Did you ever wonder where mixed martial arts came from? I don't think anyone really knows the answer, but I do have a few thoughts on the subject.
A modern popular references was in "Karate Kid II," when Mr. Myiagi taught Daniel a technique famous in the Miyagi family in Okinawa. When Daniel asked where the technique originated, Mr. Miyagi said his father took a trip and got lost in China.
This is a fictitious example, of course, but now we can take a look at one example that is real, so we can see how martial arts change and adapt.
The first person who made mixed martial arts popular was Bruce Lee. As we all know, Bruce Lee got his start in martial arts with Yip Man, who was a wing chun master. He took much of his jeet kun do footwork from Muhammad Ali (who, by the way, Bruce Lee held in high regard as the best fighter of the time).
Because Bruce Lee's fighting theories were based on many martial arts, so became the realization that to be a complete fighter, a student could stray from a single traditional style of fighting.
About the same time as Bruce Lee was beginning to get popular in the United States, martial arts tournaments also started to evolve. Tournaments started with instructors of the same style of martial art who were familiar with each other. Soon after this, tournaments in the 1970s became open invitation to all styles — however, the styles were not yet mixed. (Interestingly enough, the art that was winning most of the tournaments at the time was kenpo karate, which is Chinese in origin.)
Then came the ultimate fighting championships and the mixed martial arts tournaments. the rules were changed here to allow more ways to collect points and defeat opponents. For example (and without getting into too much detail), arm locks and throws were not allowed in karate tournaments. Punches and kicks were not allowed in judo tournaments. Now, however, these ultimate fighting championships and mixed martial arts tournaments allow it all.