PASADENA, Calif. — It was barely two hours into Day 1 of AlienCon and 500 years of accepted history and science were already being tossed out. Three thousand people had gathered inside the Civic Auditorium here for a panel discussion featuring presenters from “Ancient Aliens,” a History Channel documentary series.

Everyone had questions: about whether we were alone in the universe; about what our government really knows; about humanity’s very origins.

One of the network’s most popular and longest-running shows (Season 13 resumed on July 20), “Ancient Aliens” is itself a series of questions. Many are posed rhetorically by an unseen narrator intoning over a wide shot of a rubbly archaeological site. According to the show’s talking heads, extraterrestrials may have had a role not only in the extermination of the dinosaurs, but also in the construction of the Egyptian pyramids.

Carl Sagan, the popular scientist who captivated television audiences of the 1970s and ’80s, once said: “Extraordinary claims require extraordinary evidence.”