Martial arts history is certainly very interesting, but personally I'm most fascinated by the art and science involved in martial arts, more concretely the aesthetics on one and efficiency on the other side.
As far as the history is concerned, I agree with Diogo. Generally, the "origin" of martial arts is closely related to "fighting" overall. Besides all the wars throughout history, fighting has always been a part of life in one way or another. People were fighting over the position of a leader, they were fighting over territories, women, etc.
Since you mentioned China, well, their approach to martial arts has been heavily influenced by Buddhism, which came originally from India, the same country where Kalaripayattu comes from. Before the development of more stylized techniques, traditional forms of wrestling and basic boxing were certainly the very foundation of everything.