Is it known why there are no evolutionary intermediates between archea and eubacteria?
Nick Lane's book The Vital Questions has been highly recommended by the HN commentariat, and I think it more or less deserves its esteem. Lane gives an extensive theoretical analysis of the sequence of events wherein a eubacterium was engulfed by an archea, and the resulting endosymbiosis that was the beginning of complex (eukaryotic) life.
Apparently, the genomic evidence is strong that the original host was an archaeon an the original invader (later to become mitochondria) was a eubacterium. However, I don't think he discusses the origin of this divergence of eubacteria and archea, and why it persists so cleanly.
Archaea and Eubacteria have different Ribosomes. In the world of biology, if DNA is the source code then ribosomes are the compiler.
In other words, while both families use DNA to store information and RNA to translate it, all the rest of the machinery used by cells in that process is different. This is why there are no intermediates. Likely (and this is a guess, if an educated one) this split happened before cells were even "cells". That is, whatever the pre-cellular replicons were that eventually led to life, there might have been many variations on (what eventually became) the ribosome, but two won out and gave rise to two distinct lineages of cellular life.
So, if I'm understanding correctly, these pre-cellular replicons are distinctly less efficient at survival than both archaea and eubacteria, hence their not existing in any (recognizable) form today. Is this correct?
This opposed to the idea that the archaea existed, then the bridge (which went extinct), then archaea.
I should start by saying that we don't know, for sure, that pre-cellular replicons don't still exist. They may, and we may just not have figured out the right place to look for them.
That said, gaining a membrane is a huge advantage for life. The current working hypothesis is that life started as a series of autocatalytic chemical reactions, but for this to work there must have been just the right combination of chemicals in just the right conditions all at the same time. You can imagine that it wouldn't take much of a stray current to wash all that away, and you're back to square one. By surrounding themselves with a membrane, pre-cellular replicons could begin controlling their chemical environment, enriching it with the raw materials needed and regulating the conditions.
Again, though, just because cellular life would be expected to be more robust and, therefore, more successful than acellular replicons, you wouldn't expect those acellular reactions to immediately halt once cellular life arose.
If I had to guess, I'd say that acellular replicons may have persisted for quite some time, but ultimately succumbed to the Great Oxygenation Event (https://en.wikipedia.org/wiki/Great_Oxygenation_Event). Shortly after cellular life arose (or possibly beginning a bit before cells came on the scene), life cause a massive shift in the Earth's chemistry, freeing vast quantities of oxygen and changing what had been a reducing environment to one that is, on the balance, oxygenating.
Oxygen is, to put it bluntly, nasty stuff. It tends to break down complex molecules very readily, and so any acellular replicons that may have survived up to the Great Oxygenation Event were likely wiped out soon after.
Like others said, we don't really know, but most likely, they just got eaten, ingested, or sopped up by cellular life when it arrived. They couldn't have been much more than symbiotic molecules, not complex things like modern cells with organelles that we know today.
That's something I've always wondered about evolution in general. Why isn't there a continuum of species? For example, why are there chimpanzees and humans walking around as discrete species with nothing "in between". Seems unlikely that a few chimpanzees gave birth to human babies (or I guess some ancient human ancestor) one day, and then we just branched off and evolved separately leaving the rest of the chimpanzees behind. Why didn't the "better" DNA get fed back into the chimpanzee gene pool and the whole species evolve?
Obviously I know nothing about biology, but I'd love a high level explanation. It's the one thing that's always bothered me about evolution, because nobody ever explains it, and it seems like magic.
In general, geographic separation leads to speciation. It can be rivers (IIRC for example bonobos and chimpanzees live on different sides of the Congo river).
Another interesting example is I think seagulls along the arctic circle that are split in many species spread along an east-west axis. Most species can interbreed with their immediate neighbour, but not the neighbour of their neighbour (except at one point where the extreme populations meet and the immediate neighbours are not compatible either).
It didn't happen like that. Humans didn't branch from chimpanzees or the other way around.
At some point there was a common ancestor of both species. Its offsprings diverged into two species that continued evolving into the current humans and chimpanzees.
This doesn't really answer the question -- how come there are usually only a few clearly defined species that descended from this common ancestor? Why isn't there a spectrum?
I don't know anything about the phylogenetic tree of humans and our cousins, but I assume that at some point there was a spectrum (e.g., Neanderthals), but at some point the others got wiped out, without humans splitting any further. Perhaps this is due to the global mobility of humans? What do biologists think?
The most intense form of competition is that from your own species, nothing else requires exactly the same conditions/ecological niches/resources. The less fit individuals are out competed (dont breed, or die), and the species as a whole is "pulled" in a certain fitness direction that is more optimal.
The reason speciation arises is complex, but its often due to different subgroups of a species exploiting different niches, and thus experiencing differing fitness pulls that eventually give rise to new species.
There are no "intermediate" species because they fit more poorly into either niche occupied by the two other species, and thus are out competed or dont arise in the first place.
There is a continuum of species, but they don't exist at the same time.
Take a human woman. She holds hands with her mother. Her mother holds hands with her mother, and so on.
Take a female chimpanzee. She holds hands with her mother. Her mother holds hands with her mother, and so on.
Eventually, these two chains meet the same organism, the last common ancestor of human and chimpanzee, who lived somewhere around 6 million years ago. This (now singular) chain has somewhere around 600,000 individual organisms, where each pair next to each other holding hands are related as mother-daughter (so how could you argue they're different species?), but the endpoints of the chain are modern human and modern chimpanzee.
