From my experience, asanas seem to be a manifestation from the nervous system to release blockages and tensions, allowing for much deeper meditative states. During my DM practice, I find the body automatically moving into versions of the more common asanas as a way to help along energy flow…spinal twists, backbends etc.
Isn’t it likely that asanas evolved from early practitioners working on their spiritual practices and observing the nervous system’s natural tendencies to adjust the physical form? Then teaching these to others as the poses/exercises that help release and refine the attention?
Most likely, Mr Zuck! 