The biggest issue is that life has to perform some sort of chemical process. Any lacking any clear delineation of an inside the life in question and the outside, would make continuous chemical processes impossible.
A stray breeze blowing away chemicals would effectively kill life. A "cell" having a barrier would allow that chemical process to happen.
Life may not require cells in the way cells on Earth evolved, but they'll require cells of some sort.
I'm trying to imagine some wacky living polymer or protein that wiggles around and grabs whatever molecules it needs to force localized reactions without a protected cell environment.
But then I start to wonder if evolutionary pressure would encourage ones that form colonies and eventually closed topologies and cells. And then others might specialize to hijack those colonies...
Maybe the planet, or locality on the planet, has resources that permit continuous chemical processes to happen without cell (maybe no breeze). Maybe it doesn't need to be continuous.
Its very easy to say something like that, but impossible to provide any sort of examples, even in a fantasy context.
Like what a gas cloud with intelligence?
We live in a universe with distinct rules, saying life doesnt have to have cells is basically fantasizing about other dimensions where we're made out of hot dogs or something. Saying life doesn't require cells is just redefining what words mean, since a cell literally means the smallest functional unit of life.
> We live in a universe with distinct rules, saying life doesnt have to have cells is basically fantasizing about other dimensions where we're made out of hot dogs or something.
That comparison is ridiculous.
We live in a universe for which many of the rules are unclear and we have only grasped some small subset of the possibility space dor emergent structures those rules enable.
We have a hard enough time coming up with a clear definition of which of the emergent structures that we do know about qualify as "life". You seem to accept it as settled that viruses are not alive and are mere chemistry but it seems to me like drawing a line in the sand between two similar processes.
When we're talking about cells, that seems like an easier thing to define, but I would posit that is another definition that could prove to be slippery if an alien biology without a common ancestor is discovered.
We have to be careful about how we define life and cells in relation to each other to avoid just creating tautologies rather than useful definitions.
For example, how sure can we be that by defining life to exclude viruses, we haven't defined life in such a way that we have excluded processes that can support consciousness?
I think it is pretty easy to imagine life which has "cells" that we fail to recognize as such because they serve a similar function but with a sufficiently different form that we would fail to see or recognize them in a microscope.
Now, many of those imaginings may be scientifically impossible but that doesn't justify comparing all of them to "a dimension where we are made of hotdogs."
I think that no matter what definition one comes up with, it is possible to have counter examples (i.e., is fire alive, seeing as how it reproduces, consumes food (fuel), etc).
One of the indicators of life I like to use, that seems to be useful in the general sense is that life is good at decreasing (fighting against) entropy at a local level. So fire for example increases entropy. A living being takes in raw materials plus energy, and produces a lower entropy (higher organization) of those inputs. Like plants making sugar, and pulling apart CO2 to make free oxygen. Or animals taking in sugar and oxygen, the output is higher entropy (CO2) but you have higher organization of material (lower entropy) locally within that organism.
This also means that life doesn't necessarily need to reproduce (although that is how life that we know of works). You could theoretically have a living being that kept evolving its own form but never developed the ability to copy itself. It just keeps consuming energy and environmental items, and maintaining its own internal complex structures.
No, it actually can't. Fire will burn as hot and quickly as it has fuel and oxygen to sustain, because it's a chemical reaction, it doesn't maintain any kind of steady-state in order to extend it's survival.
If you take away my food, I will still have energy to find more. If you take away my air, I will still have air (albeit quite time-limited) to find more. That gradient is how we understand something to be alive.
You can indeed argue it does maintain certain states to help extend its survival. Think of how fire tends to burn wood into charcoal. Now a big wind may blow out the primary flame, yet it actually lives on through the charcoal for a period of time, which when the right conditions are met, are able to reignite the blaze. Think of how fire tends to fragment matter, and send it up in the updraft. Sparks of this nature are able to ignite more fires further than the primary fire could reach on its own, beyond firebreaks even. This is akin to clonal propagation to ensure survival that we see in many species, or the release of many progeny over an area banking on the reality that most will not survive, but some will.
I think if we go further down the logic hole and start questioning that by virtue of it being something you can start yourself with a tinderbox, that fire is not life, we question the entire premise of ideas like abiogenesis which argue life is inevitable when certain chemical conditions being met.
> You can indeed argue it does maintain certain states to help extend its survival.
Not really. Everything you described is a property of chemistry and physics, not biology. Coals smolder under high residual heat and a lack of oxygen, not because of gated ion channels, osmotic barriers or any other evolved structure.
> This is akin to clonal propagation to ensure survival that we see in many species, or the release of many progeny over an area banking on the reality that most will not survive, but some will.
It's really not. Fire has no genome, nor a biological urge to reproduce. Cinders are a result of physics - pieces of organic matter fragment, heat causes rising air, some fragments are light enough to rise on hot air. If they are still hot when they land, fire can start at a new location.
There is a critical point between high and low entropy where life exists: You can increase or decrease entropy, but on either end you seem to have death. High entropy at least has more information.
Homeostasis seems to be important.
Reproduction seems important, but that's "just" a survival strategy. It doesn't necessarily seem like a requirement for something to be "alive".
A system that in some context has an inheritable genotype that influences its phenotype.
I add the context part so we can say a virus is alive if the context of host cells is present, and otherwise not. Similar to how we are alive in the context of being on the Earth's surface, but not on the Sun.
It is very likely that some kind of life that doesn't require cells must be possible, because it must have existed before the appearance of cells.
Cells are much too complex to appear spontaneously, without evolving from an acellular form of life. Cell membranes can form spontaneously from fatty substances, but no cell can live without complex molecular pumps that are embedded in the cell membrane and which bring in some substances from outside and which expel some other substances from inside. Such pumps must be the result of a long evolution that preceded the first cells.
Nevertheless, any form of life that is not made of cells cannot live free in a fluid like water, but it must be attached to the surface of some suitable rocks, where a flow of nutrients must exist.
The most plausible hypotheses about the appearance of life on Earth suppose that before the appearance of cells, life was restricted to pores inside rocks from hydrothermal vents, where a flow of dihydrogen and of alkaline ions could provide the energy, and metallic sulfide minerals could provide the catalysts for organic syntheses.
