Okay, this got me curious. From the wikipedia article on viruses:
Viruses are considered by some biologists to be a life form, because they carry genetic material, reproduce, and evolve through natural selection, although they lack the key characteristics, such as cell structure, that are generally considered necessary criteria for defining life. Because they possess some but not all such qualities, viruses have been described as “organisms at the edge of life” and as replicators.
Theoretical biologist here. I consider viruses to define the lower edge of what I’d consider “alive.” I similarly consider prions to be “not alive,” but to define a position towards the upper limit of complex, self-reproducing chemistry. There’s some research going on here to better understand how replication reactions (maybe encased in a lipid bubble to keep the reaction free from the environment) may lead to increasing complexity and proto-cells. That’s not what prions are, but the idea is that a property like replication is necessary but not sufficient and to build from what we know regarding the environment and possible chemicals.
I consider a virus to be alive because they rise to the level of complexity and adaptive dynamics I feel should be associated with living systems. I’ll paint with a broad brush here, but they have genes, a division between genotype and phenotype, the populations evolve as part of an ecosystem with all of the associated dynamics of adaptation and speciation, and they have relatively complex structures consisting of multiple distinct elements. “Alive,” to me, shouldn’t be approached as a binary concept - I’m not sure what it conceptually adds to the discussion. Instead, I think it should be approached as a gradient of properties any one of which may be more or less present. I feel the same about intelligence, theory of mind, and animal communication.
The thing to remember when thinking about questions like this is that when science (or history or literature…) is taught as a beginner’s subject (primary and secondary school), it’s often approached in a highly simplified manner - simplified to the point of inaccuracy sometimes. Many instructors will take the approach of having students memorize lists for regurgitation on exams - the seven properties of life, a gene is a length of dna that encodes for a protein, the definition of a species, and so on. I don’t really like that approach, and to be honest I was never any good at it myself.
Interesting, thanks! I’m someone that has been educated on viruses to a Radiolab level, and as such I’d like to hear your take on the idea that viruses used to be more complex organisms, which then evolved to be the simple and efficient form they are now.
Wildlife biologist here, and I have to concur with just about all of this.
I think we generally look at a viruses and consider them alive but just barely. While prions are not because they (proteins) are what is considered one of the building blocks for life. Self replication being one of the major criteria we’d look for. We look at a very macro level of life but our education and work has a strong overlap down here a well.
This is such a well written post! Gets the point id like to make across in a much better way than I could
They’re not compromised of cells, can’t self regulate, and can’t replicate on their own and other organisms have to do that for them. The last point being important to our criteria for living. I was never taught as a biologist by anyone that they were alive
“Obligate intracellular parasite” was drilled and showed up on multiple exams, along with all that you mentioned. I’ve also heard “escaped cellular machinery.”
For reproduction purposes, many parasites require a specific host to reproduce in. An interesting example is a worm that mind controls a snail and gets itself eaten by a bird, and then reproduces in the bird. Surprisingly, both the snail and the bird survive this process. (Granted, the difference between this and a virus is the virus uses the RNA decoding infrastructure in the infected cell to reproduce itself, while a parasite just is adapted to reproducing in the environment of the hosts body, but uses its own cells to do the reproduction).
However, there are many, many examples in nature of some essential task (often some part of the energy production/absorption process) that are done by a different organism. Some particularly interesting examples:
there are a handful of animals that eat plants, absorb the chloroplasts, and use those to do photosynthesis
In most animals, even in humans, a lot of the digestion process is done by bacteria living in your digestive tract. Some illnesses are caused by issues with the digestive tract bacteria, such as them dying out.
There are other animals adapted to living in environments or using things produced by other organisms. Hermit crabs get their name from their behavior of borrowing shells created by other organisms.
Really the only organism that can truly live “by itself” would probably be something like algae.
Like if your computer got a glitch that caused it to burn CDs that, when inserted into another computer, gave it a glitch that caused it to burn CDs that, when inserted into another computer, etc.
It seems to fail the last criteria there. They don’t actively escape or react to predation. For the most part they aren’t actively “trying” anything other than to just float around and replicate.
Okay, this got me curious. From the wikipedia article on viruses:
Theoretical biologist here. I consider viruses to define the lower edge of what I’d consider “alive.” I similarly consider prions to be “not alive,” but to define a position towards the upper limit of complex, self-reproducing chemistry. There’s some research going on here to better understand how replication reactions (maybe encased in a lipid bubble to keep the reaction free from the environment) may lead to increasing complexity and proto-cells. That’s not what prions are, but the idea is that a property like replication is necessary but not sufficient and to build from what we know regarding the environment and possible chemicals.
