- We don’t really know of many groups of yogis in the Buddha’s time. Alara and Uddaka don’t seem to have been part of the samaṇa movement. The structure of the bodhisatta’s practice pre-awakening seems designed to show how he practiced to the utmost of what was available at the time. Clearly the austerities are Jain-like, so by elimination, Upanishadic yogis are the most likely.
- At least one of the key contexts, MN 26, is set in a brahmin’s hermitage.
- “Rāmaputta” sounds very much like a brahmin name. Uddaka (= Udraka) is the name of a brahmanical rishi.
- Uddaka’s saying criticized by the Buddha in DN 29, “one sees but does not see”, referring to a razor’s edge, is reminiscent of Upanishadic style teachings about the imminent Self; for example, Uddālaka’s teaching on the split banyan seed in the Chandogya.
- The students of Alara and Uddaka began by memorizing the texts (oṭṭhapahatamattena lapitalāpanamattena). We don’t have any evidence for any religious texts other than the Brahmanical at this time. (The Jains and others may well have had texts, but we have no evidence for it.)
- The students were practicing within a lineage or tradition. We have reference to theravāda, sakaṃ ācariyakaṃ, as well as the detail that Rāmaputta is following in the footsteps of Rāma, his (spiritual or biological) father. Most of the samana movements claimed, like the Buddha’s, to have been established by their founders (Jainism being an exception.)
- The students learned the five faculties, a set of dhammas that have many connections with things in the Upanishads, and which are featured prominently in the (admittedly later and philosophically divergent) Yogasutra (śraddhāvīryasmṛtisamādhiprajñāpūrvaka itareṣām || YS_1.20 ||)
- Having mastered the five faculties, including jhana under samādhi, the highest teachings are the arupas. These have many affinities with Upanishadic teachings. And elsewhere, advanced brahmin yogis are cloesely associated with these, especially in the Parayanavagga.
- The Pali commentary seems to assume they were brahmins. I haven’t looked into this with any detail, but a quick glance at the commentary to MN 26 shows that they depict Alara as referring to the Marks of a Great Man, which of course was regarded as a brahmanical idea.
As so often in these studies, no single criteria is decisive. But multiple independent criteria are all easily explained by a single, simple, and obvious hypothesis. Since there is, so far as I know, no counter-evidence or convincing alternative hypothesis, I regard this as probably the correct explanation.