Just a thought I had recently: People often seem to have one strict view about sects in early Buddhism, that either the Theravadins were the earliest and the Mahayanas were all corrupt, or the standard Mahayana mythology -- that Hinayana was the preliminary teaching and the higher Mahayana teachings were divinely revealed later.
These views seem to simplify Buddhist history as an interaction between the two sects of Theravada and Mahayana, but it seems likely to me to have been more complicated than that. The past importance (that is, the relevance or coherence to early Buddhism) of views held by now dead sects cannot be underestimated. Some sects now dead may have actually held views closer to early Buddhism than any living Buddhist sect today. Conversely, all living Buddhist sects today may hold views not held by early Buddhists. It seems at least historically significant that Pudgalavada (the idea of personhood, with personhood being notself) was a mainstream view at one point, while today, all Buddhist schools are "impersonalist" and reject that view. What I mean is, that's a very big change to happen -- it's impossible to suggest that Theravada is early Buddhism unchanged.
Also, the various splits may have happened for various reasons. Mahayana, for instance, may have had more than one origin -- perhaps some early Mahayanists were in fact forest monks (i.e. jhana monks), one theory of their origin, not necessarily being known as Mahayana at that time and possibly composed of various "schools" (among the early 18) and Theravadins or whatever they may have been called at that time, or whatever number, name, or sectarian division they might have been, may have been composed of scholastic monks. It seems reasonable to me that the context in which the dhamma was practiced -- the division between monks focusing on memorizing, maintaining, and teaching scripture vs. monks focusing on solitary meditation could lead to a division and social bigotry, by which you have one side of over-analytic dogmatists, while on the other side, you have blathering pseudo-nihilistic poets (see the list of the ten defilements of insight in the Theravadin commentaries and the 10 bhumis of Mahayana for examples of both).
And then, later, Nagarjuna, a scholastic comes along, and yet while he's said to have "founded" Mahayana, his most famous work, Fundamental Verses on the Middle Way, is analytical in nature and uses the Nikayas as source material (possibly everything he legitimately wrote is like this?), which is more similar to the Theravadin commentators than later Mahayana Buddhists who would write elaborate mythologies clearly diverging from early Buddhism. And then, later, or perhaps around this same time, there may have been the rise of the Mahayana practices associated with lay Buddhism -- the corruption -- the bodhisattva and buddha worship, mantras, astrology, the various superstitions... and all of this reached its peak when Tantra was introduced to Buddhism...
Now, this is all mere speculation. Somebody very knowledgeable of history here could maybe rip apart the above speculation. But the point I'm making is that it may have been much more complicated than we think -- accepting either the view that Theravada is early Buddhism in its pure form and Mahayana is simply corruption, or accepting the Mahayana mythology about the three turns of the dharma wheel. It is possible for competing theories about early Buddhism to both be wrong, because an alternative explanation or possible integration of both theories is not accounted for.
