Muhlenbein, M. (ed.) 2015. Basics in Human Evolution. Elsevier ( in press ). Chapter 27: agriculturalism

Dept. of Geography, SUNY-Binghamton, Binghamton NY 13902-6000

(607) 777-6732 (phone); (607) 777-6456 (fax); mablum@binghamton.edu

Glossary

Agro-pastoralism – any combination of farming with herding of livestock animals.

Alien species – one introduced to a region after 1492, usually due to human transport either intentional or otherwise

Allopolyploidy – A doubling of the number of sets of chromosomes that happens to occur when two species hybridize; see ‘Polyploid’

Archaeobotany – the study of botanical remains found at archaeological sites

Carrying capacity – the maximum number of individuals of a species that the environment can support

Cultivation- the intentional planting and growing of plants

Dry fallow – repeatedly plowing a field to prevent weed growth, so that rain will be stored in the soil for a crop that will be planted subsequently

Evolutionary mismatch – when a species or genotype becomes maladapted in some way to its environment as a result of environmental change (including the evolution of other species)

Holocene – the past 10,000 years or so, i.e., since the glaciers of the last Glacial period melted off

Indehiscence – the retention of seeds on the plant

Introgressive hybridization – hybridization that leads to the spread of genes from one species into another

Megafauna – wild animals weighing at least 50 kg. About ¾ to 4/5 of such species suffered extinction in North and South America about the time of the arrival of humans

Neolithic – literally, the ‘New Stone Age’, referring to a change in stone tools that occurs about 10,000 years ago, but now usually taken to refer to the start of farming

Outcrossers – species that cannot self-fertilize

Paleolithic – the ‘Old Stone Age’, before the Neolithic, when all humans apparently were hunter-gatherers

Pleistocene – the geological era preceding the Holocene, characterized by a long series of Glaciations, alternating with much shorter periods of climate similar to that of today

Polyploid – Having more than two sets of chromosomes; humans are diploid, i.e., we get one set from our mother and another set from our father, but many plant and other species have more sets of chromosomes. A tetraploid has four sets, a hexaploid has six, and so on

Agriculture; Agricultural Origins; Columbian Exchange; Cultural Evolution; Domestication; Evolutionary Mismatch; Fertile Crescent; Green Revolution; Niche Construction; Paleo Diet

Farming began in several regions of the globe at approximately the same time, suggesting an environmental rather than a socio-cultural cause. The spread of agriculture was an exercise in niche construction, essentially transferring (modified) whole ecosystems from the source regions elsewhere. Crop domestication entailed significant morphological, but relatively little genetic change, while humans took over the competitive, protective, and dispersal functions that wild plants must handle themselves. Dietary changes due to agriculture presumably have been great, and these changes produced evolutionary mismatch, to which we have responded genetically, microbially, and/or culturally.

In the original, Hebrew version of the Biblical story of Eden, it is not an apple that causes Adam and Eve’s downfall, but an unspecified fruit.According to the Talmudic commentaries, that fruit was wheat, an interpretation echoed in the Koran and in the Black Book of the Yezidis. Archaeobotanical investigations suggest that emmer wheat was the plant around which early Near Eastern agriculture centered (Blumler, 1998b; Nesbitt, 2002), and when one considers that Cain and Abel were respectively a herder and a farmer, the Eden story becomes a parable about the transition from hunter-gathering to agro-pastoralism.This view of agriculture as a fall from hunter-gatherer grace has many adherents today, notably among environmentalists who worry that we are overpopulating the planet, and also among those who promote the ‘paleo diet’ to which they argue we are adapted (Cordain, 2001). Advocates of the paleo diet argue that we suffer from evolutionary mismatch – that we are poorly adapted to our current diet and lifestyle because our genes still reflect adaptation as hunter-gatherers (see Low et al. chapter). In contrast, Zuk (2013) argues that reconstructions of pre-agricultural diets and lifestyles are ‘paleofantasies’ given the fragmentary nature of the fossil evidence, the diversity of modern hunter-gatherer diets, and because we are not genetically identical to our ancestors: we have evolved, in some respects perhaps dramatically, in response to agriculture and the changes it wrought.This review will cover the origin and spread of agriculture, beliefs about how it came into being, the evolution of crop plants, co-evolutionary interactions, and the impacts on humans including the degree to which we may be mismatched today.

