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materials all deposited on the same day. Materials deposited at different times can get mixed together, as worms and rodents and other agents churn up the ground. Charcoal residues from a fire can thereby end up close to the remains of a plant or animal that died and was eaten thousands of years earlier or later. Increasingly today, archaeologists are circumventing this problem by a new technique termed accelerator mass spec-trometry, which permits radiocarbon dating of tiny samples and thus lets one directly date a single small seed, small bone, or other food residue. In some cases big differences have been found between recent radiocarbon dates based on the direct new methods (which have their own problems) and those based on the indirect older ones. Among the resulting controversies remaining unresolved, perhaps the most important for the purposes of this book concerns the date when food production originated in the Amer-icas: indirect methods of the 1960s and 1970s yielded dates as early as 7000 b.c., but more recent direct dating has been yielding dates no earlier than 3500 b.c.
A second problem in radiocarbon dating is that the carbon 14/carbon 12 ratio of the atmosphere is in fact not rigidly constant but fluctuates slightly with time, so calculations of radiocarbon dates based on the assumption of a constant ratio are subject to small systematic errors. The magnitude of this error for each past date can in principle be determined with the help of long-lived trees laying down annual growth rings, since the rings can be counted up to obtain an absolute calendar date in the past for each ring, and a carbon sample of wood dated in this manner can then be analyzed for its carbon 14 / carbon 12 ratio. In this way, measured radiocarbon dates can be "calibrated" to take account of fluctuations in the atmospheric carbon ratio. The effect of this correction is that, for materials with apparent (that is, uncalibrated) dates between about 1000 and 6000 b.c., the true (calibrated) date is between a few centuries and a thousand years earlier. Somewhat older samples have more recently begun to be calibrated by an alternative method based on another radioactive decay process and yielding the conclusion that samples apparently dating to about 9000 b.c. actually date to around 11,000 b.c.
Archaeologists often distinguish calibrated from uncalibrated dates by writing the former in upper-case letters and the latter in lower-case letters (for example, 3000 b.c. vs. 3000 b.c., respectively). However, the archaeological literature can be confusing in this respect, because many books and papers report xncalibrated dates as b.c. and fail to mention that they are
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actually uncalibrated. The dates that I report in this book for events within the last 15,000 years are calibrated dates. That accounts for some of the discrepancies that readers may note between this book's dates and those quoted in some standard reference books on early food production.
Once one has recognized and dated ancient remains of domestic plants or animals, how does one decide whether the plant or animal was actually domesticated in the vicinity of that site itself, rather than domesticated elsewhere and then spread to the site? One method is to examine a map of the geographic distribution of the crop's or animal's wild ancestor, and to reason that domestication must have taken place in the area where the wild ancestor occurs. For example, chickpeas are widely grown by traditional farmers from the Mediterranean and Ethiopia east to India, with the latter country accounting for 80 percent of the world's chickpea production today. One might therefore have been deceived into supposing that chickpeas were domesticated in India. But it turns out that ancestral wild chickpeas occur only in southeastern Turkey. The interpretation that chickpeas were actually domesticated there is supported by the fact that the oldest finds of possibly domesticated chickpeas in Neolithic archaeological sites come from southeastern Turkey and nearby northern Syria that date to around 8000 b.c.; not until over 5,000 years later does archaeological evidence of chickpeas appear on the Indian subcontinent.
A second method for identifying a crop's or animal's site of domestication is to plot on a map the dates of the domesticated form's first appearance at each locality. The site where it appeared earliest may be its site of initial domestication—especially if the wild ancestor also occurred there, and if the dates of first appearance at other sites become progressively earlier with increasing distance from the putative site of initial domestication, suggesting spread to those other sites. For instance, the earliest known cultivated emmer wheat comes from the Fertile Crescent around 8500 b.c. Soon thereafter, the crop appears progressively farther west, reaching Greece around 6500 B.C. and Germany around 5000 b.c. Those dates suggest domestication of emmer wheat in the Fertile Crescent, a conclusion supported by the fact that ancestral wild emmer wheat is confined to the area extending from Israel to western Iran and Turkey.
However, as we shall see, complications arise in many cases where the same plant or animal was domesticated independently at several different sites. Such cases can often be detected by analyzing the resulting morphological, genetic, or chromosomal differences between specimens of the
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same crop or domestic animal in different areas. For instance, India's zebu breeds of domestic cattle possess humps lacking in western Eurasian cattle breeds, and genetic analyses show that the ancestors of modern Indian and western Eurasian cattle breeds diverged from each other hundreds of thousands of years ago, long before any animals were domesticated anywhere. That is, cattle were domesticated independently in India and western Eurasia, within the last 10,000 years, starting with wild Indian and western Eurasian cattle subspecies that had diverged hundreds of thousands of years earlier.
let's now return to our earlier questions about the rise of food production. Where, when, and how did food production develop in different parts of the globe?
At one extreme are areas in which food production arose altogether independently, with the domestication of many indigenous crops (and, in some cases, animals) before the arrival of any crops or animals from other areas. There are only five such areas for which the evidence is at present detailed and compelling: Southwest Asia, also known as the Near East or Fertile Crescent; China; Mesoamerica (the term applied to central and southern Mexico and adjacent areas of Central America); the Andes of South America, and possibly the adjacent Amazon Basin as well; and the eastern United States (Figure 5.1). Some or all of these centers may actually comprise several nearby centers where food production arose more or less independently, such as North China's Yellow River valley and South China's Yangtze River valley.
In addition to these five areas where food production definitely arose de novo, four others—Africa's Sahel zone, tropical West Africa, Ethiopia, and New Guinea—are candidates for that distinction. However, there is some uncertainty in each case. Although indigenous wild plants were undoubtedly domesticated in Africa's Sahel zone just south of the Sahara, cattle herding may have preceded agriculture there, and it is not yet certain whether those were independently domesticated Sahel cattle or, instead, domestic cattle of Fertile Crescent origin whose arrival triggered local plant domestication. It remains similarly uncertain whether the arrival of those Sahel crops then triggered the undoubted local domestication of indigenous wild plants in tropical West Africa, and whether the arrival of Southwest Asian crops is what triggered the local domestication of indige-