With modern-size brains and fully articulate language, we humans became capable of inventing all the complexities of modern civilization by 50,000 years ago. But our first settlements didn't emerge until about 15,000 years ago. For 35,000 years, we continued to live from hand to mouth in small tribes, foraging and fighting in the wilderness. Why?
First, settling down wasn't a choice on a menu; it had not yet been conceived. We had been hunter-gatherers for all five million years of our prior evolution. It seemed natural to roam, to be able to carry our few individual possessions and be free to move away from danger or walk off and start a new tribe. The very idea of living with non-kin would have aroused deep suspicion, and living under rules imposed from outside your tribe would, at first, have been unthinkable.
Second, any successful settlement would need enough manpower and defenses to reliably deter or repulse an attack by a foraging tribe. Even if that many hunter-gatherers could agree to give up their freedom and try to live with strangers, they would have had to invent a whole new social structure, as well as a new kind of food supply. Which in turn would seemingly require the simultaneous invention of agriculture, grain storage facilities, and domestication of animals like cattle.
So the real mystery is not why it took so long, but how did all those mutually-dependent advances come together at all?
Archaeologists and other scientists believed until recently that small-scale farming came first, but there is now some general agreement that agriculture was preceded by small villages located near abundant food supplies. The concept of fortifications may have started with the large mammoth-bone houses built around 18,000 years ago, and dogs would have been able to augment human sentries by 15,000 years ago.
In any case, there is evidence of successful settlements in the Syria/Israel/Jordan area as early as 15,000 years ago. These first settlements then became the catalyst for a great surge in human evolution. They served as testbeds for farming and agricultural techniques, centers for domestication of animals, and incubators of more complex social structures and new concepts about money, property, weight and measurement.
Once the process of urbanization was ignited, it accelerated steadily, leading to the first mighty civilizations of Egypt and Babylon that we've all read about. Here, in brief, is how it came together.
Notions of democracy were still a long way in the future, so city-states were governed by strong rulers. Hierarchies of power and wealth emerged, and the lives of ordinary people were often cruel. Cholera and other infectious diseases became more or less permanently established as people crowded together under less than sanitary conditions. Still, the cities provided security, and development of domesticated cereals and grain permitted large-scale agriculture and stored surpluses.
No longer did everyone have to forage for food and be a fighter; people could be farmers or merchants or house-builders or soldiers -- even musicians. And women could be full-time moms. As foragers, women had to keep their birth rate low (or resort to infanticide), being unable to carry more than one child when shifting camp. Permanent settlements, though, enabled birth intervals to drop from roughly four years to two, greatly increasing the population growth of developing cities.
Specialization led to a flowering of invention, including domestication of cattle, pigs, sheep and goats by 9500 years ago, and horses by 6000. Bronze tools appeared around 6000 years ago, and the first written tablets by 5000, soon followed by Egyptian papyrus.
Religion, such as ancestor worship in Egypt, became more organized and more collaborative with rulers by sanctioning wars, which were now fought army against army instead of tribe versus tribe.
Yet despite the conflicts, or in between them, trade flourished. Our aggressive instincts share an uneasy co-existence with an instinct for reciprocity, which goes all the way back to chimps trading favors. This instinct, shared by all humans, enables deals to be struck with strangers and generally honored, under pain of being banished from further trading.
Gradually, longer-range trading networks developed, with commodities such as grain surpluses exchanged for specialized products made by craftsmen in distant cities. This led to cross-fertilization of ideas between societies, which resulted in more inventions and further accelerated the cycle of progress.
Our ancient tool-builder instincts were now in full bloom, resulting in the wheel and a profusion of lesser inventions like the plough, the loom, bricks and breweries.
Our brains readily accommodated all these changes without any mutations or discernible evolution; the capacity to do so had simply lain dormant for 35,000 years. Once money was invented -- at first a totally alien idea -- we quite rapidly progressed to concepts of capital and economy. Scribes conceived the first writing systems, a monumental intellectual achievement, and philosophers attracted followings.
Religions grew more organized and developed symbiotic relationships with kings and rulers: rulers tolerated these separate power structures in exchange for priests providing religious justifications for wars. This unholy collaboration gave rise to a fanaticism in war that was new in human history. The idea of fighting suicidally would never have occurred to humans before the advent of states and religions. Despite being as aggressive as they were, our hunter-gatherer ancestors would always look for a winnable advantage before mounting a raid.
At this point, we were firmly set on a course that led directly to where modern civilization is today, and the story of human evolution shifts to conventional history, available in countless books. We will look into one last mystery in our evolutionary past, and then see what may lie ahead for us humans.