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.

A graph superimposes the growth–decline curves of major Early Rider Empires, from 600 BCE to 600 CE. Compared to running messengers, the Achaemenid empire sharply boosted message speed by using horse relay stations. A major rise in empire area resulted, also helped by split delegation of provincial power. The Xiongnu steppe empire broke the Achaemenid size record hugely but briefly, while Han did so longer but minimally. Rome fell short in size but lasted longer. By population size Han mostly was top, but Maurya and Rome at times surpassed it. Most of these empires profited from religious tolerance, compared to previous theocracies; Sassanids were an exception. The Qin-Han rule established a harsh centralized bureaucracy, while Rome limited it, allowing for local autonomy. Han official contact with Hellenic Bactria in 127 BCE marks the first indirect linkage of states from the Pacific to the Atlantic.

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.