Atman is termed soul in English. An atman is the spirit existing in our heart. As detailed in Bhagavad Gita we are primarily a spiritual traveler, a consciousness manifesting human form.
All atmans souls are indestructible, can never be created or destroyed. All atmans, the spirit exists from times immemorial. An atman is integral path of Parmatman, Brahman God Almighty.
If an individual grain of sand is an independent atman soul, the whole mound Parmatman, Brahman. If an individual piece of puzzle is an independent atman soul, the completed puzzle Brahman, God Almighty.
In fact Brahman, God existed in all living beings including human beings in miniscule form as our atman existing in our heart.
As an atman could not purify itself of its own it needs a medium, a body to work out its karma, remove dross impurities within. An atman manifested human form to reach state of absolute purity. Indulging in spirituality when human beings reached state of enlightenment, it announced end of cosmic journey for atman, the spirit within.