The Iberian Peninsula has been inhabited for at least 500,000 years, first by Neanderthals and then by modern humans. Prehistory Megalithic culture Galicia, northern Portugal, Asturias, western León, and Zamora formed a single megalithic area since the Neolithic and Chalcolithic Ages (also called the Copper Age), around 4500–1500 BC. This was the first great culture to appear in Galicia, with a great capacity for construction and architecture. This was combined with a deep sense of religion, based on the cult of the dead, the mediators between man and the gods. Many historians believe that the Megalithic culture had two sources: an oriental source that was predominant in the Mediterranean area, and an Atlantic source, which originated north of the Tagus River. The latter, because of its geographical proximity to Galicia, would explain the abundant traces of megalithic culture in this area. As this was the first great culture, it was an important source of Galicia's cultural personality. From this era there remain thousands of dolmens (mámoas), a type of tomb or sepulchre, throughout the entire territory. From its social organization it has been confirmed that it corresponded to some type of clan structure. Bronze Age The introduction of bronze-working techniques introduced a new cultural era, in which the new importance of metals resulted in intense mining activity. Some historians attribute this to the extremely dry and warm climate of the time, resulting in erosion which revealed the rich mineral resources of the North. Peoples from the Castilian plateau moved to Galicia, thus increasing the population, because its position near the Atlantic Ocean gave it a very humid climate. The increase in population caused certain conflicts, but also led to increased mining and production of weapons, useful objects, and ornamental objects of gold and bronze. Pieces of jewellery crafted from Galician metals circulated throughout the Iberian Peninsula and the rest of Europe. Antiquity The Gallaeci (Celts) At the end of the Iron Age, people from northwestern Iberian Peninsula formed a homogeneous and distinct cultural group, which was later identified by early Greek and Latin authors, who called them "Gallaeci" (Galicians), perhaps due to their apparent similarity with the Galli (Gauls) and Gallati (Galatians). The Gallaeci were originally a Celtic people who for centuries had occupied the territory of modern Galicia and northern Portugal; bounded to the south by the Lusitanians and to the east by the Astures. In ethnic terms, they