This chart shows the total population of the world over time. (This is from the Wikipedia article History of the world.) We see that from about 4000 BC to 1000 BC the population grew exponentially (it looks linear, but note the log scale of the y-axis). Then the curve turns upward, with the rate of growth increasing until it’s suddenly interrupted around 400 BC and the total population actually decreases from 400 BC to 200 BC.
What caused this population decline? I can see another dip around 1300 that’s probably the Black Death, but I don’t know of a similar event from circa 300 BC.