The first millennium is a period of time that commenced on January 1, AD 1, and ended on December 31, AD 1000, of the Julian calendar.
World population, which had tripled over the preceding millennium, grew more slowly during the first millennium and may have diminished. One optimistic estimate is that the world's population rose from approximately 170 to 300 million, but other estimates vary; one estimate suggests that the world population actually declined from 400 million people to 250 million people.
In Europe and the Mediterranean, the first millennium was a time of great transition. The 2nd century saw the peak of the Roman Empire, followed by its gradual decline during the period of Late Antiquity, the rise of Christianity and the Great Migrations. The second half of the millennium is characterized as the Early Middle Ages in Europe, and marked by the Viking expansion in the west, the rise of the Byzantine Empire in the east, and by the Islamic conquests throughout the Near East, North Africa and the Iberian peninsula, culminating in the Islamic Golden Age (700–1200 AD).
In Eastern Asia, the first millennium was also a time of great cultural advances, notably the spread of Buddhism to East Asia. In China, the Han Dynasty is replaced by the Jin Dynasty and later the Tang Dynasty until the 10th century sees renewed fragmentation in the Five Dynasties and Ten Kingdoms Period. In Japan, a sharp increase in population followed when farmers' use of iron tools increased their productivity and crop yields. The Yamato court was established. The Indian subcontinent was divided among numerous kingdoms throughout the first millennium.
In Mesoamerica, the first millennium was a period of enormous growth known as the Classic Era (200–900 AD). Teotihuacan grew into a metropolis and its empire dominated Mesoamerica. In South America, pre-Incan, coastal cultures flourished, producing impressive metalwork and some of the finest pottery seen in the ancient world. In North America, the Mississippian culture rose at the end of the millennium in the Mississippi and Ohio river valleys. Numerous cities were built; Cahokia, the largest, was based in present-day Illinois, and may have had 30,000 residents at its peak about 1250 AD. The circumference of the 10-story-high Monks Mound at Cahokia was larger than that of the Pyramid of the Sun at Teotihuacan or the Great Pyramid in Egypt.