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GRANDMOTHERS, GATHERING, AND THE EVOLUTION OF HUMAN DIETS


O'CONNELL, James, and HAWKES, Kristen

Department of Anthropology
University of Utah
Salt Lake City, UT 84112, USA
phone: (801)581-6251
FAX: (801)581-6252
email: hawkes@anthro.utah.edu
         oconnell@anthro.utah.edu

 

It has long been held that big game hunting is THE key development in human evolutionary history, facilitating the appearance of patterns in reproduction, social organization, and life history fundamental to the modern human condition. Though this view has been challenged strongly in recent years, it persists as the conventional wisdom, largely for lack of a plausible alternative. Recent research on women's time allocation and food sharing among tropical hunter-gatherers now provides the basis for such an alternative.

 

The problem with big game hunting

The appeal of big game hunting as an important evolutionary force lies in the common assumption that hunting and related paternal provisioning are essential to child rearing among human foragers: mother is seen as unable to bear, feed and raise children on her own; hence relies on husband/father for critical nutritional support, especially in the form of meat. This makes dating the first appearance of this pattern the fundamental problem in human origins research. The common association between stone tools and the bones of large animals at sites of Pleistocene age suggests to many that it may be quite old, possibly originating with Homo erectus nearly two million years ago (e.g. Gowlett 1993).

Despite its widespread acceptance, there are good reasons to be skeptical about the underlying assumption. Most important is the observation that big game hunting is actually a poor way to support a family. Among the Tanzanian Hadza, for example, men armed with bows and poisoned arrows operating in a game-rich habitat acquire large animal prey only about once every thirty hunter-days, not nearly often enough to feed their children effectively. They could do better as provisioners by taking small game or plant foods, yet choose not to, which suggests that big game hunting serves some other purpose unrelated to offspring survivorship (Hawkes et al. 1991). Whatever it is, reliable support for children must come from elsewhere.

 

The importance of women's foraging and food sharing

Recent research on Hadza time allocation and foraging returns shows that at least among these low latitude foragers, women's gathering is the source (Hawkes et al. 1997). The most difficult time of the year for the Hadza is the dry season, when foods younger children can procure for themselves are unavailable. Mothers respond by provisioning youngsters with foods they themselves can procure daily and at relatively high rates, but that their children cannot, largely because of handling requirements. Tubers, which require substantial upper body strength and endurance to collect and the ability to control fire in processing, are a good example.

Provisioning of this sort has at least two important implications: 1) it allows the Hadza to operate in times and places where they otherwise could not if, as among other primates, weaned offspring were responsible for feeding themselves; 2) it lets another adult assist in the process allowing mother to turn her attention to the next pregnancy that much sooner. Quantitative data on time allocation, foraging returns, and changes in children's nutritional status indicate that, among the Hadza, that other adult is typically grandmother. Senior Hadza women forage long hours every day, enjoy high returns for effort, and provision their grandchildren effectively, especially when their daughters are nursing new infants (Hawkes et al. 1989, 1997). Their support is crucial to both daughters' fecundity and grandchildren's survivorship, with important implications for grandmothers' own fitness.

 

The evolution of human life histories

These observations suggest an hypothesis for the evolution of human life histories, particularly for the prevalence of long post-menopausal life spans unique to us among the primates (Hawkes et al. 1997, 1998):

These changes in foraging, food sharing, and life history should also have had important ecological implications, notably a sharp increase in geographical range due to the relaxation of limits previously imposed by juvenile foraging capabilities.

 

Grandmothering and Homo erectus

Available data point to early African Homo erectus (cf. ergaster, dated ca. 1.8 myr) as the first hominid with a life history pattern different from that of great apes (Smith and Tompkins 1995). Brain size (correlated with life span across the primates) suggests maximum ages of 70-75 years; dental eruption schedules an age at maturity of roughly 15 years. Both values are intermediate between those of apes and australopithecines on the one hand, modern humans on the other. Body size shows the expected coincident increase, up roughly 70% for erectus females relative to australopithecines (McHenry 1994). Extended post-menopausal life spans are implied.

Geographical distribution also displays significant change: erectus is found in a broader range of habitats than any previous hominid and is the first to occur beyond Africa (Klein 1989). Early examples are found as far east as Java, and far north as latitude 50 degrees in both Europe and East Asia.

