ORIGINS OF AGRICULTURE 
 

The Problem, and Some Alternative Solutions

For tens of thousands of years, humans were foragers, yet in a relatively short period (ca. 10,000 - 5000 yrs ago) agricultural systems appeared in several widely separated parts of Old and New World, and by 2,000 yrs ago most human populations were dependent on agriculture [see Graph for time depth of foraging and agricultural systems, by region]

This subsistence shift radically transformed human ecology, soc. org., demography, and even art and religion -- yet we still don't have a widely agreed-upon explanation for why (as opposed to how) it occurred

Various hypotheses have been offered over last 50 years:

1. Cultural Progress hypothesis is oldest

It is based on assumption that agricultural life is inherently superior to foraging

Assumes that once bioculturally capable (i.e., once Homo sapiens evolved), humans would inevitably develop agriculture subsistence as part of culturally-mediated progress from simpler to more complex, from arduous nomadic life to comfortable sedentary one, from wild to more and more "civilized" or "settled" state

Archaeologist Robert Braidwood (1964) exemplifies this view; he portrays foragers as

...small groups of people living now in this cave, now in that...as they moved after the animals they hunted [with] no time to think of anything but food and protection...all in all, a savage's existence, and a very tough one. A man who spends his whole life following animals just to kill them to eat, or moving from one berry patch to another, is really living just like an animal himself. [quoted in Winterhalder 1993]

The progress scenario also assumes that people needed lots of time to develop sufficient knowledge of plants and animals to permit domestication, or the good fortune to stumble upon this knowledge

We now know that all foragers have very sophisticated ethnobiological knowledge; there are even a substantial number of cases where foragers cultivate, irrigate, prune, sow seeds, or otherwise manipulate wild plant foods, not to mention the ubiquitous practice of recurrent burning to encourage plant growth for direct consumption or to enhance browsing or grazing for animal prey

We also know of cases where foragers are long-term neighbors of agriculturalists (e.g., Native Calif. adjacent to Southwest), and thus have access to both knowledge and domesticates, yet maintain their reliance on foraging

Obvious conclusion from all this is that foragers needed motivation to develop or adopt agriculture more than they need knowledge

Progress model assumes that motivation is obvious: agriculture would be less work, more reliable, and more productive than foraging

But available evidence suggests that foragers do not necessarily work harder for subsistence, do not necessarily have less reliable resource base, do not usually face greater risk of famine (may even be lower), and do not consider their life inferior to that of subsistence agriculturalists or pastoralists; in fact, many have argued that foragers have easier and more fulfilling life than agriculturalists ("original affluence" view), though as discussed in earlier lecture notes and readings this is controversial

In any case, there is little evidence that agriculture is more efficient (has higher yield per unit labor time) than foraging, and many of the wild ancestors of domesticated crops (grass seeds, small tubers, etc.), termed "proto-domesticates," appear to have been relatively low-yielding, inefficient resources

As for greater productivity (yield per unit land) of agriculture, this is undeniably true, but only under specific ecological conditions, and only for fully domesticated resources that have been genetically modified to increase their yields

The case against the "progress" hypothesis is quite strong, but this only accentuates the explanatory problem -- if foraging economies are so successful, and proto-domesticates so unrewarding, why the repeated and eventually worldwide shift to agriculture?

2. Environmental change hypothesis also very popular in past

First, there is the obvious correlation of agriculture origins with end of Pleistocene

Terminal Pleistocene = time of rapid env. change, extinction of many game species, rise of sea level, rapidly warming climate, etc.

However, closer look by archaeologists reveals problems with this hypothesis

Not first time of major climatic change -- this occurred often in the Pleistocene (the geological epoch in which our genus evolved, lasting from about 2 million years ago to about 12,000 years ago)

Second, agriculture arose independently at several times and places -- and in particular, quite a bit later (after end of Pleistocene) in the Americas [see timeline Graph again]

However, recent detailed paeleoclimatological reconstructions (based on Greenland ice cores & deep-sea mud cores) indicate that during the late Pleistocene (last 100,000 years) climatic fluctuations were extremely abrupt on very short time scales (e.g., annual swings in mean temperature of 10º F. not unusual), and overall climate much drier, colder, and less rich in carbon dioxide (key to plant respiration) than in the Holocene (= last 12,000 years)

Given this, some have recently suggested that agriculture was essentially impossible in the Pleistocene (Richerson et al. 2001)

However, present consensus seems to be that while climate change may be factor in origins of agriculture, is not sufficient in itself

3. Population pressure hypothesis most popular view in recent years

It comes in several versions, but all share basic idea that population growth forces foragers to adopt agriculture, because wild resources become so scarce that eventually farming is worth doing

