 The table starts counting around the Late Glacial Maximum period, in which ice retreated and humans started to spread into the northern hemisphere.From the beginning of the early modern period until the 20th century, world population has been characterized by a faster-than-exponential growth.For the period of Classical Antiquity to the Middle Ages, roughly 500 BC to AD 1500, there has also been a general tendency of growth (estimated at roughly a factor 4 to 5 over the 2,000 year period), but not strictly monotonic