An accessible and incredibly insightful read 100% on this topic is "Power, Sex, Suicide: Mitochondria and the Meaning of Life"- despite the flashy title a thorough scientific read and analysis, working through various possible arguments and teaching fundamentals along the way.
My favourite book I read in the past five years or so
I haven't read that one but after reading The Vital Question I will recommend anything with Nick Lane's name on it. There are very few people who have thought about the origin of life as deeply as he has.
The Vital Question discusses Nick Lane's hypothesis on how the first cell came to exist in Part II: The origin of life. Energy at life's origin ; The emergence of cells
For those interested but new to role of endosymbiosis in evolution, it's useful to keep in mind that there's strong evidence of an endosymbiotic origin of chloroplasts, another major event in the history of life. Here's a recent review article that highlights some of the key evidence and outstanding questions:
Alberts et al. Essential Cell Biology is a bit less advanced, but I'm finding it very complete, just not quite so exhaustive (pun intended). It's still 700 pages, so you'll get your money's worth (if you buy a used copy).
Yeah Essential's more aimed for underclassman (e.g., sophomores) whereas the other text is for upperclassman (i.e., juniors and seniors) and graduate students.
It's probably more useful for laymen.
Those who find themselves wanting more can always go to the larger, more comprehensive text later.
My understanding is that there is much less evidence for this, but it seems like the question of whether or not there were multiple gradual endosymbiotic events in eukaryotic evolution would definitely depend in some way on viruses.
For some reason people fixate on the mitochondrion in the context of the origin of eukaryotes, to the near total exclusion of the nucleus that actually defines the eukaryotes. I don't understand why this is. I can only cynically guess that it's a form of streetlight fallacy: we have a pretty good idea where mitochondria came from, so let's focus on that (and conveniently forget about the nucleus, golgi body, endoplasmic reticulum, ...).
The rise of free atmospheric oxygen and oceanic dissolved oxygen levels were almost certainly a requirement for the development of complex multicellular life. Using oxygen as the electron acceptor in respiration means far more energy can be extracted from reduced fuel molecules (sugars, fats, amino acids, methane, hydrocarbons, ammonia, etc.) than by using species like oxidized sulfur and iron, CO2, or nitrate (NO3) for that role.
This is really where the mitochondria come into play, as they allowed their host to utilize oxygen for this purpose. This can be seen by looking at modern eukaryotes that have reverted back to an anaerobic lifestyle:
> "In lineages of eukaryotes adapted to low oxygen conditions, mitochondria have been drastically reduced, functionally altered and, in one case, completely lost."
"The Origin and Diversification of Mitochondria (2017)"
Hence, looking for the signature of free oxygen in the atmosphere of exoplanets orbiting distant stars is considered to be a fairly good indicator of the possibility of complex multicellular life of some sort, and at least of an active photosynthetic microbial ecosystem.
(Incidentally, the historical divisions withing academic university departments led to evolutionary biology generally ignoring the importance of early Earth's geochemistry in the evolution of life, as they saw evolution as a kind of cellular/organismal process divorced from the physical surroundings - the latter being the province of the geology department. The renewed interest in exobiology and origin-of-life research has tended to bridge this gap.)
The way I read that is that developing complex life is even more contingent than 'just wait long enough'. Given what we know for sure, you need rather narrow concentrations of specific gasses at least.
This is Nick Lane's argument - that complex cell happend only once on Earth (no convergent evolution) via this symbiotic accident and hence it's highly improbable if not almost impossible.
As usual, people always seem to foreget that in this universe, what happened once, happens all the time and we are not special.
Nick Lane's book The Vital Question is a masterpiece. I reckon he's figured out the origin of life there. I hope he gets on some podcast like Lex Fridman to talk about it all, because his ideas are just waiting to explode into the public imagination.
We're all just speculating given that were stuck at sample size = 1 until we develop near light speed travel.
I get the convergent evolution point, but so far the story is that 1) life arose very quickly 2) then proceeded to do very little for billions of years. If that's true, then it's bad news for the possibility of complex life being common.
> 1) life arose very quickly 2) then proceeded to do very little for billions of years.
