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In my world, I am attempting to design a primitive organism to be the ancestor of life on an artificial planet with all chemicals necessary for life. I want the organism to be as small as possible (say 50 nanometers in diameter). Is such a small size possible?

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It would be useful to have an idea of the reason that you want this ancestor organism to be so small. If it is to enable transportation from the planet of its manufacture to the artificial planet, bacterial spores are much smaller than bacteria. If instead the organism needs to be tiny throughout its lifecycle then this will affect the answers you get. – trichoplax Feb 23 at 3:39
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Definition of organism is rather fluid in biology (as is definition of "living"), so could you share your purpose? What they are supposed to do? – Mołot Feb 23 at 10:03
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50 nanometers is awfully small. In fact, this is on the same order as many viruses, which can be 20-300 nm in size (this figure seems, to a first order approximation, a good fit for Cort Ammon's estimate of 16-18 nm DNA strands). Some viruses are smaller along one dimension, but larger along others; Wikipedia mentions that some filoviruses can be 80x1400 nm in size.

In contrast, bacteria are usually in the 500-5000 nm (0.5-5.0 µm) size range. While the issue of whether a virus constitutes life is undecided, there is little doubt whether bacteria can be considered to be life.

Hence, the boundary for true life, at least in an Earth-like environment, appears to fall somewhere in the 300 to 500 nm range, which is about one order of magnitude larger than what you are aiming for.

It is possible that some extreme biology and evolutionary pressure might be able to support living organisms smaller than Earth bacteria, but I find it highly doubtful that it would be able to reduce the minimum viable size of an organism by such a large fraction. There is also no real evolutionary benefit that I can see to becoming that much smaller, hence very limited evolutionary pressure in that direction. If the limiting factor is indeed DNA base pair size and the number of nucleotides needed, then you may need some completely different biochemistry to make a significantly smaller organism viable.

At the very least, again to a first order approximation, any living organism must be large enough to support some form of procreation, even if that is done by cell division. Even cell division would appear to need the cell to be well above its minimum viable size before the division. Because they co-opt existing cells, viruses do not have that size constraint, and in fact would appear to have an evolutionary advantage to being smaller (less things to go wrong leading to higher probability of success).

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I'm extremely dubious of the notion that today's highly evolved life provides a useful model of the minimum size that life can operate at. It almost certain that the original forms of life on Earth were vastly smaller than today's lifeforms. – Jack Aidley Feb 24 at 10:47
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@JackAidley If you have any citations for such a claim, I would recommend that you add them to your answer and explain how they address the claims made in the answer. – Michael Kjörling Feb 24 at 11:01

There's some precedent for it. Spiegelman's Monster is a self-replicating strand of RNA, which they eventually got down to 48-54 nucleotides. I don't have the conversion from RNA nucleotides to length, but if I pretend that was 48-54 base pairs of DNA, we can use DNA's length of 340pm per base pair to arrive at a 16-18nm long strand.

It's an open question whether you can say much about our actual abiogenesis from studying Spiegelman's monster, but it's at least a point suggesting its not impossible.

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Spiegelman's Monster isn't really self-replicating, because it still needs an RNA replicase enzyme to do the job. It's just the shortest possible RNA strand that still interacts with the enzyme. You still need the replicase to kickstart abiogenesis. – Jan Dvorak Feb 23 at 11:56

That all depends on whether or not you consider viruses to be living things or not. Typically, viruses are far smaller than other living organisms. For example, Wikipedia tells us that

The smallest double stranded DNA viruses are the hepadnaviruses such as Hepatitis B, at 3.2 kb and 42 nm; parvoviruses have smaller capsids, at 18-26 nm, but larger genomes, at 5 kb.

If you don't consider viruses organisms, then the same Wikipedia page tells us

Prasinophyte algae of the genus Ostreococcus are the smallest free-living eukaryote. The single cell of an Ostreococcus measures only 0.8 μm across. The smallest genome of any Eukaryote is Guillardia theta with a genome size of only 551 Kilobases.

If you're looking for an actual animal-like organism, here you go (from the same Wikipedia article)

The smallest crustacean, and indeed the smallest arthropod is the tantulocarid Stygotantulus stocki, at a length of only 94 µm (0.0037 in).

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And also if, like a virus, the questioner's doohicky uses parasitism in order to manage to be so small, then it's going to need something larger to live on. I suppose the planetary engineers could drop two (or more) species: a small one that will eventually be the ancestor of all life, and a large one for it to initially parasitise. You'd have to somehow know how it will evolve, so that eventually the host is no longer needed and becomes extinct, otherwise the small parasite wouldn't be the ancestor of life, it'd be one of two. – Steve Jessop Feb 23 at 1:17
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Even single celled eukaryotes are considerably larger than bacteria or archaea, both of which are definitely alive. – trichoplax Feb 23 at 3:33
    
0.8 micrometers is 800 nanometers, which is 16x the OP's target of 50 nm. – Michael Kjörling Feb 23 at 8:20
    
"a small one that will eventually be the ancestor of all life, and a large one for it to initially parasitise." This reminds me of mitochondrias and chloroplasts. – Kiran Linsuain Feb 24 at 4:45

Absolutely.

Most of the size of organisms comes from their need to compete with other organisms and survive a range of hostile environments. Your ancestor, however, doesn't need to compete with anything but itself and can be seeded into the most welcoming of environments.

Evolution will handle the rest.

Consider this: the original forms of life on Earth must have been vastly smaller and simpler than today's life (due to the simple mathematics of what can plausibly have arisen by mere chance). In fact, they were likely a few molecules co-operating together (ref) and between there and modern life there must have been innumerable intermediate forms that transition the range of size between a handful of small molecules and the smallest extant cells. The minimum size of modern life is thus an extremely poor guide to the possible size of an artificially generated seed life.

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This seems too speculative. Even the source you cite acknowledges that it is speculation. – March Ho Feb 24 at 12:03
    
The exact mechanisms are highly speculative, yes, of course they are. We don't know how life on Earth got started but the underlying principles are more robust: life must have started from something simple enough to occur by mere chance and then accrued complexity and size. – Jack Aidley Feb 24 at 15:43

One famous example of a potentially biological organism which is smaller than any of the examples listed is surprisingly Martian in origin.

The meteorite ALH84001 is known for its fossils, which were thought by NASA scientists to be potentially fossilised Martian microorganisms.

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The long rod-shaped structures were analysed by electron microscopy, and they found internal structures that resembled terrestrial bacterial precipitates.

While it is disputed whether these are truly biological deposits as opposed to chemically or physically formed structures, if these objects are truly Martian microorganisms, their extremely small size (20-100nm diameter) potentially fits the range of sizes specified in the question.

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It's not very difficult for a lifeform to be thin if it is allowed to be long. Compare filoviruses which I do acknowledge in my answer. – Michael Kjörling Feb 24 at 12:53

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