I wouldn't expect populations of bacteria to give rise, without endosymbiosis, to complex morphology and the kind of intelligence that we have, elsewhere. I think that it would require (I'm going out on a limb here)... an endosymbiosis for the reasons I've been saying, and... that endosymbiosis is a) rare and b) likely to go wrong. So I can't put a number on how improbable it is. It's just that I would say that it's a factor that a lot of people would rather not think about. If you have an agenda where you'd like to find complex life out there, the SETI people for example... probably don't want to hear this kind of stuff. It says that it's less likely... it's not an inevitable outcome of physics.
British biochemist and writer (born 1967)
(born 1967) is a British and writer. He is a professor in evolutionary at University College London. He has published five books to date which have won several awards.
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We all share this basic machinery in cells, and it's not related to whether you're photosynthetic or whether you're phagocytes or whether you are a fungus or whether you're an animal cell. We all share the same machinery. Why? The possibility is that it's not about adaptation to the external world, it's about adaptation to these s. These pesky bacteria that went on to become a mitochondria. Maybe this conflict of interest... [that] had to be resolved somehow was what was driving a lot the elaboration of cellular machinery. It's a kind of local... intimate conflict.
These are all lifestyles that exist in bacteria anyway. ...Photosynthesis obviously. The only eukaryotic lifestyle that does not exist at all in bacteria is ... the ability to engulf other cells, to grow around them. That's never been found yet in bacteria. It seems to require... a lot of energy, a large complicated system capable of changing shape and moving around. ...For whatever reasons it never evolved. I would say the reason was that you need mitochondria to get that large and complex in the first place.
[A]cquiring mitochondria gives you a headache that can go wrong very easily, but here's an interesting problem in a nutshell. You look at a plant cell under a microscope, or an animal cell, or a fungal cell, or an or something, and you'll recognize the same structure in all of them. They've all got a nucleus. They've all got the s as straight chromosomes. They've all got s. They've all got s. They've all got complexes. They all do as a division mechanism. They all do as two steps where you first double everything and then half it twice. They all go through the same rigmarole. They've all got mitochondria. They've all got the same system, endoplasmic reticulum, things like that. ...[Y]ou could list page after page after page in a text book and it would be exactly the same for a plant, or a fungal cell, or an animal cell. Now they have really different ways of life. If you were to simply think, "Well, there's some inevitability that bacteria will give rise to complex life." ...You would imagine that a photosynthetic bacteria, a would give rise directly to photosynthetic , eukaryotic algae, but they didn't. It was by the intermediary of acquisition of a . There was a common ancestor of eukaryotes that was nothing like a cyanobacterium and nothing... quite like an algae except without the chloroplasts. So... why is it that we all have the same machinery inside, but we have such different lifestyles? Why don't we see multiple origins of complex life where cyanobacteria give rise to photosynthetic trees? Why don't we see predatory bacteria?
I would say that if there's a probability of life being cellular, which I think there is. Life being based, which I think there is. Life starting out with CO<sub>2</sub> because it's so common in planetary atmospheres, and , which is very common, from the kind of s which I'm talking about... and liquid water. They need liquid water for , but we know of it on ... on Europa... [Serpentinization] is giving rise to alkaline fluids with hydrogen gas. Most hydrogen gas you find in planetary atmosphere are coming from serpentinization. , which is the mineral required for that... is ubiquitous in interstellar dust... So all of this pushes you down a certain avenue, and if that's correct it gives you bacteria... and if that's correct then bacteria have a structural problem, and they're not going to get beyond bacteria except with an endosymbiosis, and that in itself is improbable, unlikely... because it only happened once, to our knowledge, on earth.
