[arrismVam]

My thought is that the scope of Buddha's teaching is the reality of human existence, Dependent Origination and Sunyata are the 'grand unified theories'. 4 Noble Truth, anicca, anatta and others are specific theories for specific observations.

At the risk of going off a complete tangent, I think the interesting question here is why Buddha's early students can obtain enlightenment upon hearing just 2 sermons, and not whether or not 4 Noble Truth, anicca, anatta are the entirety of Buddha's teaching. Because, after all, people don't become enlightened after reading the first 2 sermons anymore.

What this tells me is that Buddha's early students had a lot more in common (culturally, educational, experiences, linguistically...etc) to Buddha than we do today. It is almost like they are all trying to solve the same equation as Buddha, but they all have a mistake in their solutions somewhere. Upon hearing Buddha's sermons, they all realized their mistakes, changed a plus sign to minus sign, and viola, solution solved and enlightened.

As students of Buddha becomes more and more different from him, more methods are needed to teach people with different dispositions, cultural background, language...etc, and we end up with the numbers of schools and traditions that we have today. Furthermore, casual observations tell us that these schools are mostly cultural-regional based. There are reasons that Theravada and Tibetan Buddhism are not popular with Chinese based Buddhists, but Chan and Pure Land and now Humanistic have great support. Or that western world seems to accept schools that focuses on meditation with more ease.

My personal speculation is that if Buddha awaken in U.S. today, his first sermon might not necessary be about the 4 Noble Truth.