Are there self-sustaining chemical systems that exist without a cell membrane? Of those, are any capable of Darwinian evolution?
I can imagine placing 10 sugar cubes and 10 marbles in a bucket of water, waiting a day, and noting that the population of marbles was higher since sugar cubes dissolve. But to my knowledge neither sugar cubes nor marbles replicate on their own. Maybe sugar crystals replicate in a fashion as they come out of solution. They grow at least.
NASA’s working definition of life is in thread title: it is, “a self-sustaining chemical system capable of Darwinian evolution.” I’m wondering what the simplest form of life is according to this definition. Along the way, I’d like to improve my understanding of chemistry and natural selection. Google has plenty of material, but I can’t quite cut through it.
That seems like an odd definition of “life”. Obviously, no life forms are completely self-sustaining, they all have to take in some sort of nutrients from their environment. And I’m not sure why the adjective “Darwinian” is necessary; otherwise people might think it meant Lysenkoist evolution?
The answer is “a virus”, if you think that viruses are living; biologists are divided on this ultimately semantic question,. If you don’t think viruses are alive, then no, there are no non-cellular life forms and the answer would be whatever the least complicated prokaryote is.
Do you mean simplest now, simplest ever, or simplest even theoretically possible? And in what environment? Early life was much simpler than even the simplest bacteria today, but it lived in an energy-rich primordial soup with no competition.
And we don’t know what the first thing was that was self-replicating, but the last I heard, the best guess was RNA molecules.
A lot of naive definitions of life are foiled by fire, which grows, feeds, reproduces, and maybe even evolves under some definition of that word. But it doesn’t undergo Darwinian evolution.
The definition also becomes problematic at the level of the individual organism, at least for multicellular organisms that don’t reproduce via parthenogenesis. Individual organisms don’t evolve, but we can all clearly be alive or dead. Cell lines that have become malignant can evolve, but of course they won’t survive the death of their host organism.
My WAG is that the answer was already mentioned, that of RNA. Some proteins might also be able to reproduce in an environment with no other life to compete with. However, even if they could, they lose out by definition since proteins are more complex than RNA.
What I had in mind were chemical reactions that are not commonly thought of as living such as, oh, a mixture of acidic and alkaline chemicals fighting it out in a beaker until one reigns supreme. Followed by an explanation about why whatever is being discussed is or isn’t Darwinian evolution.
Or maybe a forest fire vs a hurricane. Both are self-sustaining chemical systems. Or stalactites vs stalagmites in an environment of gravity. Or the formation of felsic, intermediate, mafic, and ultramafic igneous rock. Or the formation of various crystals, seeded by something something.
The question of what is or is not life is topical, but it isn’t really what I was aiming at. I was looking taking the definition as it was.
Chronos, FlikTheBlue: I see that RNA World is a thing - that would be a chemical system without cell membranes.
It seems like a useful distinction to me. A chemical rection can ‘evolve’. Change states, phases, etc. Darwinian evolution implies natural selection and the ability to pass traits down to newer generations. It’s not the only way things can evolve.
Fall 2023 big think paper in Proceedings of the National Academy of Sciences discusses Darwinian Evolution in nonliving entities. They claim to have discovered a law of increasing complexity in contexts such as minerals, hurricanes and stars.
Not sure I buy this, but I haven’t read the paper.
PNAS has a summary they call “Significance”, which might be characterized as the abstract of the abstract.
The universe is replete with complex evolving systems, but the existing macroscopic physical laws do not seem to adequately describe these systems. Recognizing that the identification of conceptual equivalencies among disparate phenomena were foundational to developing previous laws of nature, we approach a potential “missing law” by looking for equivalencies among evolving systems. We suggest that all evolving systems—including but not limited to life—are composed of diverse components that can combine into configurational states that are then selected for or against based on function. We then identify the fundamental sources of selection—static persistence, dynamic persistence, and novelty generation—and propose a time-asymmetric law that states that the functional information of a system will increase over time when subjected to selection for function(s).
If I understand the OP properly, Darwinian evolution involves reproduction with information transmission. That’s seems to me like a pretty substantial hurdle: it rules out fire, right? Unless perhaps there are multiple and competing and mutating kinds of rapid oxidation of fuel.
I don’t know whether Darwinian evolution can occur in a beaker or a vat with relatively simple non-organic chemicals.
Some limited set of animals could do that, but an awful lot depend on available food and other environmental conditions. Robots could conceptually build machines to produce parts from raw materials. They may depend on a symbiotic relationship with humans.
Absolutely. The notion of self-replicating machines suffers from teh same problem other complex phenomenon do: They look simple from a distance, but the more you dive into the problem, the more hidden complexity reveals itself.
We can’t make true self-replicating robots. We aren’t close to making self-replicating robots. We don’t even know if true self-replication of robots is feasible. It might take the equivalent of an entire Earth full of agents to be able to make a robot.
Making a self-replicating robot that can rocket itself to another star system and land on a random planet with enough resources to begin making another self-replicating robot, while surviving thousands or millions of years in space is pure science fiction. Maybe it’s possible, maybe not, but nothing about today’s tech informs us which is true.
Back to the OP: This is related to the ‘is-ought’ problem in philosophy. When is it reasonable to say that something ‘ought’ to do something? Not until you have the ability to make choices. So it’s crazy to say that a rock ‘ought’ to be in a different place, but fair to say that a paramecium ‘ought’ to move in a certain direction to find food. The distinction involves agency and choice.
It’s hard to see any kind of Darwinian evolution when the objects in question do not have agency. There is nothing to pass on to future generations of the thing. There are no good choices that enable survival or bad choices that lead to death. A rock or a chemical just IS.
So the answer to the OP is probably a virus. Even proteins just follow programmed moves.
This seems to be introducing a wholly unnecessary layer of philosophy to the question. Yes, one can say a rock just is, but if a rock were able to reproduce and pass traits to the copy, then it would satisfy the OP.
Your last paragraph seems to me to be a surprising nonsequitur to what came before. Why can one say that a rock just is, but can’t say that a virus just is? A protein indeed follows programmed moves, and so does a virus. A virus is nothing more than a strand of DNA or RNA. Are you saying that a single molecule has “agency” or makes choices?
The part I would quibble with with that definition of “life” is the “chemical system” part. I can envision something based on magnetohydrodynamics that “lives” in the interior of a star, or based on nuclear-physics processes that “lives” on the surface of a neutron star, or even based on gravitational fluctuations that “lives” in the vast voids of the cosmos.