How many people have been alive since 4000 BC?

Estimations suggest that around 100 billion people have lived on Earth since 4000 BC. The world population was relatively small in ancient times, gradually increasing over the centuries due to factors like agriculture, urbanization, and improvements in health and sanitation. Significant population growth occurred particularly during the last few centuries, especially with the Industrial Revolution. These estimates can vary based on different methodologies and assumptions about historical population sizes.