Archaeological Evidence for Domestication and Agriculture
Archaeologist Graeme Barker (2006, p. 414) argues that the change from hunting and gathering to agriculture “was the most profound revolution in human history.” It also profoundly affected other species and continues to do so. The accumulated archaeological evidence reveals that humans independently domesticated local species and developed agriculture in several geographically separate regions relatively soon after the Ice Age ended. From recent applications of genetic research and other methodological advances in archaeology, it’s also clear that more independent instances of prehistoric domestication are likely to be identified, some locations currently identified as possible independent centers of domestication may be deleted from the list, and some plants and animals were probably domesticated multiple times in various places (Armelagos and Harper, 2005). Archaeologists have yet to fully explain how and why agriculture spread and eventually dominated economic life in many parts of the world. Peter Bellwood’s (2005) recent “early farming dispersal hypothesis” integrates archaeology, genetics, and historical linguistics in a highly original argument that early farming grew out of the Mesolithic/Epipaleolithic and spread into new lands with the movement or dispersal of farmers from agricultural “homelands.” Working within what is essentially an environmental approach that identifies human population growth as the main driving factor of farmer dispersal, or “demic diffusion,” this hypothesis offers a controversial explanation for the spread of agriculture, human populations, and languages, as well as major spatial patterns in human genetics (Bellwood, 2005; Bellwood et al., 2007; see Diamond, 1999 for a very similar perspective).
Building on much the same archaeological and genetic data, Graeme Barker (2006) reached different conclusions. He essentially questions the existence of such primal hearths or homelands and traces the roots of this revolution back to early Upper Paleolithic hunter-gatherers. It is too soon to tell which of these interpretations offers a more accurate picture of the spread of prehistoric agriculture. In examining independent centers of domestication around the world, it’s important to realize that the domestication of a local species or two would not necessarily trigger the enormous biocultural consequences we usually associate with the Neolithic period in the Near East. In fact, most altered species retained only local significance. For example, in the Eastern Woodlands of the United States, hunter-gatherers domesticated several small seeded species very early; still, the wild forest products obtained by hunting and gathering retained their primary importance until relatively late prehistoric times, when true farming developed in the East. In most regions, agriculture didn’t develop fully until people were exploiting a mosaic of plants—and sometimes animals, too—from different locations, brought together in various combinations to meet such cultural requirements as nutrition, palatability, hardiness, yield, processing ease, and storage. In the Near East, this threshold was reached around 11,000 ya, when an agricultural complex consisting of wheat, barley, sheep, and goats was widely and rapidly adopted. Plants In most areas where agriculture emerged, early farmers relied on local plant species whose wild relatives grew close by.
Old World cereal grasses, including barley and some wheat varieties, were native throughout the Near East and perhaps into southeastern Europe (Dennell, 1983). Wild varieties of these plants still flourish today over parts of this range. Therefore, barley or wheat domestication could have occurred anywhere in this region, possibly more than once. The same is true for maize and beans in Mexico. So, as we emphasized earlier, domestication and agriculture were independently invented in different regions around the world. As we noted earlier, it’s best to explain these separate but parallel processes from a combined cultural and ecological perspective. We’ve already seen that certain kinds of wild plants were more likely than others to become domesticated. Many of these species tend to grow in regions where a very long dry season follows a short wet period (Harlan, 1992). After the last Ice Age, around 10,000 ya, these conditions existed around the Mediterranean basin and the hilly areas of the Near East and in the dry forests and savanna grasslands of portions of sub-Saharan Africa, India, southern California, southern Mexico, and eastern and western South America. Most scientific understanding of ancient human plant use has come from the archaeobotanical study of preserved seeds, fruits, nutshell fragments, and other plant macrofossils like those shown in Figure 14-4 (Pearsall, 2000). It isn’t easy to preserve seeds, tubers, leaves, and other delicate organic materials for thousands of years. Archaeologists recover some macrofossils from depositional environments that are always dry, wet, or frozen, because all of these conditions slow down or halt the process of decomposition. Most, however, are preserved because the way they were harvested, threshed, processed for consumption, or discarded brought them into contact with enough fire to char them, but not enough heat to reduce them to ash. Once charred, macrofossils preserve well in many kinds of archaeological sites and can often be classified to genus, if not to species. Macrofossils offer direct evidence for important archaeological research, such as reconstructing hunter- gatherer plant use patterns, identifying farming locations, and determining the precise nature of harvested crops. They also provide insights of other kinds. The presence of perennial and biennial weed seeds in an ancient agricultural context may suggest that each year’s farming activities only minimally disturbed the soil; the seed planter may have used a digging stick, hoe, or simple scratch plow. If, on the other hand, seeds of annual weeds predominate, the farmer may have used a moldboard plow—one that turns over the soil as it cuts through it. The major shortcomings of macrofossil- based interpretations of past human plant use are due to potential preservation biases. Most plant macrofossils tend to be preserved because they were charred before entering the archaeological record (see Fig. 14-4 for examples). But not all plant parts char easily, and many don’t char at all. For example, it’s pretty easy to char a bean, but you’ll be disappointed if you try the same thing with a