Agriculture is the cultivation of domesticated plants for food. Cultivation can vary from a huge agribusiness field, to a pot on a patio. Today, farming is practiced almost everywhere that it can be, although it is disappearing from lands that are urbanizing. At the same time, the definition of ‘arable’ is continuing to expand, as humans find ways to farm lands formerly thought unsuitable. For example, the cerrado of Mato Grasso, Brazil, was largely restricted to cattle ranching until recently because of its infertile soils; now much of it is in soy after massive P fertilization.Although farming was always primarily about food, some crops from very early times were put to other uses such as clothes/textiles (flax, cotton) and building construction (grain straw for roofs, and mixed with clay for adobe). Today, crops such as cornand wheat have many industrial uses,besides serving as primary staples (Head et al., 2012).

Without agriculture, we never would have seen so much that followed: cities, civilization, the Industrial Revolution, and in general, social complexity and hierarchy (though some hunter-gatherer societies with abundant resources did achieve considerable complexity, including, as in the case of Pacific Northwest groups, widespread slavery). Much of the paleodiet discourse presumes that all of this is unnatural. But we are not the only farmers: several species of ants, termites, and beetles cultivate fungi (Farrell et al., 2001; Schultz and Brady, 2008). The fungiare not known to occur outside cultivation, and thus, qualify as domesticated species (Diamond, 1998).

If fully domesticated, a species will beso altered from its wild form that it no longer can survive without human assistance (Blumler and Byrne, 1991). A classic example is maize: after maturity, its seeds remain attached to the cob, which remains attached to the stalk. Unless harvested and sown, the seeds cannot reach the ground. If the stalk falls over some seeds may contact the soil, but even if they germinate they will do so in a mass, competing with each other. In contrast, the wild ancestor, teosinte, produces seeds that detach individually from the plant when ripe. Our agricultural plants and animals vary in their degree of domestication, and not all are completely helpless without our assistance. Pigs, for instance, are notorious for their ability to survive in the wild. Some herbs, such as spearmint, are little altered from the wild condition, and probably could thrive without humans. Even cabbage frequently establishes along coastal cliffs, reflecting its derivation from a wild Mediterranean sea-cliff species. Several feral ‘cabbage’populations are scattered along the Pacific Coast of California. In an unpublished investigation, Berkeley geneticist Herbert Baker (personal communication, 1994) cultivated seeds from these plants and found that one population was feral broccoli, another was kale, still another green cabbage, and so on (despite their striking morphological differences, all are the same species, Brassica oleracea). In establishing in the wild, they had converged on a common morphology, little distinct from the wild phenotype in its native habitat. (This example also illustrates that crop evolution, while giving rise to striking morphological changes, may do so through alterations in relatively few genes) (cf. Paterson, 2002).

From an evolutionary perspective, crop domestication is fascinating (Hancock, 2005; Purugganan and Fuller, 2011). Darwin (1859; 1883) drew upon what was then known about crop variation to supporthis theory of evolution. More recently, crop geneticists have played a major role in the study of introgressive hybridization, and the recognition of its importance in plant evolution (Hancock, 2005). Increasingly, domestication is being studied to shed light upon fundamental theoretical questions about evolution (Hancock, 2005; Purugganan and Fuller, 2009). Crop evolutionhas given us examples of recent speciation, contra the beliefs of Creationists: for instance, bread wheat speciated after agriculture began (Dvorak et al., 1998). If one considers the major evolutionary trends of the past 10,000 years (Table 1), agriculture figures prominently. In addition to fostering the evolution of domesticates and ‘pests’, it is largely responsible for massive invasions of alien species, which in turn are a major cause of extinction (Blumler, 2011).

