Nobody knows what the earliest word used by humans was. The general consensus is the Sumerians developed the first written language around 3500 BC, using wedge-shaped symbols called cuneiform. Scholars hypothesize that the first word was either "ouch" or "me," although "pie" had to have been up there since the shapes surely reminded the Sumerians of it.
For the last 5,500 years humans have spent l...