I just read a quotation from Imre Kertesz that linked Auschwitz to the "human degradation in modern experience." But the idea of genocide certainly predates modernity. I remembered the story of Joshua from Sunday School but wondered if there were older literary or historical references. "And they utterly destroyed all that was in the city, both man and woman, young and old, and ox, and sheep, and ass, with the edge of the sword." Joshua 6:21
While the concept of a genocide is at least that old, it's probably pertinent to point out that the execution of Jericho has no archaeological basis as far as we can determine, and this pretty much goes for most of the ANE accounts of 'genocide' or complete destruction. Mursilli II (14thC BCE), Ramses II (~1274) and his son, Mereneptah, the Moabite King Mesha (~830BCE), and Sennacherib (701-681BCE) all describe the complete and utter destruction of people/armies/areas, which are all untrue, and should be understood as what TL Thompson calls hagiographic hyperbole.
If you follow the traditional Joshua conquest date (1407BCE), then Thutmose III squeezes in before that by a few decades, as he stated on the Gebal Barkal Stela:
The great army of Mitanni,
it is overthrown in the twinkling of an eye.
It has perished completely
as though they had never existed
The problem being of course, that the Mitanni were very happily still going on 150 years later.