Magnificent. The placenta uses a protein named syncytin [1] to attach to the cells of the uterus. This protein is expressed by a retrovirus that was integrated into animal DNA over 100 million years ago.
This reminds me of Lynn Margulis’ discovery that mitochondria and chloroplasts are ancient bacteria incorporated into the cell [2].
Fun fact, Syncytin-1 is the syncytin used by primates, other branches of mammals have other forms of syncytins, meaning that the event of being infected by a retrovirus that gives mammals proteins necessary for the placenta happened several times in history following the original event and replaced the original syncytin.
(Not sure those proteins are also called syncytins in other mammals but I'm not a biologist so I'm using the limited vocabulary I have)
I'm just realizing that several shark and snake species also give birth to live young, but I don't know whether they have placentas or similar structures. I'm sure they couldn't be homologous to mammal placentas, right? (Certainly not if this article is right about the timeline.) To what extent do viviparous animals converge when it comes to the mother/child interface?
So some sharks have placentas, as mentioned in this article about tiger sharks secondarily losing theirs: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4906603/ I wonder if I'll ever run out of huge, obvious questions I haven't even begun to explore.
Sharks are viviparous, oviparous and also ovoviviparous.
[..] There are over 500 species of shark living in waters around the world and the majority give birth to live young. The remainder are oviparous, meaning they lay eggs. [..]
[..]Whale sharks (Rhincodon typus) are the largest species of shark. Although these animals produce eggs, they don't lay them. Instead, the young hatch while still in the female's body and are born as miniature adults. This is known as ovoviviparity.[..] In some species, the female will produce unfertilised eggs, which are eaten by embryos. This is known as oophagy ('egg eating') and occurs in species including the shortfin mako (Isurus oxyrinchus) and bigeye thresher sharks (Alopias superciliosus).
Embryos of other sharks survive by feeding on their smaller siblings. This is called intrauterine cannibalism or sometimes as embryophagy ('embryo eating'). This is known to occur in sand tiger sharks (Carcharias taurus).
Placental viviparity occurs in some species of shark - once the yolk sac has been depleted, it attaches to the uterine wall, acting as a pseudoplacenta.[..]
Addendum to previous comment: I am reminded of Leviathans of Jupiter by Ben Bova(we lost him to covid pneumonia and stroke on 29 nov 2020)
The Leviathans..city sized sentient whale like creatures that replicate by splitting themselves ..a process called ‘budding’. A very enjoyable read. This thread made me look it up to read again. If you like good old fashioned sci-fi, I recommend it. It’s part of Ben Bova’s Grand Tour series.
Seahorses are also interesting in this regard, not only do they also have internal pregnancy and a placenta-like structure, it's also the males that get pregnant.
And the thing that distinguishes eggs from sperm is what? The relative size? (Are there no species where they are roughly the same?) Or that the mitochondria come from the egg?
Edit: Looks like it's merely relative size, and when neither gamete is bigger biologists throw up their hands. Sort of a bummer that there isn't a more robust definition.
The relative size but also the structure and the process of fertilization. The sperm enters the egg, not the other way around. Mitochondria come from the egg, yes.
Also note that sperm vary in shape. Mammalian sperm have the familiar tadpole shape with a flagellum for swimming. Other species, such as nematodes, have ameboid sperm which move along surfaces rather than swimming freely.
> The sperm enters the egg, not the other way around.
I think this is wrong. My understanding is that the sperm cell membrane fuses with the egg cell membrane, forming a single container. The haploid nucleus of the sperm and the haploid nucleus of the egg then fuse. This processes is by no means fully symmetric (the egg has a tough coating on its cell membrane that must be dissolved by the sperm through the Acrosome reaction), but there is also no time when the sperm cell (delineated by its membrane) is inside the egg cell.
My point was not that there aren't many correlated asymmetries between eggs and sperm in particular parts of the phylogenetic tree (like mammals) that can be used for distinguishing them. My point was that there seems to be no fundamental asymmetry that applies to all two-sex sexually reproducing organisms; rather, there is a continuum of degree of gamete dimorphism that ends at organisms where there is no principled distinction between sperm and egg (isogamy).
I red this in some book I forgot the name of, so I can’t cite anything. But it was explained that female strips everything except for the DNA from sperm. This way there’s minimal risk of infecting the egg with the pathogens the sperm can carry. I’m not sure if it’s true just for humans or other organisms as well.
I am not a biologist but what I remember from biology is that female seahorses have the eggs. Sexual reproduction happens when two haploid cells combine and the sex is determined based on this system.
Another wild thought: Is it possible that the solution for harmful retroviruses, like HIV, are retroviruses itself? If retroviruses can overwrite or fuse with our DNA, can't we overwrite it back by making a retrovirus ourselves?
I'm not familiar with this topic at all, but it's super interesting.
