Hi, You've asked a question which could spur pages and pages of debates, multiple threads, even entire books...I'd say this is one of the two or three hottest points of contention on Buddhist discussion boards, along with karma and rebirth and a couple of others. It's a little hard to come up with a definite answer because the origins of Mahayana remain mysterious to a great degree. From what I've seen and read, my own take at the moment is that Mahayana teachings fall into four basic categories, as follows: a) Some are extensions of material found in the Nikayas (Pali Canon). For example, some scholars have noted a link between Madhyamaka (Nagarjuna's "Middle Way") and certain texts in the Sutta Nipata, which may be a very early part of the Canon. The devotional aspect of Mahayana can be traced to these and other suttas as well, and to stories found in the Jakata tales. b) Some are the result of deep meditation (samadhi) experiences during which the meditators received -- or believed they received -- teachings from Buddha. These visions were then transmitted and eventually written down in the form of new sutras. c) Some are the result of doctrinal debates during the first centuries of the common era (CE). For example, some consider parts of the Heart Sutra to be a response to a school known as the Sarvastivada. Likewise, the doctrine of "alaya vijnana" (storehouse consciousness) originated with a school known as Yogacara -- it was put forward as a solution to a logical problem found in the Abidharma. d) Some reflect developments that occurred within a particular cultural context as Buddhism was introduced. Certain aspects of Chinese Buddhism, for instance, are distinctive to China. Japanese, Korean and Vietnamese Buddhism basically originate with the Chinese model, so you will see the differences transmitted to those countries as well. My understanding of the above points is far from perfect, though...you may want to do some reading around. "Mahayana Buddhism: the doctrinal foundations" by Paul Williams is not a bad place to start.