Judging from evidence from recent hunter-gatherers, mortal combat would sometimes have occurred between different groups in the hunter-gatherer phase of human history, However, it would not have been a constant feature of hunter-gatherer society. Many groups would have lived at peace with their neighbours for long periods.
Archaeological research suggests that the very early farmers of the valleys of the Tigris and Euphrates Rivers lived at peace with their neighbours, as did the early farmers of central Europe. On the other hand, there is plenty of evidence of brutal warfare in Europe later in the Neolithic period. The mass grave containing the remains of 34 individuals at Talheim Death Pit, near Heilbronn in Germany, is an example. Around 5000 years ago farming people in the south of England built and attacked fortified settlements.
In Mesopotamia warfare became the norm soon after the establishment of the first cities, around 5000 years ago. People were immersed in cultural systems that glorified the military exploits of their forefathers and which categorised certain other urban populations as enemies. The heroes of society were the successful commanders and intrepid warriors.
War was not, however, an inevitable consequence of urbanisation. The remains of the township of Caral in Peru, which came into existence around 5000 years ago, show no signs of warfare. No battlements and no weapons have been found. Similarly, excavations at ancient cities of Harappa and Mohenjo-daro in the valley of the Indus River in Pakistan have revealed no indications of military activity until towards the end of their history.
The Anthropocene has seen a massive increase in the killing power of weaponry. For most of human time on Earth, one weapon could kill one person at a time. The two bombs dropped on Hiroshima and Nagasaki in May 1945 are said to have killed between 129,000 and 226,000 people. The vast numbers of weapons of mass destruction in storage today are a thousand times more powerful than these two bombs.
In conclusion, there is clearly nothing in human nature that precludes mortal combat between different groups of people. On the other hand, there is also nothing in human nature that rules out the possibility of different human groups living at peace with each other for long periods of time. Regarding the future, the major determinant of whether or not warfare and terrorism continue to be a feature of civilisation will be the extent to which people allow themselves to be blinded by narrow, pernicious and maladaptive cultural delusions.
For a more detailed discussion see Boyden, S. 2004. The biology of civilisation: understanding human culture as a force in nature. UNSW Press. Sydney.