The problem with pathological viruses is they take over the cells themselves and make them fabricate more viruses until they burst. In these cases there is no living cell left to reprogram.
In the case of HIV it's even worse since it targers certain immune cells. It will eventually kill most of them if it's allowed to replicate unchecked, crippling its host's immune response.
Pathological viral infection feels like an infinite loop. The CPU is a dumb machine doing its job to decode and execute instructions, until it bursts in to flames. The ribosome likewise transforms amino acids in to proteins as directed by RNA until the cell is destroyed.
Yes. Ribosomes are dumb machines. They have no authentication mechanism. The RNA randomly gets near the ribosome, interacts with it and the result is a whole new protein. It doesn't know whether the RNA was translated from cellular or viral DNA. In the case of cellular DNA, the ribosome makes useful proteins that helps the cell perform its function. In the case of viral DNA, the ribosome makes viral proteins which eventually get assembled into a new copy of the virus.
Viruses aren't actual living beings, they're just self-replicating structures. They are capable of injecting genetic material into other cells in order to get them to make more copies of themselves.
> Pathological viral infection feels like an infinite loop.
The same logic could be applied to biochemistry in general. It's not 100% accurate but it is a lot of fun to think about it in those terms. It's also fun to think about programming in biochemical terms! I wrote about this in the past:
This is an awesome topic. Thank you both for your response! I try to convert everything into programming terms to make it a bit easier to understand, but it's a really complex topic imo.
Just a wild thought: if aliens wanted to change a genome of some species, what are the chances they would construct viruses to do that? AFAIK CRISP uses something similar.
That would probably only work if it gave the altered species a significant advantage (or if you had a system to detect and penalize/eliminate individuals depending on their genome, at scale).
At this point in time, do you really think that viruses are incapable of sweeping the globe and infecting a significant fraction of humanity, if they don't provide a benefit to the host?
The parent was more specific and asked about changing a species's genome. Infecting everyone does not lead to a change in genome if they die (or fight it off).
Assuming a low percent of the infected get their genome changed or produce offspring with altered genome, those don't take over.
It is also being researched how endogenous retroviruses are part of aging. The very rough idea being that at some point long in past a retrovirus became endogenous and provided a pathway for aging increasing the evolutionary rate of that organism, which would then out-compete most of the immortal organisms with time.
Endo is internal, exo is external. Another example is endonym/exonym (nym is the particle for name): endonym - the name a group gives itself, exonym - the name outsiders give a group.
Genous, you can think of it as analogous with genesys, it's the root word for creation.
That's incredibly interesting! Do you know some (pop)science books that could help learn more about viruses? I found two that sound good from description: "Viruses: A Very Short Introduction" by Dorothy H. Crawford and "A Planet of Viruses" by Carl Zimmer. Would you recommend any of these or something else?
I'm not really in a position to suggest a good book on viruses. I found out about this from a pop-sci book on latest research on aging and longevity.
When it comes to viruses though, there's one more very fascinating hypothesis. That the cell nucleus itself was once actually a virus that took over some bacteria to do its bidding.
Red queen gives some perspective on evolutionary arms race between hosts and pathogen. Virus is a piece of genetic material, so to understand a virus one needs to understand how a gene evolves.
It’s not a science book, but this did bring to mind an enjoyable sci-fi book by Greg Bear called Darwin’s Radio, for reasons that’ll be obvious: https://en.wikipedia.org/wiki/Darwin%27s_Radio
And the unusual factor that certain species adopted an intergenerational social system (which makes longer lifespan slightly more advantageous).
Thus we have some species nurturing their young (most mammals, marsupials, and birds) as opposed to spawning them and then going on to other things (e.g. fish, insects).
The most extreme case of this is humans: females live well beyond their fertile age and, effectively, so do men (while technically fertile in their 70s, the offspring are not as vigorous, in the evolutionary sense, as those conceived when the father was in his 20s). It is believed that this is because older humans have learned lots of things that can be useful in keeping kids alive and effective until they can propagate their genes.
Although 99+% of evolutionary psychology is utter bullshit this is nonetheless fascinating.
Biological immortality is the technical term for organisms which do not die unless killed, or more accurately where rate of mortality from senescence does not increase with chronological age.
They're similar but distinct concepts. Negligible senescence means after a point aging doesn't negatively affect the organism. They don't lose reproductive capability or experience functional decline. Imagine being 20 forever.
Biological immortality means after some point mortality rates don't increase with age - you can still grow old, it just won't directly kill you. Biologically immortal organisms don't have a max lifespan, if they're lucky they can just keep on going, but they may be dramatically more feeble than their younger counterparts.
I think some Lobsters and Crocodiles are in the later category. Immune to cancer, their cells divide without limit over the years. So they never stop growing. The enormous ones are the oldest, and have a size fitness advantage in hunting and other combat, but a counter balancing fitness disadvantage in their increased needs.
