etología y comportamiento humano

http://press.princeton.edu/books/stanford/
2
Man the Hunter and
Other Stories
It is necessary to remember that
fossils were alive when they were
important.
S. L. Washburn,
in Anthropology Today (1967)
Around five or six million years ago, a hairy, one-meter
tall creature that looked much like an upright chimpanzee
left the security of the woodlands for life on the
open savannahs of eastern Africa. Physically defenseless
but gifted with a sharp mind, the species had carved a
niche for itself by becoming bipedal, allowing the creatures
to travel efficiently for long distances over open
ground. Its members ate mostly fruits and leaves but
also included increasing quantities of meat, both hunted
and stolen from animal carcasses that they found on the
savannah. Eventually they learned to modify stones into
tools, which over time became effective butchering implements,
and perhaps also weapons. Intelligence and
sociability were their most valuable assets in coping
16 Chapter 2
with the risks of a dangerous world. They therefore lived
in social groups or perhaps in monogamous pairs for the
safety of numbers. These first hominids flourished, and
their descendents became ever larger-brained and more
human, eventually evolving into modern Homo sapiens.
The above account of human origins has been the
standard version that students of human origins
as well as the public have learned and recited for
many years, but you should not find it very convincing.
Our knowledge of what the earliest members
of our hominid family were like is itself still
evolving. The evidence for bipedalism arising in a
fairly treeless savannah niche is, for example, considered
tenuous.1 Did the earliest hominids (members
of the human family) hunt, or did they scavenge
to obtain their meat? What precise factors
brought about the dramatic expansion in brain size?
For every simplistic portrait of human origins such
as the one above, there are dozens of researchers
eager to tackle other untested assumptions. Some of
these debates over our origins are quite fierce and
have profound implications for understanding who
we are.
The study of human origins is like the plot of the
Japanese film Rashomon. A brutal crime has been
committed and observed by a number of eyewitnesses.
A suspect is apprehended, but each eyeMan
the Hunter 17
witness account of the crime in question differs in
some critical way. Even though the facts have been
established, the police are unable to reconstruct the
event to their satisfaction. The crime is clear but the
lines of evidence do not converge due to the prism
of circumstance and perspective through which the
observers’ differing accounts pass. This is the situation
in which scientists studying human origins
find themselves; the bits of evidence from primate
behavior, fossils, and other sources do not always
build a consensual picture. Instead, they often point
to a variety of possible scenarios, all of which probably
contain important elements of the truth.
This Old House
The difficulty in studying human origins lies in
our narrow view of the ancient world. We are constrained
by our firsthand knowledge only of the
present, in which the why and wherefore of the past
are lost. Go into an old house and look around. If,
like most older homes, it has been remodeled and
refurbished many times over its long life, you will
be confronted with a jumble of architectural and
furnishing styles. The Victorian fixtures in the living
room, the Craftsman sideboard in the dining room,
and the 1960s kitchen remodel all coexist in one
place. Modern heating and wiring systems are built
18 Chapter 2
on top of the original, and beneath the wallpaper
there are other layers from previous chapters in the
house’s life. These changes obscure the original appearance
quite effectively. Today the house is a mosaic
reflecting a long history of styles and decors,
none of which could have been predicted by the previous
generation’s designers. A casual observer can no more
look back from the 1990s to see the house clearly in
its original form than he could have stood in the
kitchen in the 1890s and predicted the course the
house would take in the century to come.
The lesson of the old house is a good one for
those who theorize about the primate origins of our
own behavior and anatomy. Each piece of furniture
and each component of the floor plan is one part
of the jigsaw puzzle of evolutionary function and
form that natural selection molds. What’s more, the
puzzle is internally interactive. That is, when a new
piece is added, other pieces have to accommodate
and be accommodated. When bipedal posture is
adopted, the circulatory system, the spinal column,
the diet and foraging behavior, and even the mode
of social interaction also change. This makes assembling
the puzzle retrospectively an enormous challenge.
In constructing theories of our origins, we
amass diverse evidence from fossils, modern human
behavior, the behavior and anatomy of living
primates, and from genetic studies to develop conceptual
models of what our earliest ancestors were
Man the Hunter 19
like. But in the end we often create scenarios that
rely heavily on the behavior of just one closely related
species, which becomes an analogy for the
form that our ancestors may have taken.
The chimpanzee model has long been foremost
among these analogous models due to its obvious
appeal. We are physically very similar to these apes
and studies show our DNA sequences to be about
98.4 percent the same; this makes chimpanzees
more closely related to us than they are to gorillas.
But the 1.6 percent different DNA is enough to make
us distinct, having followed our separate evolutionary
paths for 6 million years. We can easily see how
different modern humans are from our ancestors.
Modern chimpanzees, on the other hand, appear to
have remained more similar to our common ancestor.
