The story
From the dialogue The Statesman by Plato (427–347 B.C.E.): “λέγω δὴ δεῖν τότε εὐθὺς τὸ πεζὸν τῷ δίποδι πρὸς τὸ τετράπουν γένος διανεῖμαι, κατιδόντα δὲ τἀνθρώπινον ἔτι μόνῳ τῷ πτηνῷ συνειληχὸς τὴν δίποδα ἀγέλην πάλιν τῷ ψιλῷ καὶ τῷ πτεροφυεῖ τέμνειν, […] [I say, then, that we ought at that time to have divided walking animals immediately into biped and quadruped, then seeing that the human race falls into the same division with the feathered creatures and no others, we must again divide the biped class into featherless and feathered, […]]”.
Etymology from Wiktionary, CC BY-SA 4.0.