When, exactly, was disco born? Was it when Billboard magazine began tabulating a chart devoted specifically to dance music in 1974? Or is that the tail wagging the dog, since armies of club DJs had been playing dance-conducive rock and pop music in discotheques since the '60s? And to put a finer point on it, what was actually the first disco song? We know all the big artists and hits of the original disco era, from The Bee Gees' "Stayin' Alive" to Donna Summer's "Hot Stuff." But where did that sound come from: those sweeping strings, the lush funkiness, the hi-hat-heavy beat?