The doctrine of reincarnation evolved from Hinduism. It was given to mankind by Hinduism religion. To understand doctrine of reincarnation in totality we need to understand the basics of Hinduism like:
Origination of cosmos
As detailed in Upanishads (independent spiritual treatises) the present cosmos formed when God Almighty, Brahman exploded self with a big bang. This resulted in all pure souls atmans scattering all over cosmos at unimaginable speeds. In the melee, the pure souls’ atmans gathered impurities. To cleanse self of dross impurities within starts cosmic life cycle of every soul atman.
Life journey of a soul atman
No sooner life supporting planets like mother earth evolved in cosmic system souls’ atmans immediately started manifesting life forms. The first manifestation by a soul atman is form of amoeba (single cell formation). It further evolves into multi-cell formation, insect life, plant life, animal life and finally the human form.
All human beings on mother earth evolved directly from stage of animals. There is no ambiguity in this statement per se Hinduism scriptures like Bhagavad Gita and Upanishads.
Having manifested a human form after death of present body, the released soul atman immediately manifests a new body to start a new journey of life on mother earth. This new manifestation by soul atman is termed reincarnation, rebirth.
For souls to evolve, cleanse themselves of dross impurities within they need manifesting a chain of 7.3 million bodies in insect, plant and animal life and 1.1 million manifestations in human form. No sooner death of body occurs on mother earth weather insect or human being, the released soul atman immediately manifests a new body.
This reincarnation by a soul atman is directly proportional to residual balance of karma at the time of death of earlier body. Based on residual balance of karma at the time of death, the soul atman manifests the new body. It is the child that chooses its parents and not vice versa.
Per se Upanishads if matching parents are not available on mother earth, the soul atman hibernates in heaven or hell… in heaven (swarga in Hinduism) if the residual balance of karma at the time of death of body is positive and hell, if negative. Both heaven and hell (swarga and naraka) existing in sun. Heaven in the core of sun where temperatures exceed millions of degrees centigrade, it is in such high temperatures a soul atman feels comfortable. Hell exists in the periphery of sun where temperatures are considerably less and in such low temperatures a soul atman suffers for eternity.
The cycle of birth and death, reincarnation was part of cosmic design ordained by god. In absence of karma, doctrine of reincarnation life in entire cosmos would become stagnant, decay and die.
For cosmos to sustain, for life to evolve in cosmic system doctrine of reincarnation is necessitated.
The cycle of birth and death for the manifest form in the life of a soul atman comes to an end when human beings indulging in Indian spirituality reached god, realized god in their lifetime.
Importance of Bhagavad Gita to reach state of self realization
For human beings to gain emancipation from cycle of birth and death forever, indulgence in path of spirituality as ordained in Bhagavad Gita is an absolute must. To realize self, realize god the following needs to be done:
1. We have to demolish our ego in totality
2. We have to overpower five senses and mind forever.
3. Practicing absolute celibacy for a minimum period of 12 years in continuation we have to awaken our kundalini shakti fully.
Full awakening of kundalini shakti results in opening of Sahasra Pushpa (thousand petalled Lotus) existing in our brain behind forehead. One immediately becomes an enlightened one forever. The cosmic journey for a soul atman finally comes to an end. Having reached state of absolute purity, the need for a soul atman to manifest a body any further ceases to exist! For a pure soul atman, the need for reincarnation finally ceases.
The pure soul atman having liberated self from cycle of birth and death forever, the moment an enlightened one leaves mortal frame reaches state of moksha salvation, enters Vaikuntha (kingdom of God).