The history of Pankration and the other heavy athletics, Wrestling and Boxing, goes back in time to our earliest histories of Greece and Rome and even back to the history of Ancient Egypt. A collection of facts clearly outlined in the most excellent work The Martial Arts of Ancient Greece by Kostas Dervenis and Nektarios Lykiardopoulos. An even more academic viewpoint on the heavy athletics is given in Michael Poliakoff''s book Combat Sports of the Ancient World which I consider the most important work on the history of Greek and Roman martial arts in the ancient world from the earliest days of Greece and Roman back into the days of Ancient Egypt and the Minoan civilization and back into antiquity.
These historical sources are important but not as important as the application of these arts, for if we wish to reconstruct these arts they must be living arts of use to modern humans. Of course without the rise of modern MMA we would not have the all important perspective and training ground of true all powers fighting sports to test our theories and suppositions. If we accept the history, heavy athletics of the ancient world were around as prime entertainment for about 1000 years until their dismantling as pagan idolatry by Emperor Theodosius