While various scholars agree that the life of Śākyamuni Buddha lasted 80 years, several theories exist regarding the year of his birth and death. If we accept the theory based on the dates of the reign of King Aśoka, namely that the Buddha was born in the 460s BCE and lived to the 380s BCE, we conclude that the saṃgha, the monastic community founded by the Buddha, stayed unified for about 100 years after his death, without division into different schools. This period is referred to as Early Buddhism.

Eventually, this unification broke down due to differences in thought regarding the precepts of Buddhism and how to keep the rules the Buddha established. Two groups arose. The Theravāda, or Sthaviravāda, believed that the rules should be kept unchanging. The Mahāsāṃghika believed that rules should change in accordance with the times. This basic division of Buddhism occurred roughly 100 years after the death of Śākyamuni Buddha. After this division began, further division of both schools continued from BCE to CE, finally producing about 20 different schools. Nikāya Buddhism refers to the schools resulting from this first basic division and their subsequent divisions.

In the same BCE-CE period, Mahāyāna Buddhism arose. There had been a dominant theory that monks maintained Nikāya Buddhism, and lay believers led the Mahāyāna movement. Now there is another theory that Mahāyāna Buddhism arose out of Nikāya Buddhism. We cannot consider the two groups as simple rival religions. Even so, it is evident that Mahāyāna Buddhism and Nikāya Buddhism show clear differences in their beliefs. It can be said that these differences reflect the differences of the followers who practiced the beliefs of the different schools.