Methods and Conventions
Essential to understanding the pre-Columbian evolution of Middle American culture and society is the construction of a reliable chronology. To do so depends on accurate age determinations, and while a number of methods are available to archaeologists, radiocarbon (14C) dating stands out as the technique most widely applied whenever organic materials are available for analysis. Unfortunately the accuracy of this method is affected by temporal variations in cosmic radiation and radioactive atmospheric carbon, resulting in dates that can be hundreds of years more recent in time than their actual calendar equivalents. Calibration formulae have been developed, but until recently they were not applied to published dates. Moreover, calibration for the earliest periods of Middle American prehistory is still imprecise. Consequently, even today some published dates are calibrated while others are not. To avoid confusion, in this article uncalibrated dates will be differentiated by use of the lower case suffix, bc or bp (before present) while calendar dates will be denoted by upper case BC or BP Where no other indication is given, terms such as ‘years ago’ used in the text refer to uncalibrated dates.
In addition to calibration errors, older methods of 14C determination, because of their sample size requirement, could not directly date small plant, bone, or other organic fragments but instead relied on the stratigraphic association of these materials with larger quantities of datable carbonized substances, such as charcoal from cooking areas. The inability to date materials directly led to significant errors where the archaeological specimens had been vertically displaced by animal burrowing, human activity, or other forms of soil disturbance. Accelerator mass spectroscopy (AMS), a newer radiocarbon dating technique, has solved the problem by enabling very small organic samples - seeds and even pollen, for example - to be directly dated. Many archaeological specimens have been re-analyzed by AMS technology, sometimes resulting in significantly altered dates.
Paleoindian subperiod begins, and may extend 35 000 years back in time, or even earlier.
The Paleoindian period ends and the Archaic, subdivided into Early, Middle, and Late subperiods, begins somewhere between 9000 and 7000 bc, at the onset of the Holocene, the epoch following the last major glacial advance. During this interval worldwide temperatures at times averaged up to 2 °C. warmer than today, the glaciers that shrouded the higher latitudes of North America retreated to the sub-Arctic, seawater levels rose dramatically and, in Middle America, generally drier conditions prevailed. Profound changes in plant and animal life also occurred, perhaps the most conspicuous being the disappearance of many species of Ice Age megafauna, a phenomenon that had begun toward the end of the Paleoindian period and ended around 7000 bc with the extinction of the indigenous American horse. Although the evidence for any kind of causal relationship is ambiguous, loss of these large animals occurred at a time when the inhabitants of Middle America were abandoning their nomadic hunting and wild-food gathering way of life for an agriculture-based subsistence economy, with most people eventually residing in permanent settlements.
By 2300 bc pottery was being manufactured in Middle America, which by definition marks the end of the Preceramic era and its final Archaic period. Some archaeologists, however, prefer to extend the Archaic to 1500 bc or later, by which time agriculture provided the bulk of food and the previously nomadic hunter-gatherers had settled in permanent villages. The variance in dates delineating the Paleoindian and Archaic periods, while having the appearance of imprecision, denotes ranges of years extending over centuries, rather than discrete points in time. As an example, there is no single date pinpointing when agriculture was adopted.
World History
