The Buddha taught for 45 years. But he did not teach “Buddhism”, he taught dharma to monks and nuns, lay men and women, kings and beggars.
This was the Iron Age and he started his teachings within the existing framework of understanding. He was born among Indian people who already accepted beginningless lives, karma, the 4 Brahmaviharas, rebirth and so on. But he tweaked and reshaped those existing understandings until they were something else.
There was no Buddhism For Dummies, no A to Z Buddhism. He didn’t teach the Lam Rim but he taught everything that is in the Lam Rim.
Thousands of years later, every Buddhist school of thought accepts that the 4 Noble Truths is one of his fundamental teachings. They are repeatedly emphasized by the Buddha in different Sutta contexts.
It is reasonable to think these fundamental teachings must have been repeated many times to new people who came to hear the Buddha’s message, but were not recorded every time they were taught. The same situation must apply many other teachings, so the question must have arisen again and again, how does an uninstructed person practise the Buddha’s teaching in a progressive manner?.
By the 10th century in India, teachers had responded to the need for a step-by-step orderly presentation of the Buddha’s teachings.
When he went to Tibet, Atisha took the Indian Kadampa tradition, which emphasized a gradual path to enlightenment. His principal work, “The Lamp for the Path to Enlightenment” (Bodhipathapradipa), served as a foundational text for the development of the Lamrim in Tibet.
The Lamrim has developed into a distinct and comprehensive presentation of the Budda’s teachings as conveyed by Indian masters and further elaborated by Tibetan scholars and practitioners.