The tradition's own account that Parshvanatha taught four vows later expanded to five by Mahavira rests on treating Parshvanatha as a real historical predecessor rather than a legendary figure, and this is one of the relatively rare points in ancient Indian religious history where mainstream scholarship extends qualified credence to a traditional claim of this kind.
Jain tradition places Parshvanatha, the twenty-third Tirthankara, roughly two hundred fifty years before Mahavira, the twenty-fourth and last, giving him a traditional date in the ninth century BCE, earlier than the sixth or fifth-century BCE dates modern historians generally assign to Mahavira from comparative chronology with early Buddhist sources. A number of historians of ancient India, including several who otherwise treat Jain hagiography with caution, judge Parshvanatha more likely than not to have been a real teacher who led an ascetic community distinct from Mahavira's, partly because Jain and Buddhist texts both mention followers of Parshvanatha as an already-existing group at the time of Mahavira's own activity, including an episode where Mahavira's own parents are described as adherents of Parshvanatha's community before their son's teaching began, a detail scholars regard as unlikely to have been invented since it subordinates Mahavira's novelty rather than enhancing it.
If this reading is correct, the difference between four vows and five becomes evidence of doctrinal development within a continuous ascetic lineage rather than legendary elaboration, which is why historians treat the Parshvanatha material more seriously than comparably early material in several other South Asian traditions, where no comparable external corroboration exists.