leaf of lettuce. Because the necessary conditions could only be met sometimes in prehistory and by some plants and plant parts, archaeologists are understandably concerned about the validity and reliability of many reconstructions of human plant use that are based only on macrofossils Fortunately, modern archaeologists can turn to several other important sources in their research on prehistoric human plant use. Plant microfossils, such as pollen, phytoliths, and starch grains, often survive even where macrofossils can’t—for example, as residues on the cutting edges of ancient stone tools, inside pottery containers, embedded in the pores of grinding stones, and even trapped in the dental calculus that accumulates on human teeth (Bryant, 2003; Henry and Piperno, 2008). Unlike many macrofossils, these remains preserve readily in a wide range of archaeological contexts; they can be classified as to the kind of plant they represent, if not also to the plant part they represent; and they can be archaeologically present even in sites where macrofossils were destroyed or never deposited. Pollen grains have been a valuable source of environmental and subsistence data for decades (Traverse, 2007). Their strengths are that they’re abundant (as any hay fever sufferer can tell you); the grains are taxonomically distinctive and often can be classified to genus, if not to species; the outer shell of each grain is tough; and the wind-borne dispersal of pollen from seed- producing plants continues before, during, and after humans occupy a particular archaeological site. The main shortcoming of pollen grains is that they tend to preserve poorly in many kinds of open sites, depending on soil acidity, moisture, drainage, and weathering. Phytoliths are microscopic, inorganic structures that form in many seed-producing plants as well as other plants (Piperno, 2006). Like pollen, phytoliths are taxonomically distinctive. They even vary according to where they form in the plant, so phytoliths from leaves can be distinguished from those that formed in the stems and seeds of the same plant. They are also affected by different preservation biases than macrofossils (Cabanes et al., 2011). Are the plant remains at your site unidentifiable, reduced to a powdery ash, or simply not preserved? Not a problem. Chances are the phytoliths from these plants are not only present in the archaeological deposit but also recoverable and identifiable. Starch grains are subcellular particles that form in all plant parts. They are particularly abundant in such economically important portions as seeds and tubers (Coil et al., 2003). They are a useful complement to phytoliths as a data source and are a major tool in archaeological investigations of root crops (Piperno, 2008). Like pollen and phytoliths, starch grains can be taxonomically classified, currently mostly to family or genus. A good example of the archaeological application of starch grain analysis is the recent examination of the surface of a grinding stone found on the floor of one of the 23,000-year-old huts at Ohalo II, in Israel. This study, which was based on carefully sampled residues from cracks and pits in the working surface of the grinding stone, enabled archaeobotanists to identify that it was a specialized implement used to grind wild cereal grasses, including barley (Piperno et al., 2004).
Some plant species also leave biochemical traces in those who consume them. Because temperate and tropical region plants evolved with slightly different processes for photosynthesis, their chemical compositions vary in the ratio of carbon-13 to carbon-12. This distinctive chemical profile gets passed along the food chain, and the bones of the human skeleton may provide evidence of dietary change. For example, their lower 13C levels reveal that females at Grasshopper Pueblo, in east-central Arizona, consumed mostly the local plants they gathered, while their male relatives at first enjoyed more maize, a plant higher in 13C. Later, maize became a staple in everyone’s diet at Grasshopper, resulting in equivalent carbon isotopes in males and females (Ezzo, 1993). Other biochemical analyses, using different isotopes, have been devised to assess overall diet—not necessarily just the domesticated portions—from individual skeletons. A higher ratio of nitrogen-15 to nitrogen-14 (15N/14N), for instance, corresponds to a greater seafood component to the diet (Schoeninger et al., 1983), while a higher strontium-tocalcium (Sr/Ca) ratio indicates that plant foods were of greater dietary importance than meat (Schoeninger, 1981). Other chemicals taken up by bones may inform us about ancient lifeways. For example, lead (Pb) is a trace element found in unusually high concentration in the bones of Romans who drank wine stored in the lead containers typical of that period. The interplay among culture, diet, and biology is, of course, a prime example of biocultural evolution. But to put it more simply, “You are what you eat,” and the odds are increasingly good that archaeologists can measure it. Microfossil analyses complement and greatly extend the valuable insights that archaeobotanists have achieved through the study of macrofossils. Recent key advances include the growing field of archaeogenetics, which applies the methods of molecular genetics to archaeological problems, as well as improvements in radiocarbon dating, which can yield accurate age estimates from samples as small as 100 micrograms (μg)— that’s one ten-thousandth of a gram, or roughly one-fifth the weight of a grain of rice (Armelagos and Harper, 2005). As a result, researchers are now able to more completely understand the prehistory of the human use of plants and the beginnings of agriculture. Animals The process of animal domestication differed from plant domestication, and it probably varied even from one animal species to another. For example, the dog was one of the first domesticated animals; mtDNA evidence suggests an origin between 40,000 and 15,000 ya (Savolainen, 2002), and dogs may even have accompanied late Ice Age huntergatherers (Olsen, 1985). The dog’s relationship with humans was different (and still is) from that of most subsequently domesticated animals. Often valued less for its meat or hide, a dog’s primary role was most likely as a ferocious hunting weapon under at least a bit of human control and direction. As people domesticated other animals, they changed the dog’s behavior even more for service as a herder and later, in the Arctic and among the Native Americans of the Great Plains, as an occasional transporter of possessions.
