, it's possible to imagine other solutions. The problem is dealing with gigabytes of genetic data instead of mere megabytes. Typically one would use units of base pairs, but in the context of talking about potential alternative forms of life I'd argue that units of information are more appropriate. In any case, the "easy" strategies suffer from a severe dead end that prevents them from going beyond a few megabytes:
https://upload.wikimedia.org/wikipedia/commons/e/e4/Genome_s...
Eukaryotes scale, and that's both difficult and significant.
The strategies they (we) use to scale beyond the megabyte range are highly elaborate and they touch the most fundamental, highly conserved aspects of the genome, of cell functionality, and of reproduction. Everything must change to accommodate the increased scale. Genes have to be hierarchically packed and unpacked, each level of hierarchy must be coordinated across all the basic modes of cell functionality, everything must be done in parallel, and the entire architecture of the genome changes as a result. It's like a distributed system vs a monolith. It's a tough, deep-reaching transition to pull off, it's a tougher transition to justify, and that's if you're an engineer capable of things like planning, prediction, and delayed gratification! A greedy optimizer bumbling around in the dark can't rely on cognition (or even a hype cycle!) to propel itself from one mountain range of local optima to another mountain range.
I'm sure there are plenty of strategies for scaling genomes, but if they're all difficult -- and the crazy elaborate mechanisms we see in Earth eukaryotes suggest that could be the case -- then they still constitute a filter.
But yeah, this is all speculation. Nobody really knows.
I'd actually written something very similar, without the rigour, a few years ago.
http://hopefullyintersting.blogspot.com/2018/03/the-drake-eq...
I'm fairly amenable to the interpretation that something like prokaryotic-to-eukaryotic jump is a real high barrier.
I would be pretty unsurprised to find prokaryotic life at least sometimes. I would be much more surprised to find eukaryotes possibly at all, and probably at any high frequency.
Doubt it. Cellular endosymbiosis has happened lots of times on Earth. Besides the common algae symbiosis, I once read a paper that contained a casual reference to dinoflagellate-based organnelles in a larger cell. And dinoflagellates are eukaryotes themselves!
Here's a paper that explains 3 levels of endosymbiosis. A living inside B living inside C living inside D.
Good find. I might have actually remembered the dinoflagellates on the wrong side of the endosymbiosis relationship in my example (this is either similar to our the same as the one I read). The broader point stands. :)
Why would you think that the prokaryotic-to-eukaryotic jump is higher?
If it is engulfment of organelles like mitochondria, that happens all the time. There are fairly good models of the evolutionary tree and how it could happen over time[1].
On the other hand, the ribosome is an amazingly complex machine, which I would expect would be much harder to create. Even synthetically, the path to build a synthetic eukaryote seems a lot easier than the path to build a synthetic ribosome.
>On the other hand, the ribosome is an amazingly complex machine
Just a bit:
Transcription - https://www.youtube.com/watch?v=SMtWvDbfHLo
Translation - https://www.youtube.com/watch?v=TfYf_rPWUdY
Replication - https://www.youtube.com/watch?v=I9ArIJWYZHI
It's not clear to me how much these features can be degraded without destroying the ability for organisms to reproduce with heritable traits.
Also, when listening to these videos, ask yourself 'how' and 'why' any time an action is described. For example, the following parenthetical questions from a small chunk of the 'Translation' video:
"The addition of each amino acid is a three-step cycle; (Why three step?)
First the tRNA enters the ribosome at the A-Site (Why does it only enter at the A-Site? How are other options prevented and/or made inconsequential? Was it always this way? How did the features of the A-Site evolve for this to happen. What happened before this? ), and is tested for a codon / anti-codon match with the mRNA. (How is the tRNA tested? What happens when it fails the test or is missing the amino acid? What is the energy budget of this test and what are the specific features of the ribosome, RNA, tRNA and amino acid that make it possible? What happened before this testing was done? How did we get from the lack of ability to test and the ability to test?)
Next, provided there is a correct match, the tRNA is shifted to the P-Site (What is the mechanism of this shifting? Why does it only go one direction? What is the energy budget of this process and how is it powered? How do we prevent multiple tRNA from shifting or keep it from shifting more than one spot? What occurred before the ribosome/RNA/tRNA/amino-acid had the features to allow this to occur?)"
You get the idea.
I wonder how often eukaryotes were created but then failed to outcompete the existing prokaryotes. Complexity often results in poorer algorithmical fit in the short term.
Indeed. It's probably the case for every step in the evolutionary ladder.
As for eukaryotes, often, evolution rewards larger size, simply because if you are larger you can't be eaten as easily, you can eat larger prey more easily, etc. Compared to prokaryotes, eukaryotes are gigantic. And they seem to cover the niche of huge lifeforms really well.
Engulfing new creatures is a lot easier for large eukaryotic cells without a cell wall than it is for prokaryotes or archaea. We have a model of how it happened but there were a lot of problems that had to be overcome in the process, problems that didn't later have to be overcome for chloroplasts, etc.
Eukaryotes have lots of ribosomes, so I'm confused about why it would be easier to create eukaryotes.
And while bigger cells absorbing smaller cells happens all the time, it seems extremely, extremely rare that those events lead to reproducing life. It's probably happened 2X on earth (whatever lead to the original eukaryotes, and the absorption by that lineage of cyanobacteria).
My understanding is that metabolic-induced stress (degradation, esspecially genetic) was a major factor.
It's also possible that earlier similar transitions occured but were overwhelmed by the surviving eukaryotic line, whether due to greater metabolic effectiveness, superior repair capabilities, or other factors.
"Intelligent life"? What about dogs? Dolphins?
May as well argue about "Civilizations that produce iPhones are rare" - the definition is particular to humans. Monkey-troupe intelligence may indeed be rare, and insectoid emergent reasoning the norm. Or whatever.
The metric they used was the oldest artwork. The table contains a citation to "Pääbo (2014)" in the line where intelligent life forms. Of course, question is, what do you consider as art :). Is a dolphin frolicking in the water performing a piece of art? Our own judgement might be clouded.
I don't find this argument terribly convincing, especially since it (and apparently all of the author's reasoning[1]) rest upon the unstated assumption that we are randomly distributed over possible observers of the universe. The endpoint of that whole (bs) line of reasoning is that we are all Boltzmann brains[2].
In the end I am only convinced that the authors' main specialty is grabbing headlines.
[1] from browsing a paper he co-authored with Nick Bostrom, offender #1 of bad probabilistic thinking
When you have one data point, you can fit whatever curve you want to it!
This paper is a perfect illustration of garbage in = garbage out.
Eric Drexler? Now there is a name I have not heard since the "nano machines" days ;)
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