Answer: The Achaemenid Empire

The Achaemenid Empire, more commonly called The First Persian Empire, was founded by Cyrus the Great and is notable for several achievements in early empire building. Thanks to careful guidance and calculation by Cryus, the empire had a highly centralized and efficient bureaucracy. Further, the empire expanded rapidly and readily absorbed surrounding peoples.

This efficient governmental oversight combined with a high rate of population growth fueled by expansion and absorption yielded the largest physical empire in the ancient world (encompassing around eight million square kilometers across three continents), but also a distinction that remains unrivaled: the percent of Earth’s population under Achaemenid rule.

The population of the empire in ~480 BCE was estimated at 50 million people and, based on global population estimates for that historical period, that means the Achaemenid Empire would have encompassed 44 percent of the global population: the largest percentage of any empire prior or since.