On present evidence, humans began farming inseveral regions of the globe at approximately the same time (Table 2 and Fig. 1). I have given the consensus origin date for each region, to the extent that it can be said that there is a consensus. The timing is best established for the Fertile Crescent, albeit with some variancein opinion (Nesbitt, 2002; Ozdogan, 2002; Zeder, 2011; Zeder et al., 2006; Riehl et al., 2013). For Mexico, we have Smith’s (1997) report of squash (Cucurbitapepo) domestication, supported by evidence for maize domestication within a millennium or sothereafter(Matsuoka et al., 2004; Piperno et al., 2009). Similarly, for China, genetic and archaeological studies support early domestication of rice (MacNeish and Libby, 1995; Huang et al., 2012), and foxtail and broomcorn millets (Crawford, 2009; Lu et al., 2009; Yang et al., 2012). Many archaeologists regard North (millet) and Central (rice) China as separate centers of agricultural origin, but given their proximity, I chose to combine them. The consensus date for New Guinea (Denham and Haberle, 2008)rests upon admittedly limited archaeological evidence. Finally, the Andean region is clearly ancient, but the date given here reflects the interpretation of the authors of a single study (Dillehay et al., 2007), who found domesticated plants in western Peru that must have originated earlier on the east side of the Andes. As Blumler (1992b) pointed out, the wild progenitorsof many Andean crops are located on the Bolivian side of the range, while the archaeology has concentrated on the western (Peruvian) side. Consequently, the picture is somewhat confused (see also Piperno, 2011). The comparative timing of origins matters in that it bears on highly contentious debates over the causes of agricultural origins, and diffusion vs. independent invention (Blumler, 1992a; 1996; 2002), as discussed below.

The wild progenitors of the earliest crops, for the most part, were large-seeded annuals, or plants with starchy tubers (Blumler, 1992b). They were mesophytes: fast-growing plants adapted to fertile soil conditions (Blumler, 1994). Subsequent domestications, often of edible weeds growing in the agricultural fields, also tended to be of mesophyticspecies. For instance, many of our vegetables are from the mustard and spinach families. These differ from all other plant families in that they have no association with mycorrhizal fungi. The otherwise ubiquitous co-evolutionary relationship between plants and mycorrhizae, one in which the plant sends sugar to the fungus, and the latter after decomposing organic matter passesmineral nutrients, particularly phosphorus, to the plant, is beneficial to plants especially on poor soil. The absence of mycorrhizal associates for mustards and spinaches indicates that they ‘expect’to grow in soils with excellent nutrient availability. They presumably can grow faster than other species under such conditions, since no sugar must be diverted to a fungal partner.

Competing Hypotheses

Rare events, such as the initiation of farming, can be modeled as having a Poisson distribution. As such, the likelihood of several independent origins occurring synchronously in widely separated regions is highly unlikely if the primary cause is endogenous, i.e., the result of socio-cultural developments. Synchronous origins would be more plausibly triggered by some global environmental change. Similarly, synchronicity of widely separated regions tends to rule out diffusion from one to the other. Given how ancient the Mexican center is, it is unlikely that its origin was influenced by Old World farmers (but see Erickson et al., 2005). Two regions, the Mississippi Valley and the Sahel, may have independently undertaken farming, but given their proximity to much earlier agricultural centers (Mexico and the Fertile Crescent, respectively), they also may have adopted agriculture as a consequence of diffusion.

The literature on the transition from hunter-gathering to agriculture is necessarily speculative. Despite remarkable advances in both the cytogenetic study of the crops, and in the collection and interpretation of archaeobotanical information, the empirical evidence remainsincomplete, and legitimately subject to varying interpretations. The best evidence comes from the Fertile Crescent, though even there it is not nearly as clear as one would wish (e.g., Fuller et al., 2012, vs. Abbo et al., 2013). From Mexico and China there is some good, though limited, evidence. The evidence from the other centers is sketchy.

