Human Anatomy is relatively consistent - apart from things like dwarfism and other 'anatomical irregularities' - in the ratio of various parts of the body to each other.
Did you know that if any human (and they mostly male in previous times doing this!) who holds their arm straight out in front and closes one eye, will subtend precisely repeatable angles of arc measured in degrees by holding up a thumb or palm of hand and sighting both sides of this? There are thus ~ 360 degrees of arc - 360 thumbs - in a circle. It is suspected that this method was used to transcribe sky maps on to the walls of caves in France buried deep inside mountains. Hence the ancient allegedly magic/occult number 360!
This is not supposed to be where the phrase 'rule of thumb' arises...