Reasoning of the idea: The Incas believed that life originated from Pacha Mama, the mother earth. That's quite true, according the Christian's belief, men and women were made of earth. Even scientifically, life was formed because of some in-organic materials (e.g. soil) interacting with lightening. It made sense to them that if we take from mother earth, we would need to return something back to mother earth. That's why some human sacrificed or mummies (naturally dead) was wrapped in a fetus position (you go back to the same way you came from).