The earliest sure witness, according to Peter Wiseman's Remus: A Roman Myth (Cambridge, 1995), pp. 65-71, is an Etruscan mirror dating to ca. 350-325 BCE that depicts a wolf suckling the twins, along with several other figures that can't be reliably identified. There are a couple of fifth-century-BCE objects depicting animals suckling babies, one Etruscan and one Latin, which might have something to do with the Romulus/Remus story but we can't be confident of that.

The earliest textual witnesses are

• Alkimos of Sicily, ca. 370-280 BCE?, who is reported to have stated that Romulus (or in Greek, Rhomylos) was the son of Aeneas and Tyrrhenia (thus making him half-Etruscan), and that Romulus' grandson Rhodius founded Rome (FGrHist 560 F 4);

• Kallias of Syracuse, ca. 350-270 BCE?, who is reported to have stated that a Trojan woman Rhome married king Latinus and had three children: Rhomos, Rhomylos, and Telegonos (FGrHist 564 F 5a).

Obviously the early versions are very different from the one that's familiar from Livy and the Origo gentis Romanae. There are many variants that vary wildly on the number of brothers and their parentage. In the earliest versions, the twins' parents are usually either Aeneas himself, or a Trojan woman named Rhome. It's likely that changing attitudes to Greek-Roman relations in the Pyrrhic War (280-275 BCE) caused drastic changes to the ethnic composition of the legend, so Roman versions after that date probably looked a little bit more like the more familiar legend.

All of this is unknown. The story predates any remaining recorded version of it.

Diocles of Peparethus apparently included the story in a history he wrote in the early 3rd century BCE. But that history is lost to us. Although both Herodotus and Quintus Fabius Pictor cite Diocles as a source we don't know how closely they quoted him.

But the story was also well enough known in the mid 3rd century that it was included on Roman coins. So it seems likely that the story goes back at least to the early 3rd century. And it is likely that Diocles was simply writing down what was by then a well known tradition.