The units of time first developed by humans would likely have been days and months (moons). In some parts of the world the cycle of seasons is apparent enough to lead to people speaking about years and seasons (e.g. 4 summers ago, or 4 floods ago). With the invention of agriculture in the 3rd millennium BC, people relied heavily on the cycle of the seasons for planting and harvesting crops. Most humans came to live in settled societies and the whole community relied upon accurate predictions of the seasonal cycle. This led to the development of calendars. Over time, some people came to recognize patterns of the stars with the seasons. Learning astronomy became an assigned duty for certain people so they could coordinate the lunar and solar calendars by adding days or months to the year.