This period begins with the first farmers and the introduction of domesticated animals and plants. The most striking features are the great ceremonial monuments which were erected throughout this period including communal burial places called long barrows. During this age humans discovered metal as raw material for fashioning tools, weapons and jewellery. The discovery of copper gave rise to new occupations, social groups and social hierarchy. At the top of the pyramid were the warriors, who protected their village communities. Trading of copper and copper objects allowed villages to gain contact with other villages.