In the Holocene's first millennium, the Palaeolithic began to be superseded by the Neolithic (New Stone) Age which lasted about 6,000 years, depending on location. The gradual transition period is sometimes termed Mesolithic (northern and western Europe) or Epipalaeolithic (Levant and Near East). The glaciers retreated as the world climate became warmer and that inspired an agricultural revolution, though at first, the dog was probably the only domesticated animal. This was accompanied by a social revolution in that humans gained from agriculture the impetus to settle. The settlement is the key precursor to civilisation, which cannot be achieved by a nomadic lifestyle.