Outside the Western scheme the thumb is not read, it is used, and the two great elemental hand systems of Asia disagree about what it stands for. In the esoteric Buddhism recorded in Japanese sculpture the five fingers are the five elements counted inward from the little finger: earth, water, fire, wind, and at the thumb space or the void, ku, the element that contains the rest. In the yogic hasta mudras the assignment runs differently and the thumb is fire, agni, the element the other four are brought to touch. The two schemes agree about exactly one digit, the index, and the thumb is the plainest of their disagreements: the container in one reckoning, the igniter in the other.
Both readings lean on something the hand actually does. The thumb is the only digit that opposes, swinging across the palm to meet each fingertip in turn on a saddle joint at its base that no other digit has, and it is the only one with two bones where the rest have three. A gesture that joins thumb to fingertip is therefore available in a way that no finger-to-finger contact is, which is why the seal formed by touching thumb to index is the commonest in the whole repertoire, and why both elemental schemes make the thumb the fixed term and the fingers the variable one. The anatomy does not confirm the symbolism. It does explain why the symbolism was built around this digit rather than another.
Sources Mudra: A Study of Symbolic Gestures in Japanese Buddhist SculptureE. Dale Saunders with Asana Pranayama Mudra BandhaSwami Satyananda Saraswati