If these stories could be taken to be truly narrated by the Buddha, it gives some interesting twists to the generally understood history of India and Indian philosophy.
For example, there are Jataka stories such as Dasaratha Jataka and Ghaṭapaṇṭita jataka that give the Buddha's previous birth as either a character in the popular epic Ramayana, or as kin of a character in the popular epic Mahabharata. There are also other Jatakas that narrates some of the other characters of Mahabharata, but in all such cases, with much less dramatic than their depictions in the epics.
As of now, the general Indian belief is that the epics of Ramayana and Mahabharata (and thus the philosophies depicted in them) preceded the Buddha. However, in the Suttas there is hardly any reference to them, unlike the references to many other schools of thoughts. Thus, I would assume these epics were non-existent at the Buddha's time. In my knowledge, the first reference in the Buddhist literature to these epics in its present form is in Buddhacarita, the biography of the Buddha written by Asvagosha in the 1st century CE. In that Asvagosha presumes that these epics were fully developed before the Buddha's time, and hence bring references to those in conversations people had with the Buddha. So, I was concluding that these epics would have formed sometime after the Buddha, but before the 1st Century CE. And, Asvagosha's references to Ramayana and Mahabharata are more in line with its present day form than its depictions in the Jatakas. (And, I would assume Asvagosha's narrations may be fictitious)
If Jataka commentaries can be taken as being based on the narrations passed down from the Buddha, rudimentary versions of these epics are coming from the Buddha's narrations. They come as isolated accounts, not connected together as a grand epic. They stories also lack the dramatic turns and glorification of characters that are found in those epics. In addition, the character depictions in the Jataka are inconsistent with the depictions in the epics known at Asvagosha's time.
So, I was thinking about two possibilities. 1. If the commentaries are regarded as completely based on the narrations given by the Buddha, then the epics developed later by deriving inspirations from Jataka stories. In other words, the glorious characters of these epics were created by altering the original narrations by the Buddha. (I tend to believe this. However, I need to make sure that the Jataka commentaries are authentic and in existence at least at the time of the first council to assert this.) 2. Alternatively, these stories were inserted later when the epics of Ramayana and Mahabharata became popular. In that case, characters from the epics were adopted to convey a message. I believe this is unlikely, because Jataka depictions of these characters are much less attractive and plain compared to their depictions in the epics. And, the rest of the Pali Suttas are matter of fact depictions and not later concoctions. So, I prefer to assume that Pali Jatakas also hold the same authenticity as the rest of Pali Suttas.