I'm not aware of a context where immortal refers to organisms that can't be killed by accidents and predators. In mythology and novels alike, vampires, gods, elves, ..., are all called immortal and still can be killed.
I thought crocodiles or alligators (forget which one) were immortal and they tend to die only because they get so big and can't eat enough to maintain life eventually. But, they're not on this list?
>> The very rough idea being that at some point long in past a retrovirus became endogenous and provided a pathway for aging increasing the evolutionary rate of that organism, which would then out-compete most of the immortal organisms with time.
No virus needed. Humans are currently (or were very recently) under enormous evolutionary pressure to get smarter. The ability to adapt is likely itself a trait as you suggest, but it makes sense that it can vary over time as needed depending on environmental factors.
I like to hope, if I am ever afflicted by a virus; the inevitable distress of "A virus? woe is me" will be tempered by the pithy thought, "A virus whence is me".
Unfortunately trying to get something useful from a viral infection is almost like trying to get bitten by a radioactive spider in order to get superpowers. The problem is that as useful as those new "features" may be, they always need many generations and many more mutations to be integrated and turn into something practically useful. In the meantime you would at best have to live with a painful spider bite and some radiation poisoning.
Every thing about life is so fascinating. I have always wondered how viruses have persisted for so long if they need a host to survive and it took us millions of years to go from single celled to multicellular. I wish there was a way to know everything and become good at everything. :/
I think the plain answer is that if they didn't, you wouldn't know about them. Just like all other parasites. In some sense, we're all parasites on plants, that in turn feed on the energy delivered by the giant fusion reactor in the middle of our solar system.
In the case of viruses, remember that there are viruses for much simpler organisms, like bacteria. And yes, you can use those to treat bacterial infections.
> In the case of viruses, remember that there are viruses for much simpler organisms, like bacteria. And yes, you can use those to treat bacterial infections.
Those are called bacteriophages (a misnomer, since they don't actually eat bacteria).
I have a theory that bacteriophages originated as a biochemical weapon for inter-bacterial competition.
If you think about it, the ability to produce packets that target and penetrate a competitor's cellular wall and inject a payload is pretty useful. Payloads could be as simple as a toxin to start.
From that point, you could have stepwise improvements for deliverables that disrupt reproduction in the target (for example by cutting apart DNA strands), by inserting junk genetic material to evade self repair mechanisms, by inserting or removing specific genetic sequences to create other vulnerabilities, that insert the specific genes to make copies of the delivery system and various payloads (this is useful even if the 2nd gen copies are poor and don't include those same genes), and then finally a fully self-reproducing package that can spread exponentially.
At that point the 'weapon system' can evolve independently, and indeed co-evolve with it's target/host to (probably) jointly outcompete other species including the originating species.
Even prior to full self-reproduction, proto-viroids might act as a horizontal gene transfer mechanism that can exhibit familiar 'selfish gene' patterns and mediate various feedback loops between bacterial species. After 'escape' any species remaining with proto-viroids mechanisms would likely be parasitized and bootstrapped into actual viruses through competition and crossovers, or have those mechanisms suppressed into remnants indistinguishable from endoviruses (presuming endoviruses can even be identified in organisms as simple as bacteria).
An arms-race of proto-viroid attack vectors might also be the antecedents of and/or the driving force behind the need for various organelles that distinguish the more complex eukaryotes from bacteria.
"In some sense, we're all parasites on plants, that in turn feed on the energy delivered by the giant fusion reactor in the middle of our solar system."
I think that's wrong. By definition, a parasite interacts with a living organism. For example worms that eat the remnants of fallen leafs are not parasites, they are saprophytes. Humans as well can live consuming bits of fallen fruits that do not take part in plants' further reproduction (i.e. fruit's seeds) and would otherwise just rot (and can thus be considered dead tissue).
I wouldn't say everything, but I have encountered such people too. Extremely knowledgeable and always curious. It feels as if these guys somehow have more energy to expend than most of us. On the contra, I get burnt-out pretty quickly when I try to push myself beyond an arbitrary threshold which I have never been able to measure.
Viruses are ultimately bits of genetic code wrapped in a shell and injection mechanism. IANAB, so this is pure conjecture, but: it wouldn't surprise me if it turned out that viruses are a kind of degenerative state - something evolution may just spit out as it makes mistakes trying to improve on cells. If that's the case, then viruses may exist simply because they are a waste product of natural selection of more complex life.
This implies that evolution has a goal and that we are that goal. This shows a fundamental misunderstanding of evolution of natural selection that is common.
Who is to say that viruses won't be the end result of life on the planet earth?
Complex life <> more evolved life. It's just differently evolved.
