mammal species living there, which could be domesticated. The development of agriculture allowed higher populations, with the newly dense and settled societies becoming hierarchical, with inequalities in wealth and freedom. As the cooling and drying of the climate by 3800 BCE caused drought in Mesopotamia, village farmers began co-operating and started creating larger settlements with irrigation systems. This new water infrastructure in turn required centralised administration with complex social organisation. The first cities and systems of greater social organisation emerged in Mesopotamia, followed within a few centuries by ones at the Indus and Yellow River Valleys. In the cities, the workforce could specialise as the whole population did not have to work for food production, while stored food allowed for large armies to create empires. The first empires were those of Ancient Egypt and Mesopotamia. Smaller kingdoms existed in North China Plain, Indo-Gangetic Plain, Central Asia, Anatolia, Eastern Mediterranean, and Central America, while the rest of humanity continued to live in small tribes. Middle East and the Mediterranean The first states of sorts were those of early dynastic Sumer and early dynastic Egypt, which arose from the Uruk period and Predynastic Egypt respectively at approximately 3000BCE. Early dynastic Egypt was based around the Nile River in the north-east of Africa, the kingdom's boundaries being based around the Nile and stretching to areas where oases existed. Upper and Lower Egypt were unified around 3150 BCE by Pharaoh Menes. This process of consolidation was driven by the crowding of migrants from the expanding Sahara in the Nile Delta. Nevertheless, political competition continued within the country between centers of power such as Memphis and Thebes. The prevailing north-east trade winds made it easier to sail up the river, thereby helping the unification of the state. The geopolitical environment of the Egyptians had them surrounded by Nubia in the smaller southern oases of the Nile unreachable by boat, as well as by Libyan warlords operating from the oases around modern-day Benghazi, and finally by raiders across the Sinai and the sea. The country was well defended by natural barriers formed by the Sahara on both sides, though this also limited its ability to expand into a larger empire, mostly remaining a regional power along the Nile (except for a conquest of the Levant in the second millennium BCE). The lack of timber also made it too expensive to build a large navy for power