Fundamental assumption is that agriculture has only one real advantage over foraging -- it can provide more food per unit land (tho at higher labor cost per unit food yield, and often lower nutritional quality)

Getting more food out of given unit of land is termed intensification; the declining efficiency hypothesized or observed to result from intensification (because of higher labor costs) is example of "diminishing returns" (declining marginal output/unit input)

Basic principle of diminishing returns w/ intensification applies within a subsistence mode as well as in shifts from one mode to another

For example, adding low-ranked prey types leads to more total food harvested, but at lower efficiency (greater labor time per calorie or gram harvested)

Population pressure model assumes that increase in population density will lead to greater competition for resources, thus decreased return per unit labor or even outright resource shortage

In thinking about how population pressure might affect shifts from one subsistence mode to another, might expect that each mode has distinct "cost curve" [see Cost-curve graph, from Glassow (1978)]

This model suggests there is a point at which it pays to add strategy B to the subsistence regime, or even to switch to pure B (e.g., from foraging to mixed or pure agriculture; from hunting to broad-spectrum foraging; from swidden to intensive agriculture)

Reasons agriculture might be favored over foraging at higher population densities:

1) crops, esp. seed crops, not as subject to overexploitation, whereas increased harvest and intensification of hunting or even of gathering may rapidly lead to depletion

2) efficient foraging often depends on mobility, whereas opposite holds for agriculture (sedentism necessary to tend & harvest crops, store harvest)

Main problem with population pressure hypothesis is timing -- why did it take thousands of years before population pressure came to head?

Assumes very low growth rates or long-range demographic equilibrium for entire Pleistocene -- true in aggregate, but not necessarily for each local population

I conclude that while population pressure is probably a factor, by assuming rather than explaining the slow population growth or population stability of the Pleistocene, it in effect begs a key question about agricultural origins--why did it happen when and where it did?

4. Coevolution is most recent major hypothesis about agricultural origins

Coevolution refers to mutual evolutionary effects of two interacting species on each other; in this case, the two species or interest are humans (i.e., some particular population of humans) and a plant or animal "proto-domesticate" (PD for short), meaning a wild species that is beginning to be subject to domestication process

Idea here is that certain ways of utilizing wild resources will cause the evolution of features that increase their yield to humans as a byproduct of increasing their own reproduction

Interesting fact about many PDs is that they are opportunistic, "weedy" species that thrive in disturbed locales (e.g., around human settlements); also characterized by great genetic plasticity (favoring rapid "coevolutionary" response to human utilization)

Coevolution hypothesis proposes that proximity of humans and such species would set in motion a process by which humans would modify environment of PD, favoring genetic changes in PD that would make it reproduce better in "disturbed" environment

These same changes would make PD more attractive as human resource, and in turn humans would be selected (via cultural evolution) or motivated (via rational choice) to modify their behavior so as to favor PD propagation, because this would give them more food, and hence more offspring who in turn would continue the coevolutionary interaction

For example, foragers harvesting wild barley or wheat PDs will necessarily harvest more of the variants that retain seed on stalk (tough vs. brittle rachis, to use the botanical terms)

If in turn humans accidentally disperse this variety to new locales (e.g., spilling some seed in middens), or even purposefully plant it, the fitness of this variety will be enhanced and natural selection will favor evolution and spread of tough rachis independent of human intentional selection

David Rindos (botanist turned anthropologist) has detailed several features of PD dispersal and fertilization that would likely be responsive to such coevolutionary dynamics

While coevolution model is quite clever and plausible, it seems more a theory about how the domestication process occurred once it was started than why it got started in the first place; for agricultural origins per se, additional causes (e.g., climate change, population pressure, sedentism, etc.) would seem to be needed

So once again we are faced with a model that is too general to explain specifics of why agriculture arose when & where it did, though it seems likely to be an important part of the answer to this question

Will return to question of how to put various models together, but first let's briefly look at archaeological record of domestication
 

Archaeological Data

Archaeological record indicates that plant & animal domestication arose independently in at least 7, and possibly more, separate locales (see also Table 5.1 in Diamond book):

 Two basic types of agriculture revealed in archaeological record:

1) seed-crop = cereal grains (wheat, maize, rice); occur in simple ecological communities (low species diversity); very productive, but unstable -- cultivation requires constant human intervention (e.g., burning, tilling, weeding)

2) vegeculture = root crops and tree crops (manioc, yams, taro, avocado, potato); complex plant communities (multi-species); less productive but more stable than seed crop, less labor inputs needed

Because seed-crop domestication took place primarily in arid regions with good archaeological preservation, while vegecultures originated in moist tropics (poor preservation, difficult to locate sites), archaeologists know a lot more about seed-crop origins

