The gladius has an interesting history with a lot of holes in it. Polybius says the Romans stole the design from the Spaniards. We have evidence of its use circa 380 B.C. and we know it was replaced by 200 A.D, but in between those dates the it has to be considered the most important weapon of the Roman Army.
Our story begins with the kings of Rome, most notably Severus Tullius, the fifth of the six, who reigned from 579-535 B.C. Roman folklore says that Severus built the first Roman army. Livy, who spun folklore into history, says the first three classes of soldiers in the Severan army carried swords. The fact that the other classes did not implies that they were not standard equipment until later. What this early sword looked like was unknown, although we have many sword types depicted on Etruscan pottery, so we might assume it was derived from those.