Lessons from a National Geographic Society special that contained scenes of warfare between tribes of New Guinea living much as they had in the Stone Age:
1.  War is an ancient part of human culture.
2.  Warfare is culturally-defined; each society and time-period defines its proper conduct based on such factors as current technology, the degree of social organization, or even the beliefs of what constitutes correct conduct.
Weighed against the chivalric code of the Middle Ages, modern soldiers would be found wanting; they might be characterized as cowards who shoot down their enemy from far away, rather than facing him in hand-to-hand combat with “a man’s weapons” like sword, dagger, and lance.
Medieval knights felt the same way about the forerunners of fire-arms bearing soldiers, the crossbowmen of their own period, sometimes riding down even those fighting on their own side.  In some places, the penalty for a captured crossbowman was to have his hand cut off.
A literature grew up in the troubled 1960s that attempted to explain man’s aggressive behavior leading to war: Is war inherent to humanity?
Killer Ape Thesis: a wide-spread idea in the 1960s that man was directly descended from a particularly violent and aggressive primate species and therefore had a deadly, aggressive nature built into his genetic makeup. Reinterpretation of evidence on which the thesis was based led to its collapse. Today, most scientists would not argue that man is particularly aggressive and violent, but that he shares these traits with other species, traits that help make possible survival in the natural world.
Robert Ardrey:  American playwright whose best-selling books, African Genesis and Territorial Imperative advanced the Killer Ape Thesis.
Conrad Lorenz: Austrian scientist; foremost naturalist of the twentieth century; he  identified important biological concepts such as imprinting.  His explanation for the extent of human violence has stood up better than the Killer Ape Thesis.
On Aggression: Book by Lorenz, aimed at the general public, in which he applied several of his major arguments about animal behavior to humans.
1.    Species develop normal limitations on intra-specific aggression (aggression aimed inward toward other members of  the group).  These limitations help prevent the members from wiping each other out.  Within the group, there is an on-going struggle to establish dominance that involves aggressive behavior. Individual organisms use aggression establish their place on the the pecking order.  However, in most mammal species, such intra-specific aggression does not ordinarily lead to death.
2.    Humankind has circumvented this very effective system for mitigating violence within a species.  He has escaped the natural constraints, just as he has escaped so many other aspects of his animal inheritance, through the development of tools (what is known as cultural evolution.)
3.    A great increase in population density can promote intra-specific violence.
Understanding of Human Technology:  Largely for religious reasons, it was not until the middle decades of the 19th century  (roughly 1830-1860) that western society finally began to acknowledge not only the great age of hominid species (those that are ancestors to modern man), but also how long those hominids had been using tools and what their original tools were like.  Scientists who work in paleo-anthropology (the study of ancient man) recognize that the ancestors of present day humans possessed tools more than 2 million years ago.
Homo habilis:  identified as the first homind to make fairly extensive use of tools.
Humans are not the only members of the animal kingdom to exhibit tool use, or even tool making on a very limited scale; however, we are the only species that has used tools to reshape its environment in a major way.
Before tools, human-like species probably had eating habits much like our closest  cousin, the chimpanzee, which is an omnivore.  Vegetable matter, supplemented with insects (such as grubs, ants, and termites), small animals (frogs, lizards, rodents, snakes), scavenged meat from kills of larger predators.
Animal types categorized by eating habits:
Carnivores:  meat-eaters
Herbivores:  eat vegetation
Omnivores:  eat all available food matter
Although tools were probably first developed for scavenging meat from kills of other predators, they eventually converted man into the greatest hunter of the prehistoric world, perhaps capable of driving some species into extinction.
Tools used for hunting were eventually turned on other members of the species.  They permitted man to do much greater damage to his fellow man than was possible in the intra-specific violence of any other species.  According to Lorenz, tools make natural curbs on intra-specific violence inadequate.  The result is human warfare.
Increasing population density, that can also be traced to cultural evolution (i.e. the development of tool-use) also promotes the growth of warfare as human societies have increasing contact and compete with one another for resources.  While humans continued to lead the life of the hunter-gatherer, they remained rare.  Only when man became an agriculturalist did his numbers increase astronomically.
Neolithic Revolution:  Period that that witnessed the birth of agriculture and domestication of animals.  Rivalled the original development of tool use as the greatest technological change in human history
Chronology of human culture established in 1837 by Christian Thomsen, director of the Copenhagen museum:
Stone Age
Bronze Age
Iron Age
Stone Age sub-divided into several other periods, including:
Paleolithic (Old Stone Age):  hunter-gatherer societies
Neolithic (New Stone Age):  agricultural societies
Stone Age divisions reflected changes in stone technology.  In the case of the Neolithic, it was the polishing of stone tools on natural abrasives such as sand in order to further refine them.
Neolithic Revolution occurred independently in a number of different sites, and from these original sites spread by cultural diffusion around the globe.  First occurrence was in the ancient Near East, where domestication and the birth of agriculture started shortly after the last Ice Age, some 10 to 12,000 years ago.
Other early sites:
Nile valley of North Africa;
Indus River Valley
Northern China;
Parts of Central and South America.
One major results was an immense growth of human population.  By securing increased control over the food supply, humans began to escape natures limits of population,
Thomas Malthus:  English clergyman, often thought of as the father of modern economics, and a major forerunner of Charles Darwin.  Published Essay on the Principles of Population (1798).
Malthusian Concepts:
All species, including humans, have a constant tendency to breed themselves into starvation.  Unchecked, the population of any species would inevitably outstrip the earth's ability to support it.
However, human population seemed to differ from the populations of all other animals and plants.  In the case of all other species, nature strictly limited their numerical expansion, thus preventing them from outstripping their food supply.  Only human beings seem to have escaped (at least to some extent) this natural check on population size.
Malthus had identified the effect of the Neolithic Revolution on humans.  Control of the food supply had freed human beings from the natural checks on population, allowing the  human species to proliferate rapidly since the end of the last Ice Age.
This increased population could also abandon the nomadic lifestyle of the hunter-gatherer.  It could settle down in one place, build villages and towns, and pile up possessions.
A hunter-gatherer society of the paleolithic period, numbering perhaps 25 individuals, had needed a territory of roughly 250 square miles from which to draw their living.  By contrast, a neolithic village of 150 people, using agriculture and domestication of animals, could harvest adequate food supplies from only about 6 square miles.
In the age of the hunter-gatherer, there had been limits on what one could possess imposed by what one could carry when the tribe moved.  With a sedentary (settled) lifestyle, made possible by the Neolithic Revolution possessions could be multiplied manyfold.
Results for Growth of War:
People no longer lived in their natural state.  They possessed deadly weapons, lived in much closer proximity, and had a great deal more property to protect or to seize.  As a result, war became a much more meaningful social activity.
As man entered the period of written history that began some 5000 years ago, he was already a warrior, not because of any killer ape genes, but due to his cultural evolution.  The same tool-making capacity that made possible all the great advances of the human species helped doom him to war.
In life, there is a flip side to just about everything!