So there's one example of free living ... with bacteria living inside it. It wasn't . It's got a cell wall and it's not a . So they can get inside, but we can say for sure it's rare. What does it do? In a nutshell it changes the topology of the cell. It allows you to internalize respiration and it's not just internalizing the membranes. It's internalizing a genetic control system with in our own case, which by standard selection is whittled down to a kind of minimal unit required to do the job, and that in effect allows the nuclear genome to expand up to anything it wants to be. So... it's a structural change. It's not something which you can find by genetic exploration of the evolutionary space. It's something [in] which you change the topology of a cell. And once you've got that, you've got bacteria living inside another bacterial cell. You've got a fight on your hands! They've got to get along with each another somehow. So the chances of it going wrong is quite high. So I would imagine if we know of one or two examples now, there must have been thousands, millions, billions of cases of this over earth history. The fact that... all this searching across the earth that we've done for life, we find bacteria, we find , we find these candidate phyla. We're not sure what they are, exactly, but they seem to be very simple and probably s, and we see Eukaryotic cells, all the cases that appear to be potentially evolutionary intermediates, something slightly different, have turned out to be highly derived... from more complex ancestors.
What I would say with some degree of certainty from the example of life on earth, is that if you simply have a population of bacteria... the chances of it giving rise to the kind of morphological complexity... we see in eukariotic cells, and we do not see in bacteria, is remote... because bacteria and archaea, if you look at the amount of , they dwarf the genetic variation that we see in Eukaryotes. They have explored genetic sequence space to orders of magnitude greater that Eukaryotes did, and despite exploring all of that space, they haven't come up with morphological complexity. ...[T]hey did through an endosymbiosis. ...It's rare between prokaryotes, rare to the point that we know of one example of free-living bacteria with bacterial cells living inside it. We know of two other examples where, there's a for example, which has inside its own cells... some gamma protein bacteria, with beta protein bacteria living inside them. It's a little bit of a strange system and it's hard to know, again, can you generalize from this, because it's all inside a Russian doll?
[Why a cell vs a gene or partial gene?] There's been plenty of work done on RNA replicators and they have a tendency to become smaller and simpler and effectively better able to make copies of themselves with whatever you provide them in the environment, and they end up with a thing called , which is basically the binding sequence of the which allows it to furiously replicate away. ...If you're providing in the environmwent an RNA polymerase and an infinite supply of s then... they become simpler and simpler, and faster and faster at copying. ...The trouble is there isn't ever going to be an environment that's providing that for you except in a cellular context... If you're selecting at the level of genetic replication, the replicators that are better able to make copies of themselves fast are those which are, in effect, the most selfish and the least likely to cooperate to try and convert the environment into .
[On the :] It's a bit of a sterile conversation. I suppose I think of it as the cell. That's not to say that it can't act at the level of s. Of course it can. It does all the time. Any selfish gene is acting in it's own interest. I think the trouble with looking at selection only at the level of genes is it tends to downplay the importance of genetic conflict in a strange way... [I]f you have levels of selection you can have, for example... mitochondria... They were bacteria once. They're the power packs inside eukaryotic cells... [O]nce they get inside another cell, inside another originally, then they have an agenda of their own. They're making copies of themselves, and it's the speed at which the bacterium as a whole is making a copy of itself that means whether it tends to dominate in the population or not. It's not the individual genes. They will tend to throw away genes that they don't really need. And the host cell itself has got its agenda. It needs to make sure that it's getting benefits from this symbiont. It's not being taken over. It's not being eaten, and so it's... more intuitive to think of the interests of the cells themselves. And if you simply think of all of them as genes then you don't have that discrimination between the layers. Again, if you're thinking about s at the origin of life, the unit of selection in my mind is, "Can a cell make a copy of itself?" If you have a pure RNA world...
It's interesting... that life as a rule does not use UV radiation as an energy source, and the kind of chemistry that's being done using it doesn't resemble biochemistry as I know it... [T]he kind of environment that I'm talking about is deep sea s, and the question is, "Well, does it have to be deep sea? Could it... be same systems on land?" and they exist on land. They perfectly could. So it's perfectly feasible.