a number of different groups. On historical linguistic evidence, see for example classification of Thracian. The traditional view of associating early Celts with the Hallstatt culture, and the Nordic Bronze Age with Germanic peoples. The Roman Empire spread after the first widespread use of iron outside Central Europe from the Villanovan culture area. Most likely there was trade also in these periods, e.g. with amber and salt being major products. Influences from northern Africa via Gibraltar and Sicilia cannot be readily discounted. Many other questions remain open, too; for example, Neanderthals were still present at this time. More genetic data is being gathered by various research programs. Early Holocene As the ice age ended, major environmental changes happened, such as sea level rise (est. 120m), vegetation changes, and animals disappearing in the Holocene extinction event. At the same time Neolithic revolution began and humans started to make pottery, began to cultivate crops and domesticated some animal species. Neolithic cultures in Eurasia are many, and best discussed in separate articles. Some of the articles on this subject include: Natufian culture, Jōmon culture, List of Neolithic cultures of China and Mehrgarh. European sites are many, they are discussed in Prehistoric Europe. The finding of Ötzi the Iceman (dated 3300 BC) provides an important insight to Chalcolithic period in Europe. Proto-languages of various peoples have been forming in this period, though no literal evidence can (by definition) be found. Later migrations further complicate the study of migrations in this period. Emergence of civilizations Due to the similarities between Indo-European languages spoken throughout Europe, Iran, and India, it is widely believed that a group originating in the Pontic steppe in the 5th millennium BC spread both east and west, gradually making their way towards the Indian subcontinent and China in the east and western Europe in the west. These Proto-Indo-Europeans spread their languages into the Middle East, India, Europe, and to the borders of China. Early forms of civilization in Southwest Asia began as early as the 8th millennium BC, in proto-urban centers such as Çatalhöyük. Urban civilizations began to emerge in the Chalcolithic. The earliest urban civilizations in Mesopotamia, India, and China all developed along river valleys. The Uruk period of Mesopotamia dates from about 4000 to 3100 BCE and provides the earliest signs of the existence of states in the Near East. Civilizations grew along the Indus River around 3300 BCE in