Entheogenic drugs have been used by various groups for thousands of years. There are numerous historical reports as well as modern, contemporary reports of indigenous groups using entheogens, chemical substances used in a religious, shamanic, or spiritual context. A Finnish study assayed psilocybin concentrations in old herbarium specimens, and concluded that although psilocybin concentration decreased linearly over time, it was relatively stable. They were able to detect the chemical in specimens that were 115 years old. The Maya, Olmecs, and Aztecs have well-documented entheogenic complexes. North American cultures also have a tradition of entheogens. In South America, especially in Peru, the archaeological study of cultures like Chavin, Cupisnique, Nazca and Moche, have demonstrated the use of entheogens through archaeobotanical, iconographic and paraphernalia. The Olmec (1200 BCE to 400 BCE) lived in Central America and are largely viewed by many as the mother culture of Aztecs and Maya. The Olmecs left no written works on their belief structures, so many interpretations on Olmec beliefs are largely based on interpretations of murals and artifacts. Archaeologists state three reasons for believing that the Olmecs used entheogens: Burials of Bufo toads with priests The use of entheogens in later Olmec-inspired cultures Sculptures of shamans and other figures have strong Therianthropic imagery. Entheogenics and the Maya The Maya (250 BCE to 900 CE) flourished in Central America and were prevalent even until the arrival of the Spanish. The Maya religious tradition was complex and well-developed. Unlike the Olmec, the Maya possessed religious texts that have survived to this day. The Maya religion displayed characteristic Mesoamerican mythology, with a strong emphasis on an individual being a communicator between the physical world and the spiritual world. Mushroom stone effigies, dated to 1000 BCE, give evidence that mushrooms were at least revered in a religious way. The late Maya archaeologist, Dr Stephan F.