During the three phases of the Jōmon period the population was primarily hunter-gatherer with some primitive agriculture skills and their behaviour was predominantly determined by changes in climatic conditions and other natural stimulants. Early dwellings were pit houses consisting of shallow pits with tamped earth floors and grass roofs designed to collect rainwater with the aid of storage jars. Later in the period, a colder climate with greater rainfall led to a decline in population, which contributed to an interest in ritual. Concentric stone circles first appeared during this time.