An essence can make use of a physical body on this planet, to move forward on its path. At first it remains as an unconscious form in the body of an animal a great many lives and this way it can get used to growth and to the developments that are attached to the life forms on this planet.
It gains all kinds of experiences and gradually it comes to a point where it can slide into a human form.
With the help of a Teacher or a spiritual helper the essence chooses a body with a particular thinking hat, a particular atmosphere and social environment where it can learn its lessons.
It will connect to a physical body, because its lessons can only be learned through experience, and experiences are related to emotions and feelings associated with being human on this planet.