Some old ideas, such as that farming was invented as part of the march of ‘progress’, are no longer accepted. The publication of Man the Hunter (Lee and DeVore, 1968), which demonstrated that the hunter-gathering !KungSan work few hours in comparison to members of modern societies, let alone traditional farmers, was highly influential.Consequently, Malthusian, population pressure hypotheses (e.g., Cohen, 1977) became popular, and remain so today (Cohen, 2009; Lambert, 2009). Cohen and Armelagos (1984) gathered evidence that Neolithic farmers in the Near East suffered from malnutrition and for the women, even from the physical act of grinding grain for long periods of time. In contrast, Gage (2005) showed that there is little evidence this was other than a temporary effect related to the initial adoption of farming.

It is difficult to reconcile population pressure hypotheses with the evidence for very early origin in Mexico, which had only recently been peopled, and where population density clearly was very low. Kent Flannery, who dominated agricultural origin theorizing for a long time, and who inclined towards scenarios involving human manipulation of environment with population pressure, was aware of this problem. He suggested that although Mexico was not densely populated when farming began, it had reached the point where people had “settled in” (Flannery, 1986:11). Alternatively, perhaps the elimination of the native megafauna through overhunting (Martin and Wright, 1967; Flannery, 2001), more or less coincident with the Younger Dryasclimatic episode (see below), created a depletion of food resources that necessitated experimentation with cultivation.

Numerous scholars have advocated some version of a gradual intensification or ‘continuum’ hypothesis, in which proto-agricultural practices such as replanting of tubers grade into horticultureand eventually, full-fledged farming (e.g., Harris, 1977; Denham et al., 2009; Zeder and Smith, 2009). These ideas derive in part from so-called ‘evolutionary’ models in anthropology, which are not reallyin accord with evolutionary theory, and are more properly termed developmental (Orlove, 1980; Blumler, 1996). Since they propose a gradual, incremental growth of pre-agricultural practices, they also do not align well with the current consensus that New World origin is almost as early as in the Fertile Crescent and China. Moreover, the early cereal domesticates in the Fertile Crescent and China seem poorly suited to a horticultural system (in contrast, the New Guinean crops, and squashes in Mexico and Peru/Bolivia, do seem appropriate). In contrast to the gradualist paradigm,Ozdogan (2002: 156) concluded: “The rate of change in the Neolithic of the Near East can only be compared with that of the industrial revolution…”

Another set of hypotheses involves environmental change, and has become more popular as the concern over global warming has increased appreciation of the magnitude of climate change in the past. Several scholars have argued that farming was not feasible during the glacial periods of the Pleistocene, when it was not only colder, but drier, and far more variable as well (Sherratt, 1997; Richerson et al., 2001; Bettinger et al., 2009). Sage (1995) pointed out that the low CO2 levels in the atmosphere during the Pleistocene would have posed a problem for plants, which obtain their carbon from the air. Byrne (1987) argued that climates became more seasonal during the Pleistocene-Holocene transition, and that farming began in regions with seasonal drought. The first domesticates, annual plants and those with starchy tubers, are well-adapted to seasonal drought. Blumler (1992b; 1998a; cf. Diamond, 1997: Chapter 8) showed that large-seeded annual grasses are strongly associated with regions of seasonal drought, and that such plants contributed disproportionately to early agriculture. The Fertile Crescent has the most extreme seasonality of rainfall in the world, both Mexico and the central Andes have pronounced seasonal drought, and China has a monsoon climate; but New Guinea does not fit the pattern very well. It does not have a seasonal drought, and none of its early domesticates were annual seed crops.

