We designate as an empire a state that stands out by area and population, as compared to most neighbors. The same population–technology interaction that enables world population to grow also enables states and then empires to form and expand, mostly by brutal force. Empires form where people are. The joint population share of the top five empires reached 50% of the world population by the year +1, and it did so on just 10% of the dry land area. The areas of top empires expanded in three phases, dependent on message speeds but also skills in delegation of power: Runner, Rider and Engineer Empires. The Rider age produced a new type of nomadic “area empires,” with low population density, in contrast with standard high-density “people empires.” The areas and populations of people empires tend to follow a square root relationship: Their share of world population is the square root of their share of dry land area.

World population growth has proceeded in two steeper-than-exponential phases, with an intervening standstill from 1– 400 CE. Our interaction model of population, technology, and Earth’s carrying capacity projects to a peak of 11 billion people by 2100. Yet, our impact on Earth’s biosphere may undo our very existence. Then projections in this book, such as a single world state by 4600 CE, become moot. Over 5000 years, the number of states has fallen and top empire sizes have increased exponentially but also in three phases triggered by breakthroughs in message speed: Runner, Rider and Engineer Empires. This approach can lead to a non-Eurocentric periodization of history, with cut-off dates at 3000 BCE, 600 BCE, 600, 1200, and 1800. Various relationships connect world population and top empire and major city sizes, but they have tended to fail since 1800, as the world becomes a single, rapidly interacting system. A distinction of Talkers, Doers, Regulators, and Followers serves to characterize the internal structure of empires. An initial human self-domestication (slavery) seems to be later followed by self-taming. Lists of world history events put the midpoint of history around 1500 CE.

Over 5000 years, the world population has grown from 10 million to 8 billion. Yet the number of separate tribes and states has shrunk from tens of thousands to a few hundred, despite an increase in the number of states during the 100 years. This book traces these historical macro-trends, contributing a new perspective through multidisciplinary exploration. The present chapter outlines its structure. Population growth was boosted by the population–technology interaction but is now rapidly slowing down, limited by Earth’s ultimate carrying capacity. Hence, today’s centenarians are unique in human history: the world population more than quadrupled during their lifetimes. The number of states has been reduced by empire growth. Empire sizes have increased in three main phases triggered by breakthroughs in message speed: Runner, Rider, and Engineer Empires.

Over 5000 years, the world population has grown from 10 million to 8 billion. This has taken place in two distinct phases that begin with steeper-than-exponential growth due to the population–technology interaction but later came to a halt, blocked by limits in Earth’s carrying capacity. These two phases stand out when data are graphed in a novel way, with time as well as population on logarithmic scales. Each phase can be fitted using a novel “tamed quasi-hyperbolic” T-function. The ancient phase reached a population of 220 million by year 1, dropping to 190 million by year 400. The present phase slowly picked up speed, reaching its maximal relative (percent) growth rate around 1970 and absolute (in million) around 2010. It projects to a population maximum of 11 billion. The entire process conforms to a “PCT model”: population–carrying capacity–technology interaction. Population growth comes from population itself, as long as population is smaller than capacity. Capacity growth comes from increase in technology, as long as capacity falls short of an ultimate carrying capacity. Technology growth comes from technology itself but also from population.

The world population growth curve shows a kink from 100 BCE to 200 CE. It occurs in all major populated regions: China, Mesopotamia, Mediterranean, and (least of all) in India. Graphing of population doubling times over time shows utter discontinuity. It looks as if Earth’s carrying capacity for humans reached a limit that no increase in technology could overcome, but by 400 CE a unique breakthrough occurred. An exponent value in “T-function” fits to population data before and after the kink shifts in a way that suggests a crucial change in innovation practices. From isolated innovators, humankind shifted to moderate interaction over space and time, due to literacy. A sudden increase around 400 CE in ability to make use of written records is implausible, yet it remains the only conceivable way to explain the kink in world population growth curve.