In this world we hear about two kinds of births. One is being born is human being and second is being born as an animal, bird, insect or tree etc. Human being are of three kinds, to be a Yogi or a scholarly person or ordinary person. If you have performed more good deeds than sins you will belong to the first two and if your sins and good deeds are even, you will live an average life. If your past life was mostly sinful, you will be born subhuman life forms to suffer. Because of these differences, people enjoy the rewards or punishments according to their Karmas.
(Yajur Veda Chapter 19 verse 47)