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Comment by CRUDite | Hacker News Reader

If we assume single celled life is common, and exists in either all star systems, or in all systems without a hot Jupiter close to the star; then given the length of time involved here before eukaryotes, can we estimate how many planets in the galaxy have eukaryotes? Or how long it will be before some do? Presumably increasing the length of time increases the probability of occurrence as would increasing the number of stars involved. Though how can we know if the event was statistically likely after that amount of time or whether we are deviations from the centre of the distribution (other than observing the quiet galaxy). Surely we can have a rough idea now of where we stand

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Maybe, but there are other narratives that are easy to spin. Our solar system has 3 planets all of which might have had single-celled organisms, at some point: Venus, Earth, and Mars. And for the first 3.7 billion years, the 3 planets might have followed a similar path. And then all 3 planets reach old age and basically die, Venus becoming too hot, while Earth and Mars become mostly dead ice covered snow balls. And maybe that is the normal history of most planets, even planets that develop single-celled life.

In that narrative, the emphasis is on the extraordinary re-birth of Earth, after the end of Snowball Earth. Almost everything that we regard as interesting about Earth happens after this late-in-its history revival. That raises some other questions, such as, why did Snowball Earth end? Why does multicellular life take off then, but not before? What is it that makes the Earth/moon system so unusually dynamic that it hasn't settled down to some dead equilibrium, even after 3.7 billion years? What allows Earth to have such an extraordinary additional era?

> can we estimate how many planets in the galaxy have eukaryotes?

Not yet. In the history of life on Earth, this has happened once. Knowing what we know about cellular biology, it’s stupidly unlikely. Beyond our present theories’ ability to quantify.

By the way, I think this is one of—if not the—great filters. It’s unlikely to happen, to not promptly get smote by its primordial planet’s tantrums and to get it so right it perpetuates for billions of years.

> Not yet. In the history of life on Earth, this has happened once.

1) That we know about.

2) Not unlike startups vs. established business, any newly emerging "eukaryotes" have to out-compete the already-evolved incumbents, which are already quite good at harnessing energy. You're much more likely to find success in business than in an entirely new evolutionary branch, though I doubt biological "gray goo" is outright impossible [1].

[1] Reverse chirality autotrophs sound like a scary sci-fi novel plot https://news.ycombinator.com/item?id=28038505

Additionally the environment changed significantly since then (for one example oxygen which was highly toxic to most forms of life that existed back then is now over 20% of atmosphere).

> Reverse chirality autotrophs sound like a scary sci-fi novel plot

Very ice-9-like.

>Reverse chirality autotrophs sound like a scary sci-fi novel plot

I doubt this. 'Not being digestible' is very far from 'being invulnerable' or even 'being able to spread quickly'. The kingdom of life has many ways to kill stuff, ways which don't care about chirality, and our typical R-sided lifeforms have all the evolutionary 'motivation' to come up with new ways just the off the competition. That's before humans get into the picture, which we have the tech to do.

There may be an accumulation of non-digestible stuff until nature reaches a balance. However, there's a very large recent accumulation of non-digestible materials called 'plastics', and while somewhat harmful, they're not a life-ending threat. Nature is already finding ways to process these materials[0].

[0] https://en.wikipedia.org/wiki/Plastic_degradation_by_marine_...

Sure, but I did say "sci-fi". And I think there are a lot of unaddressed points.

Plastics don't self-manufacture. You might not be able to control the rate.

Just because you kill something doesn't mean you break down its carbohydrates. Reverse chiral organism skeletons could bioaccumulate and we could have a situation similar to the Carboniferous.

Someone might be able to synthesize a bacteria in the lab given enough time and effort from an organism that proliferates quickly. It doesn't have to capture all the carbon. Just out-compete a keystone species. Plankton, mycorrhizae, etc. Or attack a large percentage of the plant biomass.

>Plastics don't self-manufacture. You might not be able to control the rate.

The rate is limited by the process. Since no precursors exist, it must 'self-manufacture' from scratch. This has inherent limits even before introducing competition for food, poison, predators that eat you even despite them not being able to really digest, etc.

>Just because you kill something doesn't mean you break down its carbohydrates. Reverse chiral organism skeletons could bioaccumulate and we could have a situation similar to the Carboniferous.

So you don't break it down. Nature will have plenty of time to adapt. Humans will step in if needed.

>Someone might be able to synthesize a bacteria in the lab given enough time and effort from an organism that proliferates quickly.

