Mitochondria and the origin of eukaryotes
knowablemagazine.org
knowablemagazine.org
My favourite book I read in the past five years or so
Sibbald SJ, Archibald JM. Genomic Insights into Plastid Evolution. Genome Biol Evol. 2020 Jul 1;12(7):978-990. doi: 10.1093/gbe/evaa096. PMID: 32402068; PMCID: PMC7348690.
The article, and yours too I would assume, mentions https://en.m.wikipedia.org/wiki/Paulinella which is really cool for being a second known primary symbiosis.
Greatest rerun of all time!
It's probably more useful for laymen.
Those who find themselves wanting more can always go to the larger, more comprehensive text later.
https://en.wikipedia.org/wiki/Viral_eukaryogenesis
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.
I've often used this "fact" as an argument against complex extraterrestrial life. Glad to see it might be wrong after all.
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)"
https://www.cell.com/current-biology/pdf/S0960-9822(17)31179...
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.)
As usual, people always seem to foreget that in this universe, what happened once, happens all the time and we are not special.
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.
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.
We know very little about modes of sentience with N=1. Perhaps life is generally mutualistic.
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?
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.
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?
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.
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.
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.
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.)
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.
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.
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
> Reverse chirality autotrophs sound like a scary sci-fi novel plot
Very ice-9-like.
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_...
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.
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.
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.
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.
One clue would be leftover genes from when it was there. Eukaryotes often find it hard to prune out excess DNA, vs. just turning it off.
Because biology isn’t complicated enough. :)
A most famous example is red blood cells, which are emptied of most organelles to make space for hemoglobin.
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.
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.
This is a little more than a tabloid article.