evidence: Ancient North Americans gouged elaborate rock art into a heap of big boulders northeast of Reno, Nev., more than 10,000 years ago and perhaps 15,000 years ago. That makes the carvings the oldest known petroglyphs on the continent, according to a paper published in the Journal of Archaeological Science. [...] "I think it's really amazing that people that far back were creating such wonderful things," Benson says. [...] The new discovery shows that some of the earliest North Americans were surprisingly creative artists.