But the burial of a puppy with a Natufian person who died some 12,000 ya in the Near East suggests that dogs may have also earned a role as pets very early (Davis and Valla, 1978). Most other domesticated animals were maintained solely for their meat at first. Richard J. Harrison’s (1985) insightful analysis of faunal collections from Neolithic sites in Spain and Portugal concludes that meat remained the primary product up until about 4,000 ya, when subsequent changes in herd composition (age and sex ratios), slaughter patterns, and popularity of certain breeds all point to new uses for some livestock. Oxen pulled plows, horses carried people and things, cattle and goats contributed milk products, and sheep were raised for wool. Animal waste became fertilizer in agricultural areas. Leather, horn, and bone—and even social status for the animals’ owners— were other valued by-products. Of course, animals are more mobile than plants, and most of them are no less mobile than the early people pursuing them. So it’s unlikely that hunters could have promoted useful genetic changes in wild animals just by trying to restrict their movements or by selective hunting alone. Possibly, by simultaneously destroying wild predators and reducing the number of competing herbivores, humans became surrogate protectors of the herds, though this arrangement would not have had the genetic impact of actual domestication. Since domestication is a process, not an event, it’s nearly impossible to say precisely when a plant or animal species has been domesticated. The process involves much more than an indication of “tameness” in the presence of humans. More significant are the changes in allele frequencies that result from selective breeding and isolation from wild relatives. People may have started with young animals spared by hunters or, in the case of large and dangerous species such as the auroch (the wild ancestor of domesticated cattle), with individuals that were exceptionally docile or small (Fagan, 1993). Maintained in captivity, these animals could be selectively bred for desirable traits, such as more meat, fat, wool, or strength. Once early farmers were consistently selecting breeding stock according to certain criteria and succeeding in perpetuating those characteristics through subsequent generations, then domestication—that is, evolution— clearly had begun. Overall, not many mammal species were ever domesticated. Those most amenable to domestication are animals that form hierarchical herds, are not likely to flee when frightened, and are not strongly territorial (Diamond, 1989). In other words, animals that will tolerate and transfer their allegiance to human surrogates make the best potential domesticates. Several large Eurasian mammals met these specifications, and cultures of Asia, Europe, and Africa came to rely on sheep and goats, pigs, cattle, and horses (listed here in approximately their order of domestication) as well as water buffalo, camels, reindeer, and a few other regionally significant species. Even fewer New World herd animals were capable of being domesticated. Aside from two South American camelids—the llama and the alpaca—no large American mammal was brought fully under human control (Fig. 14-8). Dogs had probably accompanied the first people into the New World. None of these animals were suitable for transporting or pulling heavy loads—llamas balk at carrying more than about 100 pounds, and Plains Indian dogs dragged only small bundles—so the people of the New World continued to carry their own burdens, till their fields by hand, and hunt and fight on foot until the introduction of the Old World’s livestock in the 1500s. Archaeological evidence of nonhuman animal domestication is subtle and difficult to assess from the bones themselves .
Archaeozoologists, who study the skeletal remains of nonhuman fauna from archaeological sites, have also shown that many of the morphological changes long viewed as good proxy measures of domestication (for example, smaller adult size, horn shape) actually follow, by as much as thousands of years, other archaeological measures of domestication in the Near East, such as distinctive demographic patterns typical of managed herds (Zeder, 2008). For example, at Ganj Dareh, in Iran, around 9,900 ya, villagers slaughtered a high proportion of young male goats, but adult females survived long after reaching reproductive maturity. This culling pattern is typical of herded animals (Zeder, 2008). If these villagers were hunting wild goats, the demographic pattern evident in the goat bones would have been different, emphasizing large adults, mostly males. Among important recent developments, DNA analyses of modern animals and ancient animal bones found in archaeological sites are greatly enhancing our understanding of the domestication of individual animal species as well as the development of farming (Larson, 2011). Although not without their own methodological and interpretive problems, population genetic analyses, coupled with new morphometric analyses and recent advances in small sample radiocarbon dating, hold considerable promise for answering some of the big questions about the origins of domestication.