There are a bunch of characteristics that, while fairly arbitrary, can't be changed once they are locked in. The mappings from RNA to proteins are a good example. Changing it is instantly lethal to the cell involved. Others include the chirality of amino acids, and the choice of bases for DNA.

If we look at the entire tree of life, we see no deviations in these deep characteristics. This implies that they were fixed before we all split. There might have been alternative variations in the past, but none have survived to the modern day (that we have identified).

This goes double of eucaryotes (basically everything other than bacteria). The design is so unique that convergent evolution is highly unlikely to stumble into the exact same layout from 2 sources.