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.
As gathered from remnants of Ancient statues, Ancients appeared 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 Of course, not many printed books survived, and electronic media disappeared without a trace except for corroded hardware.