The first life form may have been RNA. verything after that point may be up for grabs–even the structure of DNA. Synthetic biologists have created DNA chains that function, but are made of different sugars and acids than the stuff in our cells. It’s even possible that DNA-like molecules could work in ammonia, not water. The number of basic amino acids used to make proteins may also be flexible–just because we’re stuck with 20 doesn’t mean that’s how it has to be. From there on out, there’s no reason tons of things couldn’t have happened differently.
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