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What is surprising to me is not just that it's always DNA, but that it's always translated in the same way (codons to aminoacids, and the mapping function is fixed). Is there any simple reason for that?

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The simplest reason is that the ribosome evolved very early on (and is probably the best preserved remnant of whatever the earlier "RNA world" looked like) and it was always easier to leverage that machinery than to evolve another whole system from scratch. Once you have a world populated by any form of proto-life that can self-reproduce, even with crappy fidelity, it's basically game-over for any of the other semi-closed metabolic pathways that might have been competitors up until that point - they're now just food. There's no opportunity for another system to arise because all the raw materials are being scooped up by the new superpredator on the scene - the ribosome & whatever associated machinery it drags along. The competition now heats up between different ribosome-equipped complexes - improve metabolism, increase independence, increase reproduction rate, improve reproductive fidelity, etc.

Changing it makes the organism non-viable, so a change there never enters a population. In contract DNA/RNA-mutations can be viable so happen all the time.

There's a tiny bit of variation across the tree-of-life, see https://en.wikipedia.org/wiki/Genetic_code#Alternative_genet...

But the variation is in the one place you could possibly have it - by changing the interpretation of one or two of the three available STOP codons.

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