The prehistory of Ohio is generally divided into four major time periods: the Paleoindian, Archaic, Woodland, and Proto-historic periods, with the Archaic and Woodland each further subdivided into Early, Middle, and Late periods. The earliest inhabitants of the Ohio region, known as Paleoindians, arrived around 12,000 years ago shortly after the last glaciers receded and lived as highly nomadic small bands following herds of large game over seasonal routes that could cover several hundred miles. During the Archaic period, which began approximately 10,000 years ago and lasted about 7,500 years, Native American populations in the Ohio region increased gradually and became less nomadic as they exploited a broader diversity of plant and animal resources. Around the beginning of the Late Archaic period about 5,000 years ago, the regional population increased significantly, with larger bands occupying semi-permanent villages, often in river valleys, during spring and summer before dispersing into smaller family-based groups in fall and winter to use nut supplies and deer populations in forested uplands. The Late Archaic also saw the invention of fired-clay pottery. The Woodland period in what is now northeastern Ohio began approximately 2,500 years ago, and during the Early Woodland period grain or corn agriculture first appeared in the Ohio region. By the Late Woodland period, archaeologists believe the area from the Grand River Valley east to modern Erie County, Pennsylvania, was inhabited by the Iroquoian-speaking Eastwall Complex culture, which settled in large villages sometimes fortified by defensive ditches, suggesting rising social instability and conflict between groups. During the Proto-historic period, ca. A.D. 1550-1650, European trade goods began reaching the region from the east even though Europeans had not yet entered Ohio territory. As Iroquoian populations indigenous to western New England were gradually pushed westward by European encroachment, they in turn displaced existing Iroquoian and Algonquian populations in northeastern Ohio, leading to further conflict just before European explorers and traders entered the Great Lakes region. The newer Iroquoian populations became known as the Erie tribe when Euro-American settlers first arrived in northwestern Pennsylvania and northeastern Ohio.