So where are the essential amino acids actually produced?
And to follow on to that how could life get started without being able to produce amino acids? It doesn’t seem like something early life could evolve gradually. Are they then naturally occurring?
“Essential” amino acids are those that humans can’t synthesise. Other life forms can, which is why we ingest them with our food.
As for how early life started, presumably with a strongly reduced set of amino acids. Some of these are simple enough chemicals which form spontaneously given the right conditions (technically all of them can, but probably at very low rates). This was famous established by the Miller–Urey experiment (https://en.wikipedia.org/wiki/Miller–Urey_experiment).
And to follow on to that how could life get started without being able to produce amino acids? It doesn’t seem like something early life could evolve gradually. Are they then naturally occurring?