If Buddhist concepts are a revision of older Hindu ideas, then the idea of prana > chi > ki would therefore be traced back to India's prehistory. However the idea of a life force can be seen in western culture. Pagans, Gnostic, New Age and alternative thinkers like Wilhelm Reich or T. Galen Hieronymus have all used a concept like the life force in their theories. You can also see this concept being used by both past and present-day shamans.
The connection between life,death and breath giving a conclusion of some kind of life force is fairly evident to a casual observer. You run out of other options, especially if you aren't "distracted" by the theories of modern medicine, like ancient man was. I don't believe any one culture can claim having "discovered" this concept, since so many people across so many centuries have re-discovered, re-tuned and re-invented this idea.
I don't know the mechanics of unbendable arm, since I'm not a physiologist. You might be better asking Craig about this. As for the human bridge, that was explained in Ki in Daily Life
where K. Tohei is photographed doing it.
Speaking of Chi, I'm looking for anything about the origins of Tai Chi and Qi Gong with anything about the development of Qi Gong. Please email me if you have anything.