Earliest well-documented domesticates occur ca. 11,000 yrs ago in Near East (Iran/Iraq area); main domesticates = wheat & barley (along with goats & sheep)

At time when plant and animal domestication occurred in Near East, following conditions prevailed:

1) climatic conditions favorable to wild cereals (barley, wheat, and rye), though not the first time this had happened

2) sedentary settlement in small villages (though still purely foraging economy)

3) higher population densities than had previously occurred in area

4) subsistence based on very broad spectrum of resources (wild cereals; nuts; ungulates = goats, sheep, deer; lots of small game, even invertebrates like snails, clams)

First 2 conditions allowed domestication to proceed, while last 2 suggest adaptive advantage for doing so may have been some factor (population growth, prey depletion, or climate change leading to extinctions) favoring wider diet breadth
 

Optimal Foraging and Agricultural Origins

A detailed argument along these lines has been developed by Bruce Winterhalder & Carol Goland (1997; summarized in Winterhalder & Kennett 2006)

Winterhalder-Goland model explicitly incorporates optimal foraging theory (in form of prey-choice/diet-breadth model) along with population ecology and archaeological information on characteristics of many proto-domesticates

According to this model, since climate deterioration or resource depletion would favor a wider diet breadth (as per prey-choice model), in particular times & places this might mean incorporation of low-ranked but abundant resources (e.g., PDs) into the diet; if this diet-breadth expansion persisted, it could then be followed by intentional or coevolutionary enhancement of yield (i.e, genetic modification of PD into a domesticate)

Although the initial stages of this process would look like just more of the same (foraging intensification), those PDs that were abundant (not same as high-ranked or efficient) and capable of rapid reproduction (e.g., weedy plants) might rapidly become dietary staple (abundant, plus taken whenever encountered, as per prey-choice model) [see Table]

This would lead to rather significant increase in human population and a "ratchet effect" preventing return to original (narrow) diet breadth

As increased human population put greater pressure on high-ranked resources, many of these (i.e., ones that are relatively scarce or have low reproductive rates) might become depleted, causing even greater reliance on PD and intensified coevolution of domesticates

Result would be a gradual evolution of subsistence economy and resource ecology, which we can summarize as series of steps:

1. Pure foragers w/ intermediate diet breadth
2. Foragers w/ wider diet breadth utilizing PDs
3. Incipient agriculturalists with depleted wild resources (narrowing diet breadth) and coevolving PDs
4. Agriculturalists relying on staple domesticates

As it intensifies, agriculture increasingly becomes a strategy of creating artificial ecosystems of domesticated plants & animals almost totally dependent on human intervention to reproduce

These ecosystems are initially maintained by human labor, but eventually require the addition of labor inputs from draft animals, and then machinery running on fossil fuels (tractors, etc.)

This takes us to the next topics in the syllabus:  swidden agriculture, and agricultural intensification

References Cited

Bellwood, Peter (2005) First Farmers: The Origins of Agricultural Societies. Oxford: Blackwell

Braidwood, Robert (1964) Prehistoric Men. Chicago: Chicago Natural History Museum.

Glassow, Michael A. (1978) The concept of carrying capacity in the study of culture process. In Advances in Archaeological Method and Theory, Vol. 1, ed. M.B. Schiffer, pp. 31-48. New York: Academic Press.

Richerson, Peter J., Robert Boyd, and Robert L. Bettinger (2001) Was agriculture impossible during the Pleistocene but mandatory during the Holocene?  A climate change hypothesis. American Antiquity 66(3):387‑411.

Rindos, David (1980) Symbiosis, instability, and the origins and spread of agriculture: a new model. Current Anthropology 21(6): 751-72.

Rindos, David (1984) The Origins of Agriculture: An Evolutionary Perspective. New York: Academic Press.

Rindos, David (1989) Darwinism and its role in the explanation of domestication. In Foraging and Farming, D. R. Harris and G. C. Hillman, eds, pp. 27-41. London: Unwin Hyman.

Winterhalder, Bruce (1993) Work, resources, and population in foraging societies. Man 28:321‑340.

Winterhalder, Bruce and Carol Goland. (1997) An evolutionary ecology perspective on diet choice, risk, and plant domestication. In Kristen J. Gremillian, ed. Peoples, Plants, and Landscapes: Studies in Paleoethnobotany. Tuscaloosa, AL: U. of Alabama Press.

Winterhalder, Bruce and Douglas Kennett (2006) Behavioral ecology and the transition from hunting and gathering to agriculture. In Behavioral Ecology and the Transition to Agriculture, ed. D. Kennett and B. Winterhalder, pp. 1-21. Berkeley: U. of California Press.