Himanshi Dhawan

“In the Brhadāraṇyaka Upaniṣad7 the first form of the doctrine of transmigration is given. The souls of those who have lived lives of sacrifice, charity and austerity, after certain obscure peregrinations, pass to the World of the Fathers, the paradise of Yama; thence, after a period of bliss, they go to the moon; from the moon they go to empty space, whence they pass to the air, and descend to earth in the rain. There they “become food,… and are offered again in the altar fire which is man, to be born again in the fire of woman”, while the unrighteous are reincarnated as worms, birds or insects. This doctrine, which seems to rest on a primitive belief that conception occurred through the eating by one of the parents of a fruit or vegetable containing the latent soul of the offspring, is put forward as a rare and new one, and was not universally held at the time of the composition of the Upaniṣad. Even in the days of the Buddha, transmigration may not have been believed in by everyone, but it seems to have gained ground very rapidly in the 7th and 6th centuries B.C. Thus the magnificently logical Indian doctrines of saṃsāra, or transmigration, and karma, the result of the deeds of one life affecting the next, had humble beginnings in a soul theory of quite primitive type; but even at this early period they had an ethical content, and had attained some degree of elaboration. In”
― A.L. Basham, The Wonder That Was India: A Survey of the Culture of the Indian Sub-Continent Before the Coming of the Muslims