As a side note, categorizing life via the idea of species seemed like a good idea when it was developed, but we now know it's not. There's other (real life) examples that show that the idea of species is just a human invention that humans try to fit onto a reality that's more complicated and doesn't respect human abstractions.
Basically if two species occupy the same ecological niche in the same physical location, one will ultimately win out, because it will be slightly more fit. This creates a pressure to either quickly branch into a new and different species, or remain the same. This also taps into the species problem, because the whole concept of a "species" is an artificial human construct, for which nature has no respect.
The terms you want to look up are "allopatric speciation" and "sympatric speciation". In short, events can occur that segment population, and from that point forward you have two species. Given enough time, these two will continue to diverge and split, until you have two completely separate subtrees. It just so happens (or is very likely) that the split between archaea and eubacteria is the earliest of these splits.
The first and only reason there's no transitionary species between humans and chimpanzees is that we simply are not descendants from them, instead we share a common ancestor with the apes (which was probably more chimp-like if you want to envision what it was). We both evolved from the common ancestor because we fit different environments better. Eventually we developed sufficient intelligence, stemming from some environmental pressure chimps did not have, to manage survival in almost any environment, leading to us leaving them "behind".
From our common ancestor, there is a lot of continuum of hominids in the fossil record however. At some point humans and neanderthals did coexist as a spectrum of hominids coexisting at the same time, and we did "feed back" the most-fit DNA by interbreeding with them until we bred them out of existence.
Arm-chair opinion, but there are - just that chimpanzees/humans are a particularly bad example 'cause it happened so long ago. Once you have a common ancestor species split off into different species those species, by definition, can't share mutations between each other. Separately, chimpanzees and humans are "different", not necessarily better than each other. I don't know enough about the evolutionary history to say why but superficially they're better at tree-living and we're better at Savannah-hunting. Two different environments, so sharing mutations probably wouldn't help much.
Its an excellent question for an automotive analogy and if you accept that market pressure evolves cars similar to the process of biological evolution, then its interesting to contemplate various "family tree of car ancestors" while cars evolve into distinct and interesting niches, while at the same time there are no or very few chimera cars on the roads today that are an intermediate form between manufacturers and successful models.
Its actually very unusual in the car world, and I guess in evolution in general, for something like the SUV crossover to evolve where its basically SUV appearance on a car chassis, more or less. I think the closest biological analogy to a crossover SUV is a platypus. Although SUVs aren't nearly as cool as a platypus.
Not an evolutionary biologist but my guess would be it has something to do with niches in an ecosystem and lack of evolutionary pressure to change in an environment in which you have reached some equilibrium.
Chimps for whatever reason were able to satisfy their biological imperatives within their given environment early on and have never needed to change. The human line, not so much.
I guess the way I see it is that some mutation in early humans (perhaps selected for by the environment) created a priority on brain development. I'm sure there were many dead-end branches along the way to us that we haven't noticed. We just look back and see a straight line from chimps to humans. Perspective is key here.
A species (at least in the mammal sense) by definition is discrete, as it refers to organisms capable of producing fertile offspring. This is why dog breeds (which exist on a continuum are not different species.
In terms of speciation (chimpanzees or their ancestors giving rise to humans) this usually occurs due to an event such as a seperation between groups of the same species. They are then subject to differing external pressures as they may live in isolation, and their genetics continue to diverge until they are so far apart that they can no longer procreate and have become a species.
In many cases there are. The Burke Museum in Seattle currently has a display about the Pacific Northwest. In one part, they have a row of birds arranged based on where they were found on the Cascade Mountains. You can see them smoothly blend from one kind of coloration to another.
In cases where there are more distinct species, it's because the groups of animals don't interbreed for any of many reasons and then have drifted apart from each other.
Lane has a good story and tells it nicely, but I don't think he represents the mainstream, and he conveniently throws all the evidence that doesn't agree with him under the rug.
Depends on how specific you are as endosymbiotic theory can be very complicated. For example, the nucleus itself could be regarded as a seperate cell itself (which would be the archea), but what was the sourding cell?
I like Lane's book, but it is annoying how he praises Bill Martin and basically ignores the actual pioneers of the field. Carl Woese, who discovered the archaea and who fought a long battle to get the world to understand that they weren't just bacteria, gets like a sentence in the whole book.
Yea OK, but it's really hard to expect authors to dole out praise of scientists in proportion to the size of their contributions as assessed by the community. Individuals will naturally disagree with the community's assessment (which needn't always be correct), and will like their friends disproportionately, etc.
A lower bar is just that the author correctly represent the evidence and arguments about the scientific issues, especially with regard to identifying the consensus position and representing it fairly. If Lane fails to do that, it's a more serious charge.
Nick Lane's book The Vital Questions has been highly recommended by the HN commentariat, and I think it more or less deserves its esteem. Lane gives an extensive theoretical analysis of the sequence of events wherein a eubacterium was engulfed by an archea, and the resulting endosymbiosis that was the beginning of complex (eukaryotic) life.
Apparently, the genomic evidence is strong that the original host was an archaeon an the original invader (later to become mitochondria) was a eubacterium. However, I don't think he discusses the origin of this divergence of eubacteria and archea, and why it persists so cleanly.