Being some of the oldest religions known to date is not the only thing Hinduism and Buddhism have in common. They are also similar because they both started in India, and because they believe in rebirth. From the beginning of time Hinduism and Buddhism have been some of the most supported religions with millions of followers. Hinduism and Buddhism are different because they have different Holy Books and different restrictions, but are similar because they both believe in reincarnation and polytheistic religions.
One of Hinduism and Buddhism's main commonalities is their place of origin. They both originated in the country of India. However Hinduism did not have a specific founder, the culture of Hinduism flourished throughout India which then took the form of a religion. On the flip side, Buddhism was founded by a man named Gautama Buddha also known as Siddhartha. Another main commonality is their belief in the cycle of life and reincarnation.
According to Hindu beliefs, a person has to undergo multiple lives until they are truly perfect and achieve Enlightenment and Nirvana. The Hinduism reincarnation system tends to be more complicated then the Buddhist system. Dharma, Karma, Reincarnation, repeat. That is the path in which reincarnation happens. Dharma is what determines your karma. If you perform your duties to the best of your ability, you will have good karma. If you have good karma then you will move on to your next life. Then everything is repeated to start another life. A caste system is what determines your duty in that current life. One last similarity between Hinduism and Buddhism is that both religions are polytheistic. Hindus believe in Shiva, Vishnu, and Brahma. Buddhism has multiple deities as well, however since the Buddhist religion does not strictly revolve around the distinct worship of certain Gods,