Fascinating research into the question of how the first eukaryotic cells arose from prokaryote antecedents.
The ancestors of the mitochondria organelles you see today in every cell in your body were once free swimming bacteria, in a world where there were no animals, and no eukaryotic cells. Planet earth then, and for its first 2 billion years, was populated exclusively by bacteria, or single celled organisms. These cells were about one hundredth the size of the cells in your body.
How did life on earth become more complex?
Billions of years before the Cambrian explosion, when the number of animal species exploded into the millions, something much more basic happened at the cellular level. Cells became more complex.
How?
Maybe cells engulfed other cells, and the engulfed cells survived, forming a partnership or a colony.
Or maybe different types of cells hung out in close proximity, in colonies, which eventually merged into a network of DNA exchanging nodes that keep growing in complexity and interdependency.
We don't know.
This new effort to grow and study archea in vitro gives us something to study and experiment with.
I very warmly recommend the book "Sex, power, suicide" which explores different theories on how we came to have mitochondria and their impact and role. Dense reading but I learned something on every single page.
The author more or less concludes (highlighting also limitations of the science and current knowledge) that our cells were arachea which absorbed a methanophile bacterium that became both our today's mitochondria and a parallel process for the chloroplasts in plants (with possibly a common ancestor if I remember well).
This made perfect sense to Lyn Margulis, an evolutionary biologist who was once married to astronomer Carl Sagan. She put up with decades of skepticism and derision from older scientists before Woese and Doolittle identified the genetic data in chloroplasts that confirmed the theory.
The History of Evolutionary Thought → 1900 to present → Endosymbiosis: Lynn Margulis
Already said in another comment, but I recommend the book "Sex, power, suicide" which explores the history of mitochondria and their evolutionary and todays role. The author also discusses Lyn's theories and partially absolves her (in particular on the symbiosis) but also highlights where her further theories don't hold up.
No, these are cell organs that also exist in simpler lifeforms, and they don't hold DNA. Key is that mitochondria have their own DNA and multiply independently of the host cell.
My understanding is that the really interesting thing about eukaryotes is their complexity, especially of their genomes and all the machinery needed to maintain them. Eukaryotic cells are just really complex overall compared to bacteria, tons of weird organelles all over the place. The arrival of mitochondria is kind of a red herring by comparison, in my view; we know cellular endosymbiosis has happened a bunch of times so it's not special.
I'll grant my view is mainly driven by the question of how much complex life exists off of Earth, and therefore how hard/unlikely it was for Earth life to reach its current complexity. If it happened a bunch of times on Earth it's probably not a blocker, and therefore not relevant to my interests. :D
Anyway, the evolutionary gradient that drove complexity in the genome is what I'd really like to know about. How it works, how it got started, why it apparently only happened once, etc.
> The arrival of mitochondria is kind of a red herring by comparison, in my view; we know cellular endosymbiosis has happened a bunch of times so it's not special.
The arrival of mitochondria, or the "merger" of the bacterial ancestors of mitochondria and the archea "host" was pivotal.
Bacteria and archea have been around for billions of years, ~a billion years longer than eukaryotes. Only once, as far as we know, did they blossom into the complexity of the eukaryotic cell, via a very specific endosymbiotic event. Most of the bacterial DNA ended up in the (newly developed) nucleus, where, in addition to coding proteins for the mitochondria, it could also take on lots of new functions.
We don't have a bunch of intermediate states between prokaryotes and eukaryotes-- it's a huge gulf that suggests a one time event that left most offspring unable to reproduce.
Whether we are talking about life on earth or other worlds, this seems very, very lucky, and very relevant to your interests-- it is why we got the big complexity gradient, and why, even billions of years later, you don't have "complex" bacteria or archea.
> blossom into the complexity of the eukaryotic cell, via a very specific endosymbiotic event.
Citation needed. There is a hell of a lot more to being a eukaryote than having mitochondria. They're defined by having a nucleus, not mitochondria, and I think that's a good definition.
If mitochondria were a necessary step to modern eukaryotic complexity, it could only be by providing a bigger power budget, which could have happened lots of ways. Again, cellular endosymbiosis is common, other notable examples being chloroplasts and red algae serving as chloroplasts for brown algae.
So, I'm not saying mitochondria are not important, but I don't think they were the blocker to eukaryote evolution.
Eukaryote does mean "true nucleus", but all* eukaryotes have mitochondria. They only became eukaryotes via the merger. Again-- archea (and bacteria) have existed for billions of years, and never became "complex", even by unicellular eukaryotic standards, let alone multicellular creatures, except when the two got together (once, as far as we know) to form eukaryotic cells with mitochondria.
(Among other things, it suggests that the nucleus itself developed because of the inflow of bacterial DNA and the need to parse out meaningless intron segments of genetic code.)
Note also that the subsequent endosymbiotic absorption of chloroplasts is not a "common" event, although of course it is very important-- at least to us, as we wouldn't be here without it.
* Some have lost them, when they are involved in symbiotic relationships that supply the required energy, or, as part of multicellular organisms-- for example, our red blood cells. Note that our red blood cells also lack nuclei.
You're not even attempting to make an argument about why the origin of mitochondria deserves all the press it gets, just restating the intro biology story. I already know they're "the powerhouse of the cell". Biology has lots of powerhouses but only one nucleus.
> The arrival of mitochondria is kind of a red herring by comparison, in my view; we know cellular endosymbiosis has happened a bunch of times so it's not special.
On the contrary! In billions of years of evolution it happened only very few times (successful), and only once really successfully. It is a rare event.
"Duggin studies cell division in the archaeon Haloferax volcanii. It’s a lover of salty conditions, such as those in the Dead Sea, and not of volcanoes, as the species moniker suggests. (It was named after microbiologist Benjamin Elazari Volcani.)" Indeed.
It traces how we got to some of these new ideas from pre-Darwin, through Lynn Margulis, to present day nanopore sequencers via the guy who discovered Archea.
Archaea strike me as the kind of thing we'd find on Mars. I didn't realize it until doing some searching, but some archaea are methanogenic (they generate methane), which has been detected on Mars.
Look into Panspermia if you're not aware already. Hoyle and Wickramasinghe wrote a lot on this. I don't know how seriously it's regarded in the science world.
If life somehow hitched a ride from one planet to another, it seems far more likely that it would have gone from earth to Mars, than the other way around. ;-)
The ancestors of the mitochondria organelles you see today in every cell in your body were once free swimming bacteria, in a world where there were no animals, and no eukaryotic cells. Planet earth then, and for its first 2 billion years, was populated exclusively by bacteria, or single celled organisms. These cells were about one hundredth the size of the cells in your body.
How did life on earth become more complex?
Billions of years before the Cambrian explosion, when the number of animal species exploded into the millions, something much more basic happened at the cellular level. Cells became more complex.
How?
Maybe cells engulfed other cells, and the engulfed cells survived, forming a partnership or a colony.
Or maybe different types of cells hung out in close proximity, in colonies, which eventually merged into a network of DNA exchanging nodes that keep growing in complexity and interdependency.
We don't know.
This new effort to grow and study archea in vitro gives us something to study and experiment with.