As anticipated by the grandmother hypothesis, these changes in life history are ecology are correlated with sharp adjustments in climate, notably a marked shift (at ca. 1.7-1.9 myr) toward cooler, drier, more seasonal conditions of the kind likely to have reduced the availability of resources easily taken by juveniles, especially in the dry season (Cerling 1992; O'Connell et al. 1998).

Resources newly or more intensively exploited in connection with provisioning are not easily specified, but may have included "underground storage organs" (USOs), or tubers. USOs are widely available in tropical through temperate latitudes, are high in carbohydrate content, were commonly exploited by ethnographically-known human foragers, but are not much taken by other primates, probably because of their high collecting and processing costs. Though there is no direct evidence of tuber exploitation by erectus, the availability of lithic technology suitable for the manufacture of digging sticks, early evidence for fire (necessary for cooking and the elimination of toxic compounds in some taxa), and the general coincidence between the distribution of erectus and of historic foraging economies based on heavy tuber use are all consistent with the notion they were (O'Connell et al. 1998).

 

Implications for social organization

A model of Homo erectus ecology grounded on adult female foraging and food sharing practices has unexpected implications for current ideas about early hominid social organization. In recent years most analysts have appealed to alleged patterns of male philopatry in common chimpanzees and patrilocality among ethnographically known hunter-gatherers in suggesting that similar forms of sex-linked co-residence and dispersal characterized all ancestral hominids. Despite widespread acceptance(e.g. Rodseth et al. 1991), there are good reasons to be skeptical of this argument (Hawkes et al. 1997).

The "grandmother hypothesis" suggests a very different pattern: heavy reliance on high cost/high yield resources in connection with offspring provisioning should have given daughters a strong incentive to remain with their natal group. As daughters grew, they acquired the strength and skill needed to feed younger siblings. When they matured, the assistance of aging mothers continued to enhance the benefits of proximity. From this perspective, mother-infant food sharing, combined with assistance from grandmothers, points to persistent co-residence among related females. The stronger the pattern, the greater the incentive for males to leave their natal group. Matrilocality may have been the dominant model of hominid social organization throughout most of the Lower and early Middle Pleistocene (1.8-<0.5 myr) (O'Connell et al. 1998).

 

References

Cerling, T. E. (1992). Development of grasslands and savannas in East Africa during the Neogene. Palaeogeography. Palaeoclimatology, Palaeoecology 97, 241-247.

Charnov, E. L. (1993). Life history Invariants: Some Explorations of Symmetry in Evolutionary Ecology. Oxford: Oxford University Press.

Gowlett, J. A. J. (1993). Ascent to Civilization: The Archaeology of Early Humans (2nd ed.). New York: McGraw-Hill.

Hawkes, K. (1993). Why hunter-gatherers work: An ancient version of the problem of public goods. Current Anthropology 34, 341-361.

Hawkes, K., O'Connell, J. F. & Blurton Jones, N. G. (1989). Hardworking Hadza grandmothers. In (Standen, V. & Foley, R., Eds.) Comparative Socioecology of Mammals and Man, pp. 341-366. London: Blackwell.

Hawkes, K., O'Connell, J. F. & Blurton Jones, N. G. (1991). Hunting income patterns among the Hadza: Big game, common goods, foraging goals, and the evolution of the human diet. Philosophical Transactions of the Royal Society, series B, 334, 243-251.

Hawkes, K., O'Connell, J. F. & Blurton Jones, N. G. (1997). Hadza women's time allocation, offspring provisioning, and the evolution of post-menopausal lifespans. Current Anthropology 38, 551-578.

Hawkes, K., O'Connell, J. F., Blurton Jones, N. G., Charnov, E. L. & Alvarez, H. (1998). Grandmothering, menopause, and the evolution of human life histories. Proceedings of the National Academy of Sciences (USA) 95, 1336-1339.

Klein, R. G. (1989). The Human Career: Human Biological and Cultural Origins. Chicago: University of Chicago Press.

McHenry, H. M. (1994). Behavioral ecological implications of early hominid body size. Journal of Human Evolution 27, 77-87.

O'Connell, J. F., Hawkes, K. & Blurton Jones, N. G. (1998). Grandmothering and the evolution of Homo erectus. Unpublished ms.

Rodseth, L., Wrangham, R., Harrigan, A. & Smuts B. (1991). The human community as primate society. Current Anthropology 32, 221-254.

Smith, B. H. & Tompkins, R. L. (1995). Toward a life history of the hominidae. Annual Review of Anthropology 24, 257-279.

 

 






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