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Comment by Enginerrrd | Hacker News Reader

If DNA is found, it almost certainly indicates a common ancestor. The likelihood of DNA evolving twice from whatever separate self-reproducing protolifeforms came before it has to be unbelievably low.

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It's more complicated than that. A few of the considerations:

* DNA/RNA are not the only options. Other nucleobases potentially exist and artificial ones have certainly been created (1)

* A completely unique sequence would indicate no shared ancestor even if it was DNA.

* We're not sure how life started on Earth either. It could've been more than once in more than one way. We don't know.

* Earth and Mars shared material as asteroid impacts ejected material that eventually landed on the other. Some microorganisms could potentially survive the journey.

(1) https://en.wikipedia.org/wiki/Nucleobase

Why is this DNA/RNA/polynucleotide chauvinism so prevalent? It's one complex molecule in an infinite space of complex molecules, some of which might be more suitable as basis of life on other planets with other chemistries. Life does not equal chain of heterocycle-sugar-phosphates.

Based on the current sample size of one life bearing planet, DNA/RNA is the most durable example we have that produces results at this time. As soon as a second nucleotide is discovered as functional, this will be up for discussion.

"Based on the current sample size of one writing system having civilization, hieroglyphs are the most durable example we have that produces results at this time. As soon as a second way of record keeping is discovered as functional, this will be up for discussion."

But we can infer that life could exist without DNA. It could be similar to our machines, made of different materials and replicating through other ways.

There is a definition of life that comprises any mechanism that controls its own entropy. Maybe there are energy-based interactions that fit this description like systems with no matter involved, or very high-scale/long-term systems.

Maybe I'm a DNA chauvinist, but I think this is one of the most important questions in the universe, but incredibly difficult to answer without massive-scale space travel.

We only have a single sample of a life encoding mechanism such as DNA. We can't know if it's one of many possibilities, or the naturally emergent phenomenon for life, like stars and quartz crystals that would form the same way anywhere from the same basic components of our physical universe.

Silcon is another possible mechanism similar to DNA. I forget the research on this.

EDIT: Found this https://en.wikipedia.org/wiki/Hypothetical_types_of_biochemi...

Sure, I wouldn't be surprised if it's possible, but the chances DNA emerging elsewhere aren't necessarily "unbelievable low".

The AI in this computer agrees ;D

Or this speculation:

https://en.wikipedia.org/wiki/The_Devil_in_the_Dark

Silicon is a post-ferrous element. There may be silicon-based life in the universe. But abundance of carbon and hydrogen, together with the superior stability of hydrocarbons relative to silanes, sort of seals the deal for everything but theoretical edge cases.

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