Stone Circle at Nabta Playa, Egypt, ca. 4800 B.C.E. This circle of stones, erected when the Egyptian desert received much more rainfall than it does today, may have been a type of calendar marking the summer solstice. Circular arrangements of stones or ditches were constructed in many places during the Neolithic era, and most no doubt had calendrical, astronomical, and/or religious purposes. (Courtesy of Raymond Betz)