[dekhn]

This is just wrong (as you say, careless). The criteria is "metabolism", not growth and reproduction. Similarly, viruses are not alive although they can reproduce. Interestingly, there are some highly parasitic organisms with limited ability to metabolize, that is, they have lost their genes and instead depend on their host organism (but I think all those examples still have some level of metabolic capability).

[Ericson2314]

I don't really know what "metabolism" means, abstractly. I am deader than a virus without chloroplasts existing elsewhere. Viruses need a host for the productive bit, I had food for the non-reproductive bit. Viruses don't break down fast enough compared to their abundance to cause extinction, so why fault them for optimizing?

People commonly theorize that virus came from rump parasitic cells. I wouldn't be surprised if at some point a huge virus with a membrane accidentally picked up enough stuff to become a cell again. To go back to chloroplasts again, it would be like how brown algae is an ex-heterotroph.

[bonzini]

There used to be a hypothesis that DNA originated in viruses and that the three branches of life (archea, bacteria and eukaryota) evolved when three different DNA viruses encountered preexisting RNA-based cells. (https://www.pnas.org/content/103/10/3669)

Nowadays however it is more widely believed that eukaryota descend from archea, but anyway it's pretty much a given that viruses are not and have never been alive. And while the opposite has happened, gene transfer usually goes from viruses to cells, endowing them with new abilities, more than the opposite direction.

[geomark]

Metabolism has two parts: Catabolism breaks down complex molecules into smaller building block molecules - think proteins -> amino acids. Then anabolism uses those building blocks to build up larger molecules for growth and maintenance. Crystals don't do the catabolism part.

[kubanczyk]

Nah, this seems more like an implementation detail than an actual core of life's "functionality".

[BlueTemplar]

Looks like cell's phospholipid bilayer is anabolized just like an "endothermic" crystal : no need to cross a potential barrier (and releases energy instead of consuming it)?

On the other hand, other forms of anabolism require crossing an energy barrier (and consumes energy) ?

While catabolysm is the inverse operation : crosses a barrier, but releases energy?

The formation/destruction of "exothermic" crystals like dissolution of Sodium Hydroxide if put in contact with water is I guess similar to above two reactions ? (Or better example: explosives)

[Ericson2314]

While I don't think it's a good definition for life, I do think it's a good definition for metabolism.

That simply means "metabolism" isn't the concept to generalize further, and I'm OK with that.

[webmaven]

> People commonly theorize that virus came from rump parasitic cells.

I have an alternate theory, that I've never seen articulated elsewhere: Viruses might be inter-microbial biochemical warfare gone wild; Among primitive and not-particularly robust cells, one competitive strategy might be to package toxins or other disruptive molecules into capsids that cleverly get past the 'enemy' cell membrane, perhaps aimed simply at disrupting the initially brittle reproductive process. Packaging random genes in with these capsids would be the beginning of the Horizontal Gene Transfer function that viruses perform in the ecosystem. A refinement would be to add in the specific genes for producing capsids as a deliverable so a subverted 'enemy' then takes out several neighbors (but 2nd gen capsids don't have to necessarily contain the same genes in order to accomplish their 'mission', or even be entirely identical to the 1st gen, as long as they work). A further refinement would be increasing fidelity, and packaging the responsible genes into the 2nd gen capsids produced by subverted cells so the 'weapon' can spread exponentially.

And voila, the 'weapon' has now escaped the originating organism and can evolve independently parasitizing the target. Each incremental step can at least hypothetically provide an evolutionary advantage so the whole thing doesn't have to spring forth fully formed.

Depending on how widespread variants of the original strategy were, viruses might have independently emerged more than once.

If I'm right, we ought to still be able to find cells that attack or suppress competitors with capsid-like molecules that don't particularly resemble existing virus lineages, possibly containing non-genetic payloads of various sorts.

We might also find bacteriophage endoviruses that as their 2nd stage only produce capsids sans-genes or that otherwise aren't identical to the endovirus, or ones that aren't very good at packaging up the correct genes, or that are much better at disrupting cell reproduction than hijacking it, or that the subsequent copies are only low-fidelity ones that can similarly penetrate the host but not spread any further; basically any indicators that the mechanisms for penetration of the membrane, attacking the cell, and even for making poor 2nd gen copies, have been evolving longer than capabilities only needed for indefinitely repeated copying, retransmission, and reinfection.

I suspect though that the oldest reconstructible endoviruses will no longer be capable of penetrating current members of their host cohort, which tells us little about the virus itself, but may offer clues about whatever ancestral cells were still prone to infection were like.

[pmayrgundter]

Neato!

[sterlind]

Viruses need a host environment to replicate. Fish need water and plant life or other fish to replicate. Algae needs nutrients to replicate. Every lifeform can only exist in some host environment. Viruses simply happen to have biological host environments, right?

[bufferoverflow]

You're equivocating environments and hosts.

Hosts do the replication for the viruses.

Environments do NOT do the replication for the fish.

[tw25508394]

Single-cell organisms replicate.

Single cells of multi-cellular organisms replicate.

Shoot me in the head and all my cells will eventually stop replicating.

To a cell in my body, is the body a host or an environment? Is the distinction useful?

[est31]

I would say neither. Your body is a colony, like an ant colony or bee colony. In all instances, most individuals can't start a colony of their own but it's limited to a set of queens/gametic cells.

As for shooting into the head, some multicellular life forms can actually heal from that. From planaria you can remove any part of their bodies, it will grow back. Shooting into an ant colony won't do much harm to them either, nor is removing the branch of a tree. It's only a property of animals I'd say who have given up this property (for the most part) in exchange for more complex metabolic systems. (heart, nervous system, etc)

[jacquesm]

In fact, for quite a few fish they do, ditto amphibians and insects too.

Mammals are the exception here, there the environment for replication is an integral part of the body of the parent. But in all those other cases, including, technically, marsupials it isn't.

[kaba0]

Just because it happens outside the ‘parents’ body doesn’t mean the environment does it anymore than a mother being only a filter around the womb and the environment.

An egg is a complex life that can reproduce the whole organism (and actually with energy stored INSIDE, that’s why they are great nutritios value for others)

[cdogl]

The distinction between environment and organism here seems a little arbitrary. The organism’s cells have membranes, but they’re permeable and the organism can’t do its job without an interaction between itself and the environment. Even with the energy stored inside, it requires warmth and atmospheric pressure. I suspect there are any number of counter examples of life forms that challenge the simplicity of this formulation. You could just as easily argue that each cell is a life form unto itself, right?

[kaba0]

Isn’t each cell a life form unto itself? Like there are cell lines from cancer cells that has been living outside their ‘host’ long after the human they were aquired from died (HeLa) They just specialize in an specific environment created and maintained by cells with the same genetic material

[Ericson2314]

We are all just environments for ribosomes.

[plutonorm]

You've not understood the depth of the original comment's thinking.

[dekhn]

this is certainly not the mainstream way of thinking; it's certainly not specific enough.