Simes generally accepted that both Simes and Gens were mutants. Juncts used this to prove that Gens weren't human. Householdings said that it took a Sime and a Gen together to be the equivalent of an Ancient. Ancients LOOKED exactly like Gens.
It couldn't be proved whether or not Ancients produced selyn -- Simes had a vague tradition that in times of chaos there were a few people who looked like Gens but who didn't have any selyn field potential.
The Ancients left behind many books.
(Source: Index card fiile -- SIME SURGEON -- http://www.simegen.com/sgfandom/rimonslibrary/surgeon/SURGEON1.html -- An early draft of Unto Zeor, Forever) Neither Simes nor Gens. Orim Farris thought they were a little of both. Produced selyn and used it themselves.