Aryans migrated from Central Europe and settled in the very fertile Indus Valley. Aryans were very clever. They implemented division of labor in their society. The most clever members of society were selected and given the task to teach others. The bravest people were selected and given the task to protect society. The dullest types of individuals who were able to tolerate a heavy workload were selected and given the task of supplying labor to others whenever needed. The rest of the people of the society were given the tasks to grow food, produce materials, trade, and supply the goods to others.
As the Aryans were very clever, they found ways to keep the people doing what they had been doing. Thus, they bred four types of new generations. The ones considered most clever whose duty it was to teach were called "Brahmins". The protectors of society were called "Chetris". The group of producers and suppliers were called "Baishyas". And the group of laborers were called "Sudras".
In Hindu philosophy, every creature in this world is created by the God, Brahma. Brahmins are thought to be created from the mouth of Brahma to symbolize teacher groups. Chetris are thought to be created from the arms of Brahma to symbolize warriors. Baishyas are thought to be created from the lap of Brahma to symbolize a mother feeding her baby. And Sudras are thought to be created from the feet of Brahma to symbolize a weight bearer. Therefore, the Brahmins are the uppermost caste of society, followed by Chetris, Baishyas, and Sudras. The source for proof for this setup is the "Bhaagabad Geeta", one of the holy books of Hindu religion.
In Nepal however, there are truly only three categories of caste: Tagadhari (twice born), Matwali (liquor drinking) and Pani Nachalne. The Tagadharis include Nepalís highest castes: Brahmins, Thakuris, and Chetris. The mid-ranking Matwalis include most of Nepalís Tibeto-Burman and Indo-European tribal groups. The Pani Nachalne are untouchables associated with specific traditional occupations. These castes are ranked along an axiom of purity and pollution This caste system was codified in Nepal by the National Legal Code (Muluki Ain) of 1853 by Nepalís Rana rulers. It lasted until 1951 when the Ranas were overthrown. It heavily favored the Brahmins and brought ruling against them when violating caste rules.