I consider a virus to be alive because they rise to the level of complexity and adaptive dynamics I feel should be associated with living systems. I’ll paint with a broad brush here, but they have genes, a division between genotype and phenotype, the populations evolve as part of an ecosystem with all of the associated dynamics of adaptation and speciation, and they have relatively complex structures consisting of multiple distinct elements. “Alive,” to me, shouldn’t be approached as a binary concept - I’m not sure what it conceptually adds to the discussion. Instead, I think it should be approached as a gradient of properties any one of which may be more or less present. I feel the same about intelligence, theory of mind, and animal communication.
The thing to remember when thinking about questions like this is that when science (or history or literature…) is taught as a beginner’s subject (primary and secondary school), it’s often approached in a highly simplified manner - simplified to the point of inaccuracy sometimes. Many instructors will take the approach of having students memorize lists for regurgitation on exams - the seven properties of life, a gene is a length of dna that encodes for a protein, the definition of a species, and so on. I don’t really like that approach, and to be honest I was never any good at it myself.
Thanks for posting this! While my knowledge of biology is quite limited, it’s always great to get an informed person’s take on an interesting topic.
Interesting, thanks! I’m someone that has been educated on viruses to a Radiolab level, and as such I’d like to hear your take on the idea that viruses used to be more complex organisms, which then evolved to be the simple and efficient form they are now.
Wildlife biologist here, and I have to concur with just about all of this.
I think we generally look at a viruses and consider them alive but just barely. While prions are not because they (proteins) are what is considered one of the building blocks for life. Self replication being one of the major criteria we’d look for. We look at a very macro level of life but our education and work has a strong overlap down here a well.
This is such a well written post! Gets the point id like to make across in a much better way than I could
They’re not compromised of cells, can’t self regulate, and can’t replicate on their own and other organisms have to do that for them. The last point being important to our criteria for living. I was never taught as a biologist by anyone that they were alive
o7
“Obligate intracellular parasite” was drilled and showed up on multiple exams, along with all that you mentioned. I’ve also heard “escaped cellular machinery.”
Absolutely fascinating…if a tad frightening.
There are plenty of organisms we generally consider “alive” that can’t replicate or do other key functions without other organisms.
Like what? Sorry my comment posted so many times my phone was messing up
For reproduction purposes, many parasites require a specific host to reproduce in. An interesting example is a worm that mind controls a snail and gets itself eaten by a bird, and then reproduces in the bird. Surprisingly, both the snail and the bird survive this process. (Granted, the difference between this and a virus is the virus uses the RNA decoding infrastructure in the infected cell to reproduce itself, while a parasite just is adapted to reproducing in the environment of the hosts body, but uses its own cells to do the reproduction).
However, there are many, many examples in nature of some essential task (often some part of the energy production/absorption process) that are done by a different organism. Some particularly interesting examples:
there are a handful of animals that eat plants, absorb the chloroplasts, and use those to do photosynthesis
In most animals, even in humans, a lot of the digestion process is done by bacteria living in your digestive tract. Some illnesses are caused by issues with the digestive tract bacteria, such as them dying out.
There are other animals adapted to living in environments or using things produced by other organisms. Hermit crabs get their name from their behavior of borrowing shells created by other organisms.
Really the only organism that can truly live “by itself” would probably be something like algae.
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There you go defining humans as not alive again
Are these requirements for your definition of life? Is it possible for us to reproduce without relying on other organisms?
Perhaps an artifact from an earlier abiogenesis event that cannibalized itself before our own evolutionary tree started?
I believe the theory is that viruses have evolved from other life forms multiple times. Basically a DNA sequence gone rogue.
Like if your computer got a glitch that caused it to burn CDs that, when inserted into another computer, gave it a glitch that caused it to burn CDs that, when inserted into another computer, etc.
This explanation is excellent but feels incomplete. What happens after it burns the CD?
It runs out of all CDs and thus stops working. A PC can not work without CDs.
I’m no scientist but I’d say, “Do it reproduce? Do it evolve? Do it try to survive? Bruh, it’s alive.”
I’m no scientist though. Just an idiot watching thangs. :p
It can’t reproduce on its own, though. It needs a living cell to do that.
It seems to fail the last criteria there. They don’t actively escape or react to predation. For the most part they aren’t actively “trying” anything other than to just float around and replicate.
ok i’m not a biologist but having a cell structure as a prerequisite for defining life sounds very arbitrary to me.