Here's something you might like, recently submitted to Trends in
Evolutionary Ecology (TREE). Comments welcome.
Lisa
***
The Behavioral Ecology of Modern Hunter-Gatherers and Human Evolution
Kristen Hawkes
James F. O'Connell
Lisa Rogers
Department of Anthropology
University of Utah
Salt Lake City, Utah 84112
Key Words: hunter-gatherers, human evolution, foraging, division of
labor, sexes, sharing, hunting, menopause, life histories, fertility
Recent work on the fitness related tradeoffs people face when
foraging for a living demonstrates key relationships between ecology
and behavior with important implications for human evolution.
Problems posed by the temporal and spatial distribution, capture
costs, morphology and composition, processing requirements and
economic defendability of locally occuring plants and animals engage
direct time and energy tradeoffs. These have associated mating and
parenting costs and benefits and often result in notable conflicts of
interests among individuals. Foraging tradeoffs are linked not only
to changes in subsistence practices, but also to patterns of
cooperation and sharing, the sexual division of labor, the role of
hunting in human evolution, and distinctive features of human life
histories including long post-menopausal lifespans.
The idea that present-day hunter-gatherers are an important source
of information about human evolution has long been disputed.
Currently, many anthropologists see modern foragers as part of a
world-wide, dispossessed "rural proletariat" with no special
connections to the distant past. That view is widely regarded as the
informed alternative to the popular myth that contemporary foragers
are isolated, unchanged relics of the Pleistocene, a proposition
falsified by all of world (pre)history.1 Modern human anatomy does
not evolve until the last 100,000 years; modern behavioral capacities
are reflected in the archaeology only after 50,000 years ago; key
features of recent hunter-gatherer technology and subsistence appear
no more than 20,000 years ago, in some instances even later.2 All
parts of every occupied continent have witnessed massive changes in
the distribution of human populations since the onset of modern
climatic conditions 8-10,000 years ago. Migration, war, trade, and
conquest have been pervasive. Many contemporary hunters have recent
farming or herding ancestors. In light of this historical
complexity, recent global economic and political processes are widely
seen to determine patterns of culture, including those of modern
hunters.
There is however a baby in the bath of "unchanged primitives." When
modern people subsist on wild (i.e., non-domesticated) resources,
they encounter problems in daily life broadly comparable to those
confronted by any hominid forager, no matter how ancient. These
problems, the constraints they pose, and the solutions adopted are
all open to direct observation. By abandoning the conventional
social science concern with cultural "systems," investigators can
take advantage of this opportunity to focus instead on the daily
behavior of individuals, specifically on the effects of age, sex, and
immediate ecological circumstances on the fitness-related tradeoffs
they face. Modern actors and environments differ from those of the
past, and represent only a fraction of some larger possible range of
variation. But each case offers a chance to see whether critical
variables are related in predictable ways.3 If so, results provide a
basis for hypotheses about situations in which those variables take
different values, including some outside the modern range.
Which resources?
Much research undertaken from this perspective has been directed at
questions of resource choice.1-2,4-7 In general, foragers have been
found to select prey that maximize mean rates of nutrient
acquisition. They routinely bypass resources yielding relatively low
post-encounter rates when they do better seaching for more profitable
items, but take a broader array of prey when encounters with high
ranked resources are rare.
Patterns in the archaeological record of resource choice also
reflect this tradeoff between search and handling.8 After the last
glacial maximum, many human populations began to exploit locally
abundant, nutrient-rich but previously unused resources, notably
seeds and other plant foods that require extensive processing to
improve digestibility or remove toxic components. This "broad
spectrum revolution" probably marks a decline in encounter rates for
higher ranked prey, which is in turn the result of terminal
Pleistocene climatic change, human population increase, human-induced
habitat change, or some combination thereof.2
The use of resources requiring substantial handling also had
implications for initial experiments in domestication. Broad
spectrum foragers spend more time processing than searching and thus
have more to gain from improvements in processing efficiency
(including those gained from actively manipulating resource
characteristics) than do foragers with narrower diets.9 Broad
spectrum diets were thus not only a common, but probably necessary
precursor to agriculture. Prominent in the list of initial
domesticates were plants whose reproductive habits and genetic
make-up allowed the selection imposed by human harvesters to promote
relatively rapid improvements in handling efficiency.
Assessing diet breadth archaeologically is complicated by forager
processing tactics, particularly the discard of more durable (hence
archaeologically more visible) resource components at or near the
point of acquisition. Relationships between actual diet and food
waste left at the point of consumption (usually the residential base)
may be skewed accordingly. Ethnographic observation10 and related
modeling11-12 indicate that differential transport and discard are
highly systematic, and perhaps often consistent with the goal of
maximizing the nutrient value of loads transported, given constraints
of time and transport capacity. Complex links between diet and its
archaeological reflection are thus in principle open to more accurate
interpretation. Models that combine resource characteristics with
transportation constraints can also be used to predict the location
of residential bases and other sites.13
Foraging goals
Despite the broad pattern of mean rate maximization, hunter-gather
foraging is not always consistent with this goal. Men often favor
large animal prey, ignoring plant foods and other "small package"
resources profitable enough to increase their mean nutrient
acquisition rates; women frequently do just the opposite, taking
plants, small game and shellfish more often than large animals.
Two hypotheses are offered to account for this pattern. One
attributes it to the effect of macronutrient composition on resource
value and the incompatibility of hunting and child care. Men may be
maximizing their mean rate of nutrient gain in a currency that gives
higher weight to fat and protein than carbohydrates, while costs in
child welfare cut women's potential net benefits from hunting.7,14
The second hypothesis focuses on differences in the effects that
predictability and defendability of plants and animals have on the
payoffs foragers can expect from these resources. Animals are
generally larger and less predictably acquired than plants, making
their carcasses (once acquired) costly to defend from other
claimants. These factors link resource choice to food sharing.