This is understating the feats of unicelular life. Those tiny organisms essentially terraformed the whole planet in those billions of years, oxidizing nearly all the iron close to earth surface, burying most carbon and then pumping so much oxygen in the atmosphere that at a point it comprised 30% of Earth gaseous envelope by weight. I wouldn't call that very little. To come close we'll need at least to terraform Mars or Venus, to match the feats of our microbial forebears.
Well, it's quite possible that once it happens, it voids any chance of it happening again just because life filling that niche already exists. If so, it only happens once, and this says absolutely nothing about the odds of that first occurrence.
> what happened once, happens all the time
That's a fallacy people do all the time here when talking about exobiology. It's basically states that all small numbers and all big numbers are alike, and that all small numbers are like the inverse of all big numbers.
Most other features evolved independently several (or even many) times so this is not a good argument. It’s not as if once there was a eukaryote it instantly spread across the whole planet, taking up every ecological niche. Even today eukaryotes don’t exist in certain environments.
It’s not wrong, at least I don’t think this article is arguing that multi cellular life evolved any earlier than we thought. Just that the mechanisms were different than we expected.
Nothing in the article contradicts that 'fact'. The question is only whether the development of eukaryotes involved other significant steps in addition to the mitochondrial endosymbiotic event, or whether the latter event was all that really mattered. In both cases, subsequent development of eukaryotes and hence multicellularity depended on the seemingly very rare endosymbiotic event.
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
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.
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.
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 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.
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.)
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.
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.
> 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].
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
>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].
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.
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.
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.
How simple are Prokaryotic Cells really? Like "simple" enough that we understand all the intricate workings of it, or "simple" enough to be made from scratch?
Imagine giving a scientist in 1900 a modern CPU. He may be able to disassemble and study it, but it’s impossible to make it from scratch because it takes a chain of steps that lead to complicated modern factories that then produce the chip. It’s hard to skip those steps. We can swap DNA in living cells and produce modified cells, but we lack the tools to create such complicated and microscopic structures in a pure synthetic way.
DNA doesn’t help as much as you think with single celled organisms thanks to HGT. Bacteria, at least, share DNA and thus we can’t use it to determine a linear evolutionary sequence.
So does anyone know how Lokiarchaeota fit into this? I remember reading that Lokiarchaeota were evidence against the mitochondrion-first hypothesis, but I have no idea how that claim has held up over time.
Evolution did a terrible job there. So many diseases linked to mitochondrial dysfunction nowadays. We have this amazingly complex machinery, and a single point of failure, no redundancy whatsoever.
Evolution did a great job? I highly recommend reading Nick Lane's "Power, Sex, Suicide". Mitochondrial function is a complicated question. It's amazing nature makes it work at all. Two aspects he discusses:
1) Mitochondrial selection is multilayered and aggressive. Mitochondrial population selection goes on during formation of egg cells [1]. Mitonuclear compatibility is heavily selected for during ontogeny [2]. Many miscarriages are are related to mitochondrial dysfunction. Once someone is born they've already passed a high selective bar for mitochondrial health.
2) There is a tradeoff between mitochondrial selectivity and fertility. Birds have much greater energetic requirements than mammals (flying is hard) and therefore have much greater mitochondrial selectivity. This means fewer offspring though. Pigeons and rats have similar size and metabolic rates, but a pair of rats can have 80+ offspring in a year while a pair of pigeons can only manage under 20.
Presumably like we do - (I'm heavily simplifying the complex regulation involved) the mitochondria have their own DNA which gets duplicated when the cell prepares to undergo mitosis, then the two new cells split the results.
There are lots of mitochondria in a cell (almost always, ignore red blood cells, for example), and the mitochondria have their own mechanisms for detecting when they need to divide (exercise and the resulting stress being one of them, which is why working out makes you more fit).
Also note that the DNA for most of the mitochondrial proteins is in the host cell's nucleus-- the mitochondrial DNA is stripped down to coding for just a few essential proteins.
I know the cytoplasm between cells is split on division and corresponding the mitochondria are roughly as well. They could technically have different times of reproduction than the overall cell and it would still work. I’m not certain what actually happens to the mitochondria during cell division though.
My favourite book I read in the past five years or so
https://en.wikipedia.org/wiki/Power,_Sex,_Suicide