I know that evolution doesn't have a goal. I may have antropomorphosized it too much, but my goal was to follow a sort of temporal/casual trail that we find interesting.
The way I imagine my idea is that, if you pick a species and its interesting descendants, and order on a timeline like such:
A --> B --> C --> D
What really happens is more like:
[bunch of other spinoffs]
^
|
A --> B --> ...
And that some of those spinoff could be viruses. Imagine a mutation to A causes a bunch of functionality in the organism to shut down (and in subsequent generations disappear), while leaving a viable organism. For a sufficiently uncomplicated A, the result of such mutation could be a bundle of genes wrapped in a shell with an injection mechanism. That's a virus.
This is also described in a recent book called “world of viruses.” Worth reading, as there is way more fascinating things I learned about viruses reading this book. Like that there are super big viruses the size of bacteria that are the closest point between a cell and a virus, a fascinating discussion about if viruses are dead or alive, and that there are 55 types of viruses found in a random blue birds that could hop in to humans like the West Nile virus did not so long ago.
There seem to be a lot of different levels of maternal support for unborn offspring, ranging from „shelter“ by carrying eggs until they hatch to full life-support like mammals offer it.
More specifically, the gene that enables syncytiotrophoblasts to do what they do enabled placentas to be compatible with an adaptive immune system. So the evoution of the placenta needs to be seen in the immune system context, a more simple immune system may not have a problem with a non-self embryo in the body. So the virus gene could be one of the factors that enabled a proto-placent to evolve along an adaptive immune system. [Note this is pure speculation, I haven't read up on the evolutionary timescales of either.]
I think this article is a bit misleading. The placenta did not evolve from an ancient virus. A protein found in primate placentas (but, interestingly, not found in other placental mammals) is clearly related to endogenous retrovirus proteins. (I do not think that HERV and HIV have much to do with each other -- they are very different viruses). But it is pretty interesting that primates started using this protein relatively late in mammalian evolution.
For anyone fascinated by this topic, the book I, Mammal[1] by Liam Drew is highly recommended. A whole chapter is dedicated to the importance of placenta in evolution of mammals, along with other chapters on other mammalian features like lactation, hair, locomotion, homeothermy etc.,
What’s fascinating is that genome is highly dynamic. The transposons rearrange themselves all the time and are believed to be remnants of ancient viruses. And there’s probably many more pieces of DNA which are domesticated viruses. Alu element comes to mind, which is the most common transposable element in human genome and probably has a role in regulating gene expression.
The baby's cells are not wholly walled off from the mother's system. Like the best parasites, babies suppress their mother's immune system when it comes to their own cells.
It's amazing how quickly people leap to conclusions in the field of evolutionary biology. Fact: a portion of the placenta's protein sequence is very similar or identical to a virus. Unsubstantiated (and pretty wild) conclusion: the placenta is the product of that virus. We would never accept that kind of mythology in software, medicine, law, etc. – why do it here?
Former biochemist here. There may be flaws in the study (i haven't read it), but none of what you're gesturing to strikes me as evidence of "jumping to conclusions". A couple thousand basepairs is a row (a fair assumed length for encoding proteins) is plenty enough to draw conclusions about evolutionary provenance, and this is totally common.
For lots of ho-hum reasons that don't require any spectacular justification, you are full of virus DNA -- it's just mostly never mentioned in popsci articles that come under your skeptical lens :)
The parent link is a journalistic version of the information, not the scientific paper(s) with the evidence.
Besides, the closest equivalent in software is:
If you have ~20 lines of code, written twice with the same function (perhaps crossing multiple functions in the same order), written with identical style and syntax, with maybe some variable names being slightly different.
In a copyright case, would you believe that these originate from different authors?
> conclusion: the placenta is the product of that virus. We would never accept that kind of mythology in software
Seems like a very reasonable hypothesis if that virus is a retrovirus [1]:
> Endogenous retroviruses (ERVs) are endogenous viral elements in the genome that closely resemble and can be derived from retroviruses.
> ...not all ERVs may have originated as an insertion by a retrovirus but that some may have been the source for the genetic information in the retroviruses they resemble.
Weird how antivaxxers use this obscure bit of knowledge to get scared about similarity of covid S-protein to syncytin and immune response to covid vaccine (why not covid itself?) making pregnancy harder or impossible.
Thank you! Have you found anything that would indicate any level of actual similarities between S protein and syncytine? Beyond both of them being of virus origin and starting with letter S?
I couldn't find anything stronger than "german doctors say" which means nothing.
This reminds me of Lynn Margulis’ discovery that mitochondria and chloroplasts are ancient bacteria incorporated into the cell [2].
[1] https://en.wikipedia.org/wiki/Syncytin-1
[2] https://en.wikipedia.org/wiki/Lynn_Margulis#Endosymbiosis_th...