The timing of agricultural origins, which has been pushed back repeatedly so that now it follows only shortly after the Younger Dryas, has given rise to the suggestion that farming was a response to severe, natural, environmental disruption (Moore and Hillman, 1992; Moore et al., 2000; Bar-Yosef, 2002; Harris, 2003; Wells, 2010). The Younger Dryas was a dramatic, probably global, reversal back to glacial-like conditions that lasted about 1500 years, and thatended with extremely rapid warming – in Greenland, temperature may have increased seven degrees Centigrade in less than ten years (Alley et al., 1993)! (Compare to the present day, when global temperature has risen about one degree in one century). It also appears to have been drier, and one study has suggested that the Dead Sea dried up completely (Yechieli et al., 1993). Since the Dead Sea is 400 km deep, and receives only about 50 mm of rainfall per year, the aridity necessary to eliminate it is difficult to conceive. In a sense, then, Gordon Childe’s (1951) oasis hypothesis, long fallen out of favor, is now back in play.

Finally, there are theories specific to the Fertile Crescent, emphasizing trade networks (Runnels and van Andel, 1988), or religion (Ozdogan,2002). The obsidian trade extended over hundreds of kilometers, including to Greek islands (i.e., the traders had boats); the major deposits are located in southeast Turkey (Wright and Gordus, 1969), close to where agriculture apparently began (Lev-Yadun et al., 2000; Zeder, 2006). Religious temples existed in southeast Turkey before cultivation is attested there, and Ozdogan reported that the society was already highly stratified. Thus, the old, Marxist view, that organized religion was anoutgrowth of agriculture, is also called into question (Blumler, 1993b).

While each scholar has her/his predilection, it should be emphasized that much depends on the origin dates, and that these are subject to change. For instance, some scholars had suggested that agriculture came to the Mississippi Valley from Mexico, given the early dates initially reported from the famous site of Tehuacan (Byers, 1967). But then the more accurate AMS dating technique was applied, and the site turned out to be several thousand years younger than formerly thought (Long et al., 1989; Kaplan and Lynch, 1999). Fritz (1994) noted, correctly, that this strengthened the case for independent origin in the Mississippi Valley. Subsequently, however, Mexican origin dates were pushed back again, and are now even earlier than before. Even the sunflower, long believed to be an American domesticate, has its earliest date from a site in Mexico (Pope et al., 2001; Lentz et al., 2008); this suggests either that it was domesticated in Mexico, or more likely in my view, that there were maritime trade relations between Mexico and the Mississippi Valley. Smith (1992; 1996) and others have argued that Mexican crops came to the Mississippi late, via the American Southwest. But a maritime route would shorten the necessary time for dispersal. Given that the sunflower find is located near the Gulf of Mexico, and since there are other indications of interactions between the two regions, a maritime connection seems possible (Blumler, 1998a). The sunflower find is datedlong after Mexican farming began, and about the time that the earliest agriculture is attested in the Mississippi Valley (cf. Smith, 2006).

Currently, all five (six, if one separates north and central China) early centers cluster chronologically. The coincidence in timing suggests some global impact on humans, most likely the Younger Dryas. It is difficult to reconcile other theories with this synchronicity. But since further revision of the dates is not only possible but likely, it would be premature to rule out other explanations.

Once it appeared, agriculture was self-reinforcing because it enabled population growth and craft specialization (technological advances) (Diamond, 2002). Farming increased the food supply by removing vegetation that humans could not digest, and replacing it with plants that we could eat. The adoption and diffusion of farming raised human carrying capacity dramatically. One estimate is that a global hunter-gatherer population of one to five million increased to several hundred million as farming spread (Zuk, 2013).