That's an incredibly messy way - create an entire L-chiral biochemistery - to get a weapon which doesn't have a setting between 'kill everything' and 'do rather little' (IMHO, the second being much likelier). There are far worse and more directed things one can do with a lab. Even the absurd 'kill everything' goal is far more likely to be reached in different ways.

per the article, endosymbiosis has happened a bunch of times. multiple different kinds of chloroplasts, several prokaryotes, a parasite etc.

this was all when eukaryotes engulfed prokaryotes, but still, how does this mean unlikely? it seems imminently likely, since.. it happened a bunch of times.

seems to me like prokaryotes evolve a strategy of engulfing others for their resources, then one day engulf a prokaryote infected by a virus, which transfers DNA across, rinse and repeat.

how is this more of a filter than abiogenesis?

Yes, this (nonsense) discuassion is when you have computer progammers discuss biology. Is strange because they know nothing but all seem to think they must be experts of evrything because they get paid a lot to sit infront of a computer all day.

> how is this more of a filter than abiogenesis?

Common chemistries get us very close to molecular systems subject to evolutionary pressure. (Simplest: RNA world hypothesis.) We are missing links. But the pathway is plausible.

Chloroplasts, as you mention, are a potent counter argument. But once you have surplus cellular energy, additional endosymbiosis has a lower threshold. Based on current research, all life has a similar mitochondria. Different kingdoms didn’t nom their own and go. That uniqueness suggests difficulty.

Or simply that success produces logarithmic returns: in the context of when this was happening, the first species to do it rapidly outcompeted all others and functionally ended exploration of the possibility space.

The thing that only happened once on Earth and that's a prerequisite to developing complex life forms is not endosymbiosis, it's life going multi-cellular. They are not the same thing.

Endosymbiosis is not identical with going multi-cellular, and it seems that all but perhaps one of the known instances of endosymbiosis didn't play any role in us going multi-cellular anyway. In fact this article makes the case that it may not have been critical at all.

Multicellularity looks relatively easy. From https://en.wikipedia.org/wiki/Multicellular_organism#Occurre...

> Multicellularity has evolved independently at least 25 times in eukaryotes, and also in some prokaryotes, like cyanobacteria, myxobacteria, actinomycetes, Magnetoglobus multicellularis or Methanosarcina. However, complex multicellular organisms evolved only in six eukaryotic groups: animals, symbiomycotan fungi, brown algae, red algae, green algae, and land plants. It evolved repeatedly for Chloroplastida (green algae and land plants), once for animals, once for brown algae, three times in the fungi (chytrids, ascomycetes and basidiomycetes) and perhaps several times for slime molds and red algae.

It’s relatively easy once you have evolved the biochemical infrastructure to support it, but on Earth that took several billions of years to achieve. There’s no way around it, no amount of hand waving how easy it is negates the fact it took billion years of evolution to do it.

Also most of those forms of multicellularity are extremely basic, little more than tangles or sheets of cells, even after hundreds of millions of years of further evolution. That’s not likely to get to intelligent life.

it took at most a billion years. there may have been predecessors that were lost - the only evidence we have from that era is what made it into the DNA of surviving ancestors.

I guess I still feel like abiogenesis should be the bigger filter. we have ideas and suggestive experiments about how it happened, but nothing's come close to convincingly demonstrating how fully self-replicating life can evolve through simple steps. whereas endosymbiosis just seems to require two prokaryotes from different trees surviving in the same membrane, and it's not difficult for me to imagine a plasmid slipping in and making copies of enough enzymes to get by.

As I pointed out though, endosymbiosis isn’t by itself enough. It can provide many benefits, one of which might be the ability to develop complex sophisticated multicellular organisms, but it doesn’t guarantee that capability.

This sub thread is specifically about developing intelligent life, and of all the branches of multicellular life on earth only one has achieved that capacity, animals. None of the others seem to be anywhere close, or ever likely to be, for all their fancy biochemical tricks. So it seems like the vast majority of endosymbiotic events really don’t help much towards that outcome.

I think we agree. One you have prokaryote it's probably easy to get multi cellular prokaryotes.

My guess is that the transition form eukaryotes to prokaryotes in the hard step.

Also, photosynthesis seams to be more complicated than what I expected. Perhaps that is the hardest step. (It's an indirect step to intelligent life, but perhaps a lot of free oxygen to burn food efficiently is necessary for intelligent life.)

You make a big assumption, that there must be only two kinds of cells, "simple" and "complex", like on earth.

That could well be an accidental fact. Maybe on some planets we have a gradient of cell complexity.

Also, we don't really know what even simpler kinds existed on earth but were lost, since bacteria, the "simple" kind, it's obviously too complex to have been the first ever life form.

That's not even true on earth. Archaea exist, and eukaryotes span a pretty wide range of complexity themselves.

Well, that's not a good assumption to make. Our most capable life detector¹ is analyzing the atmosphere of planets, captures stuff similar to our simplest single celled life. So, it's not a good guess that it's common.

1 - Actually our second best. The best one is the fact that nobody colonized Earth before we existed, that is tuned in space-faring life.

Just us if we live in a simulation

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