The earliest of the major Meso-American civilizations was the Olmec culture, which is often regarded as the fostering influence behind the Mayan, Aztec, and other later societies. The Olmec were prominent in eastern coastal Mexico between 1200 and 400 B.C. and are remembered for constructing massive earthen mounds, sculpting giant basalt heads, and building large and prosperous cities that existed for hundreds of years. As the Olmec declined, the Mayans rose to prominence. Historical evidence seems to indicate the presence of a Mayan culture in present day Mexico at least as far back as 1800 BCE, but their greatest influence was exerted between 200 B.C. and A.D. 1000. Settlement was extensive in the Yucatán Peninsula and stretched southward into Central America. Unlike the later Aztecs, the Mayans did not exercise strong administrative control over an empire, but instead developed as a series of largely autonomous city-states, such as Palenque, Tikal, and Chichén Itzá. Fortified residential areas were often surrounded by meticulously cultivated farmlands. Mayan contributions were many. They developed an advanced writing system. Their history, entrusted to cactus fiber parchment, fared poorly against the ravages of time and Spanish censors saw to the destruction of much of the remainder. However, many of their carvings on stone have survived and provide much of what is known today about their civilization. The Mayans also were gifted mathematicians who independently developed the concept of zero, and astronomers who deduced that a solar year was slightly more than 365 days. Despite these achievements, the Mayans and other Meso-American cultures failed to discover the utility of the wheel. The decline of Mayan civilization was well under way by 1100 B.C. The causes are uncertain, but speculation points to warfare, crop failures, and disease as leading possibilities. The society was also enervated by its religion, which emphasized that human blood was extremely pleasing to its gods. Nobles mutilated themselves and their blood flowed onto fabric, which was burned as an offering. As time passed, those gifts were deemed insufficient and human sacrifice became commonplace. Victims had their still beating hearts cut from their chests and displayed to throngs gathered for these spectacles. Most of those so dispatched were captives from battles, but others were Mayan volunteers seeking to placate the gods. By the time of the Spanish arrival around A.D. 1520, the Mayans were a starkly diminished civilization. Their great cities were abandoned and the remnants of their population widely scattered.