Agriculture spread (diffused) from the origin centers, with the diffusion being most dramatic in the case of the Fertile Crescent (Fig. 2) (Diamond, 2002; cf. Bellwood, 2009). As agriculture spread, farmers moved into territory already occupied by hunter-gatherers. The latter would have had three choices: a) fight; b) retreat; or c) become farmers themselves. While they might have won some battles initially, in the long run they would have lost to the superior numbers and technological superiority of the farmers. On the other hand, if they retreated they would have eventually run out of land. Thus, in the long run their only option, other than extinction, would be to become farmers. (I am presuming here that the territory in question is arable). Diamond (2002) argued that hunter-gatherers would have mostly rejected agriculture, and I agree that at first that would have been so, but ultimately they would have folded into the system or been wiped out. Diamond and others also have related the spread of language groups such as Indo-European to the invention of agriculture, but this is contested to some extent by linguists. There is no reason to think that the original farmers were the ones who also spread the technology; it might more likely have been a group or groups on the edge of a given agricultural origin center who became mobile. Certainly some of the examples that Diamond (2002) offers, such as the Bantu, the Polynesians, and the Korean migration into Japan, seem to fit the latter scenario. In any case, asimilar process is playing out today, which we call globalization: traditional societies, such as hunter-gatherers, slash and burn farmers, and nomadic pastoralists, are being forced to join the world system whether they wish to or not.

As farming spread from the origin centers, it was increasingly likely to encounter environments to which the crops were not well-adapted, creating an evolutionary mismatch. In some cases, this mismatch could not be overcome: Near Eastern cereals could not penetrate Africa south of Egypt and the Ethiopian highlands, because they require cool weather during the growing season (Fig. 2). Near Eastern crops initially also were delayed in spreading north from the Mediterranean region into Europe, perhaps due to a need to adjust phenologically to daylength differences, or to winter cold, but eventually they succeeded (Colledge et al., 2005). But in northwest Europe, which has a cool, rainy climate, there continued to be a mismatch for Near Eastern plants during ripening, when they ‘expected’hot, dry conditions. Wheat and barley have upright spikes that are subject to fungal attack if rain falls upon them - not a problem in the bone-dry early summer of the Fertile Crescent. Also, rain at this season can cause sprouting within the ear. In contrast, oat species produce hanging spikelets that shed rain water. Thus, they are better adapted to cool, wet conditions post-anthesis. Although wild oat is a dominant species across the Mediterranean, the domesticates, common oat and sand oat,are not Near Eastern in origin (Vavilov, 1926). The Near Eastern grains can grow in northwest Europe, and over time presumably evolved to be better adapted, yet were beset with problems such as ergot, especially during cooler, wetter epochs such as the Little Ice Age. The connection between ergot poisoning and the medieval witch trials illustrates that our domesticated plants have had complex influences on our societies. Much has been made of the characteristically low cereal yields in northwest Europe during the Middle Ages; they probably should not be taken as representative of yields in the Near Eastern Neolithic, however.

One way to address evolutionary mismatch is through ‘niche construction’ (Odling-Smee et al., 2003). The beaver is the classic example, which literally constructs a habitat favorable for itself. Today, dense wild cereal stands are characteristic of annual grasslands onhard limestone and basalt in the Fertile Crescent (Zohary, 1969; Blumler, 1993c; 1999). Near Eastern farmers attempted to create the equivalent of such stands in other habitats, sowing their grains where the wild plantsnormally would not thrive, and nurturing them at the expense of the natural dominants of the vegetation. Sherratt (1980) pointed out that cultivation took place initially on hydromorphic soil. Such soil is not part of the natural habitat for wild Near Eastern cereals and legumes (Zohary, 1969; Blumler, 1999; 2002). In all likelihood, perennials are more competitive on such soils than on the soils that the wild cereals naturally dominate (cf. Blumler, 1993c). Blumler and Waines (2009) showed that evolutionary changes in phenology and growth habit would have been necessary to adapt cereals to Near Eastern hydromorphic soils.

