Big Era Four Expanding Networks of Exchange and Encounter 1200 BCE ? 500 CE Welcome to Big Era Four! Population Growth Expanding Networks of Exchange Let?s focus on two key developments of this era. ? Between 1000 BCE and 1 CE world population rose from about 120 to about 250 million. ? This rise was fueled by an acceleration in the rate of growth during this time. ? Between 3,000 and 1,000 BCE, it took about 1,600 years for world population to double. ? Between 1,000 BCE and 1 CE the doubling time was less than 1,000 years. Population Growth What caused this surge in population? Population Growth In Afroeurasia, iron axes, hoes, spades, and plows enabled farmers to clear and cultivate millions of acres never before used for farming. #1 The invention of iron!