This timeline of military history, taken primarily from George C Kohn's "Dictionary of Wars," indicates that from roughly 2925 B.C. to, oh, now, an unbroken period of hostility between one group of people and another has existed.
A Short History of War, published by the U.S. Army War College, actually dates the advent of societal warfare back to 4000 B.C. Prior to that, "...warfare itself had not in any meaningful sense been invented.
There were only the embryonic beginnings of a warrior class still loosely embedded in a tribal social structure that lacked both the physical and psychological requirements to produce war on any scale."
So do the 5000s B.C. and before represent a pax pre-historica? Maybe, maybe not. According to Wikipedia, "The beginning of prehistoric wars is a disputed issue between anthropologists and historians." The controversy includes purported archaeological evidence of a battle on the Nile that some sources date as far back as 12,000 B.C. 59 bodies were found at that site; compare that to the well over 100 million deaths attributed to war in the 20th century, and the pre-historic era does start to sound like the good old days. Perhaps this century will be the one to reverse the trend.