with an amazing variety of unique customs,

religious practices, social structures and technologies.

These differences are fascinating,

but Big History teaches us to look at the past differently,

to seek out commonalities and identify patterns

that unite seemingly unrelated phenomena.

Often this produces insights that are even more interesting.

One of the most important patterns we see

among agricultural civilizations is the need to expand.

Many agrarian civilizations grew to control

a lot of territory.

The Roman Empire at its peak controlled

around 2.5 million square miles,

the Persians more than three million square miles

and the Mongols a whopping 10 million square miles

with an empire stretching across Asia.

Though these empires existed in different places

and at different times, they shared a strong drive

to expand geographically.

And when they could no longer grow,

their territory shrank and eventually

their civilizations collapsed.

So why was geographic expansion so essential to their survival?

Maintaining a state was expensive.

Increasing the large populations required more infrastructure,

more resources as well as bigger governments and militaries.

Ambitious monumental architecture like the Pyramids

added even more expense because land productivity

had its limits and leaders could face rebellion

if they try to squeeze subjects too much

with heavy taxation.

There were limits to how much states could grow

using internal resources.

Thus growth necessitated taking what others had produced

rather than trying to increase productivity within the state.

During the era of agrarian civilizations

this type of military expansion was common.

We see it in civilizations around the world

with the Persians, the Romans, the Chinese dynasties

and with the Aztec and Inca empires in the Americas.

Large professional armies required food, weapons

and complex infrastructures like roads, forts

and defensive walls.

All of these came at great expense,

which itself increased the need for expansion.

Some military innovations lead to inventions

that would advance entire civilizations.

Iron is a good example.

The hard metal was initially used for weapons

but once iron plows appeared,

crop yields increased, and so did populations.

Roads, initially designed to move armies,

became important trade routes and building techniques

were refined after the construction of so many walls,

forts and watch towers.

Throughout this era, borders were constantly contested

but there was another side

to the interactions that took place.

Outposts and border regions often became centers

for commercial and technological innovation.

This was because different cultural groups

connected and many non-military exchanges occurred,

intensifying collective learning.

We see this accelerated change very clearly in Europe

when the Roman Empire splintered into a series

of small competitive states.

This increase in commerce and the exchange of ideas

drove a transition of power

from large agrarian civilizations

to these smaller commercial states.

The pace of collective learning further intensified.

Traditional agrarian civilizations in other parts

of the world lost power and in many cases were colonized

by the same European states with dramatic implications

for today's world.

Empires were shaped less by physical borders

and more by spheres of commercial influence.

The modern world had begun to take shape.