Rojo, to directly answer your question as to how one might know that humans of the past had superior vision, one exmple of such evidence may be found in certain cave paintings such as Lascaux depicting stars and constellations that most people alive today would be unable to see with the naked eye even in perfectly darkened skies. From Wikepedia:
"In recent years, new research has suggested that the Lascaux paintings may incorporate prehistoric star charts. Michael Rappenglueck of the University of Munich argues that some of the non-figurative dot clusters and dots within some of the figurative images correlate with the constellations of Taurus, the Pleiades and the grouping known as the "Summer Triangle".