“One who entertains desires, thinking of them is born again here and there on account of those desires”. (Mundukya Upanishad 3:2:2)
The fundamental doctrine of Sanatana Dharma and its offshoots — Buddhism, Sikhism and Jainism is the doctrine of reincarnation or transmigration. This process is called Samsara which translates as “the ocean of relative existence” or the “flow of life”. It is the process of the jiva’s involution back to Brahman — its true state. The general paradigm of this doctrine is as follows:
The Darwinian concept of evolution of the species, where one form of life evolves into another ever more complex form, may be appropriate to the physical body, but Hinduism applies a radically opposite theory to the evolution of consciousness.
The consciousness which is perfect, eternal and blissful “devolves” into the lowest species becoming more and more gross, and then begins a process of involution towards it’s natural and essential state of being known as sat chit ananda — existence, consciousness and bliss absolute . It passes through various lower life forms until the attainment of the human state which is the highest and most desirable birth in the Universe. Birth in the heavenly realms or as gods (Devas) is not desirable because it is only in the human form that one can work out one’s Karma and can achieve liberation through Self‐realisation.
There are some schools and gurus which teach that retrogressive rebirth as an animal or a plant is an option, but this is doubtful due to the fact that animals are not bound by the moral law of Karma and Karma is the driving force of rebirth.