Civilization depends on agriculture. Early in the development of civilization, humans were hunter-gatherers. There was little or no agriculture. Each population grouping had to be small, a set of extended families or a small tribe. The amount of land needed per person was large, and that land had to be nearby, since food would be hunted or gathered on a near-daily basis. Estimates of the amount of land needed for a hunter-gatherer food economy vary greatly, depending on what the land and climate provide.
“Based on the preceding calculations, a family of five would require an estimated 200 ha of habitat from which to gather animal and plant food. This estimate is based on an ideal ecosystem, one containing those wild plants and animals that are most suitable for human consumption. Researchers report that, in fact, modern-day hunter-gatherers need much more than 40 ha per person. For instance, Clark and Haswell (1970) estimate that at least 150 ha of favorable habitat per person is needed to secure an adequate food supply. In a moderately favorable habitat, these scientists estimate that 250 ha per person would be required. These estimates are four to six times greater than those in the model presented earlier. In marginal environments, such as the cold northwestern Canadian region, each person needs about 14,000 ha to harvest about 912,500 kcal of food energy per year (Clark and Haswell, 1970).” [Pimentel, ‘Food, Energy, and Society’, p. 45-46.]
A hunter-gatherer society is a rural society. A vast amount of land is needed for even a small population. And this requirement limited the development of civilization. Civilization requires many human persons to live in close proximity, to form cities, to divide labor into many different tasks, to cooperate on a larger scale (as a city or nation), to develop technology and to spread that development.
As long as humanity was limited to hunting and gathering, most persons had to spend most of their time obtaining food, and they could not gather in cities as there would not be enough land nearby. They could not spend much time on other tasks, and so there was little development of new technologies. And any new ideas would not spread quickly and widely, since humanity was broadly and sparsely distributed across the world.
Anthropologists place the beginning of anatomically modern humans as long ago as 200 ka BP (200,000 years before the present). But they also distinguish between anatomically modern humans, with modern or nearly-modern bodies, and the emergence of modern human behavior. Anthropologists generally date the arrival of humans with modern behavior in Europe as early as 44 ka BP, in Southeast Asia by 46 ka BP, and in Africa by 50 ka or as long ago as 70 ka BP.
During the entire time from the start of the human race, whether we mark that origin from anatomical modernity or behavioral modernity, the climate of the world was cold. The Quaternary glaciation (or ice age) began about 2.5 million years ago and continues to the present day. An ice age includes glacial periods, when glaciers advance, and interglacial periods, when glaciers are in retreat (as today). Most of the Quaternary ice age has been marked by a series of glaciations, called the Pleistocene Epoch, in which ice covered much of the world. Anatomically modern humans emerged about 200 ka, during the middle Pleistocene, when the climate was much colder and drier than today. During glaciations, an immense quantity of water is locked up in ice, the oceans are hundreds of feet lower, and the climate is significantly drier.
Agriculture did not develop, to any substantial extent, during the Pleistocene because the climate was not only too cold, but also too dry to favor agriculture. However, the warming of the world that occurred with the end of the Pleistocene and the onset of the Holocene Epoch, about 12,000 year ago, brought new opportunities in food sources for humanity. The melting of the glaciers released vast quantities of water, increasing the amount and frequency of rainfall and the number and size of rivers, streams, and lakes. Hunter-gatherers needed fewer ha of land per person, as the climate allowed more plant and animal food sources to exist in a smaller area. They began to live in closer proximity to one another.
Then, too, agriculture became not only possible, but probable. Even today, farmers prefer warm weather and ample rainfall in order to grow food crops. But at the onset of agricultural development, those two conditions were not merely preferable, but essential. The Holocene offered optimal conditions for nascent agriculture.
As agriculture began to develop, with the domestication of the first few crops and livestock species, fewer ha of land were needed per person to obtain food. Humanity began to rely less on hunting and gathering, and progressively more on domesticated plants and animals. Groups of humans could live in closer proximity, making communication, trade, and other interactions more likely.
As the warming of the Holocene Epoch progressed, agriculture developed further and began to supplant hunting and gathering. There may well have been sporadic and limited domestication of wild plants and animals, or the attempt, prior to the Holocene. But the cold dry weather limited the extent to which the human race overall could rely on this type of food source. Warm weather with ample rainfall makes agriculture much more productive, and therefore provides a much greater incentive to transition away from hunting and gathering food. Without warm wet weather, the amount of time and labor needed to grow a full diet (absent modern mechanized agriculture) may have exceeded the work-hours available. In other words, it may have been literally impossible for the human race to transition to agriculture prior to the Holocene.
The Holocene permitted hunter-gatherers to live in closer proximity. And as agriculture (including domesticated livestock) began to develop, more humans could live in an even smaller area. But, conversely, the development of agriculture also requires more humans living in a smaller area. An isolated individual today can grow a garden. But at the onset of agricultural development, knowledge and seed sources were very limited. The growth spurt of agriculture in its earliest days required communication and trade between human groupings, so that any seeds discovered to be suitable for domestication and food, and the knowledge of how to grow those seeds, would spread quickly and increase exponentially within humanity. So the warming of the Holocene Epoch brought groups of hunter-gatherers closer together, allowing their pooled resources to jump-start agriculture.
And with that increase in agriculture came certain advantages pertaining to land and time resources. In an ideal ecosystem, each person in a hunter-gatherer society needs at least 40 ha of land, perhaps more. But for a society based on agriculture (by manual labor), the amount of land needed falls drastically. So the pressure on human civilization to transition to manual agriculture was very compelling. One tenth to one hundredth as much land was needed per person to produce the same amount of food.
The transition from hunting and gathering to agricultural means of obtaining food allowed civilization to advance. Cities could now exist, whereas before, this would be impossible due to the amount of land needed nearby for each person to hunt or gather. Less labor could be spent on searching for food, freeing many work hours for other types of labor, including teaching, studying, building, and rudimentary forms of manufacture. The increase in the person-work-hours available for non-food and non-survival related tasks jumped sharply, allowing civilizations to rise up throughout the world. Societies moved beyond tribes to become nations and groups of nations.
And all of this was dependent on the warming of the climate at the start of the Holocene Epoch. Without ancient global warming, humanity might never have transitioned from hunting/gathering to domesticated plants and animals. And without that transition, civilization would not have developed. We would still be hunter-gatherers, living in small tribes.