Initially, farmers re-created the annual grassland/wild cereal plant community in other environments within the Fertile Crescent; subsequently, as agriculture spread out of the region, into places with climates more favorable to perennials, plowing served to remove these competitors, thus in effect enabling the Fertile Crescent annual grassland to spread. Many of the natural associates of wild cereals came along as weeds, and some of them such as wild oat, black mustard, and wild carrot, were later domesticated. Thus, ultimately an entire plant community was able to expand its range, albeit with significant deletions and additions of species.

A striking illustration is the ‘rabi’system in India (Fig. 2). Supposedly from the Arabic word for spring, reflecting the season at which harvest occurs, rabi also is an Indo-European word for mustard (as in broccoli rabe, rape seed oil, etc.), one of the most important of the rabi crops, as well as a major rabiweed. Lowland India is tropical, but it cools off slightly in the fall especially in the north. The monsoon rains come in summer. But the Near Eastern crops, so important in Indian agriculture, are adapted to winter rain and summer drought. As they moved east, farmers could find close climatic analogues as far as the Indus, but not beyond. Instead, they learned to dry fallow in summer, enabling the soil to accumulate monsoon moisture, and then planted the Near Eastern grains and legumes in fall as the temperature cooled slightly. The plants grow on the stored moisture, and normally will produce a good crop so long as there is a small amount of additional rainfall during the winter and spring (e.g., Howard and Howard, 1909). The better-known system characteristic of Egypt’s Nile Valley in ancient times – indeed, up until the construction of Aswan Dam - is a similar example of ecological niche construction to favor the cereals (and unintentionally, their associated weeds), and consequently, farmers.

In this section, I discuss only a few of the many consequences of agriculture’s origin and spread. Perhaps the most significant consequence of agriculture, and the population growth that followed, was the emergence of serious epidemic diseases (McNeill, 1976; Diamond, 1997). ‘Crowd’ diseases such as measles require a minimum human population size to remain established. Cholera is spread through water polluted with infected feces, which only becomes an issue when people must return to the same water source repeatedly because they are sedentary. (Sedentism is generally thought to be an outgrowth of agriculture, except in the Fertile Crescent where permanent villages such as Jericho may have preceded farming). Many of the major epidemic diseases are zoonotic (cf. M. Little’s chapter), spreading from domesticated animals to humans: examples include smallpox, tuberculosis, diphtheria, and influenza (Crosby, 1973; McNeill, 1976; Diamond, 2002). Farmers lived in such close proximity to domesticated animals, sometimes sleeping in the same building with them, that it is not surprising that some diseases were able to adapt tohumans. Still other diseases, such as malaria, were present before agriculture, but became more prevalent during periods of forest clearance or expansion of swamps due to accelerated soil erosion off hillslopes.

As Crosby (1973; see also Diamond, 1997) pointed out, animal domestication, occurring as it did primarily in the Old World, by giving rise to a host of epidemic diseases ultimately was a major reason that Europeans were able to conquer much of the world after 1492; it also explains why they were not able to maintain control of Africa and Asia, but did retain control over the Americas and Australia. In the former, the inhabitants were just as disease resistant as Europeans, if not more so, while in the latter, isolated from events in the Old World for many millennia, the native populations suffered enormous mortality from the diseases that Europeans brought with them.

Cochran and Hardenning (2009) argued that population growth since the Agricultural Revolution must have speeded up human evolution, because positive mutations would have appeared more frequently. Consequently, they argued, humans probably have evolved greater intelligence. In contrast, Diamond (1997) argued that people in advanced societies have less need for intelligence, because in effect their needs are well taken care of by the larger society; consequently, he argued that humans have become stupider since agriculture began. Diamond’s argument illustrates that a ‘positive’ mutation can in theory produce a decrease in intelligence just as readily as an increase. In fact, given how much energy it takes to nourish a human brain, the former seems more likely than the latter.

Finally, vegetarianism was not an option for hunter-gatherers because their primary protein sources, dairy and legumes, are post-agricultural, on the whole (see discussion re legumes, below). In meat-poor regions such as South India, vegetarianism became a widelyadopted practice.