People often think that their existence begins simply with a birth on this planet, but the old Vedas explain that we also existed before, and according to that previous existence we get a new opportunity. Let's see what A. C. Bhaktivedanta Swami, one of the greatest authorities in that field, has to say on this topic.
Srila Prabhupada explains: "Through the corse of time child becomes a boy, the boy becomes a young man, the young man becomes an adult and the adult man becomes an old man. Through all that time, although the body is changing from the form of the child to the form of the old man, he is considered to be he same person with the same personality. The body is changing but the inhabitant of the body, the soul, stays the same. So we have to make a logical conclusion; when our body dies, we will get a new body. That is the process of the transition of the soul. There are 8 400 000 different types of bodies. Depending on one's desires and karma, living being gets appropriate body, according to the principles of nature.
During this life you associate with various modes of material nature and that association will decide what kind of body you will get in the next life. This develops strictly by the principles of nature. Everyome is under the control by principles of nature; they are completely dependent but because of an ignorance people think they are free. They are not free; they only imagine they are free, but they are completely under control by the principles of nature. Your actions, religious or irreligious, define what will your next life be like.
The process of the transition of soul is very subtle. Spiritual soul is invisible to our material eyes. It has the size of an atom. After a gross body that is made of senses, blood, bones, fat, etc. gets destroyed, a subtle body made of mind, intelligence and ego, continues to function. At the time of death subtle body transmits small spiritual soul into another gross body. The process can be compared to n air that carries over a smell. Nobody can see how it's happening, but still it's happening. Similarly, the process of the transition of soul is very subtle. According to the state of mind at the moment of death, very small spiritual soul enters in uterus of a mother with semen of a father and then develops specific form of a body that gives the mother. It can be a human being, a cat, a dog or anything else."