But it would be a mistake to assume that early
humans were very similar to chimpanzees simply
because they looked more like chimps than like
modern people. There have been, in effect, 12 million
years of evolution (6 along each prong of the
fork) separating humans and chimpanzees—long
enough for profound changes to have occurred. The
ape and the human were quite similar at the earliest
stages of the early hominid-chimpanzee ancestor
branching, when the lines had been going their own
way for no more than 2–3 million years. We must
compare the modern chimpanzee with the likely
anatomy and behavior of our early common ances20
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tor that lived before the node occurred in our families’
branches. It is this gap that we are trying to
span when reconstructing our family roots.
What is a model of human evolution? Primatologist
Jim Moore distinguishes a model from a theory.
A model is an explanation that uses one member of
an analogous pair as a point of reference in understanding
the bigger picture.2 Thus craters on the
moon can be used to model the way in which celestial
objects crash into celestial bodies in our solar
system. We can study the moon in detail because of
its proximity and then use a lunar model for crater
formation. In time, space probes may prove this
model to be completely wrong if, for instance, we
learn that craters on other moons in the solar system
are formed by other means. The lunar model is
an analogic model; it uses one well-known example
to predict conditions elsewhere. A theory, by contrast,
should be an intellectual construct that uses
facts from disparate sources to build a prediction
about some natural phenomenon. For instance, one
might theorize that based on studies of the velocity
and impact rates of meteors that have been recorded
throughout the solar system, the appearance of
craters on celestial bodies everywhere should be
similar. This is different from the lunar model of
crater formation because it casts its net more widely
for relevant data to test the hypothesis about craters.
Moore points out, however, that, in practice,
Man the Hunter 21
models and theories are often treated as one and the
same.
A theory ought to have predictive power. In
the physical sciences, researchers claim they have
proved the existence of objects, whether elementary
particles or celestial bodies, that have never been
observed visually. Showing theoretically that a
black hole or brown dwarf ought to exist is enough
to provoke other researchers to take up the search.
This is because of the consistent and predictive laws
of the physical world. Human evolutionary scientists
hypothesize retrospectively what extinct creatures
may have been like, choosing from a wide
range of possible adaptations. Researchers have attempted
to create predictive theories, plausible scenarios,
or even simply stories that contain some
well-informed guesses about a long-gone reality.
But in truth, evolutionary models are, in spite of
their tremendous power to explain past history, not
predictive. Models of the evolutionary process can
describe and explain the history of organic change
in a lineage even though they could never have predicted
that those changes would occur.
Primate Models of
Early Humans
When we want to build a model of something
that no one has ever seen, it helps a great deal to
22 Chapter 2
have another structure on hand that is similar to
what is being modeled. However, having that similar-
but-not-the-same model handy may mislead us
in important ways. We could use the principles of
primate ecology combined with what we know
about the relationship between anatomy and behavior
to reconstruct the social lives of early hominids.
This approach, however, is fraught with
uncertainties. If the solitary-living orangutan were
not alive today, primatologists examining ancient
orangutan skeletons would be perplexed to note
that males were dramatically larger than females,
which is typical of polygynous, group-living primates
but completely unexpected for solitary ones.
Similarly, no primatologist could determine the behavioral
differences between the chimpanzee and
the bonobo based on the animal’s anatomy without
having a fully fleshed version to study in life as
well. In fact, if the chimpanzee and bonobo existed
only as fossils they might be mistaken for one species
because of their anatomical similarity. These are
important reminders of the limitations in modeling
the behavior of extinct animals. When a researcher
constructs a grand conceptual model of early human
evolution, she or he has no choice but to resort
at some point to the use of a strong analogy with
some living, well-studied primate relative.3 There
are four great apes, and any one of them could be a
good analog for the human ancestor. But of course
Man the Hunter 23
the human ancestor was not exactly like any of them,
and perhaps was so different that to use any as exemplars
may be counterproductive.
We can construct a portrait of a hominid ancestor
based on the behavior of one or a few living and
highly analogous species. Most models that attempt
to use empirical evidence from a range of disciplines
implicitly use a chimpanzee model as the
psychological starting point. Primatologist Richard
Wrangham has argued that of the four great apes,
the chimpanzee is the best model of our own origins
because it has more features that link it with
gorillas and humans than with bonobos. He posits
the chimpanzee as the direct best analogic model
for the last common ancestor, citing molecular, anatomical,
and behavioral links between chimpanzees
and gorillas that leave bonobos as an outgroup.4 Because
we know that we are more closely related to
chimpanzees than to gorillas, the former would
then be the last living word on our ancient selves.
Adrienne Zihlmann and her colleagues, meanwhile,
have long proposed the bonobo as the best
model for an early hominid, citing morphological
similarities in this ape to the human bipedal adaptation.
5 Other researchers have seen these similarities
only as interesting but coincidental parallelisms;
the apparent adaptation to bipedal posture
may be an adaptation to an arboreal lifestyle.6 Bonobos
are probably not a more appropriate model
24 Chapter 2
for the last common ancestor with hominids than
chimpanzees.
It is impossible to overstate the role of the chimpanzee
and bonobo in the evolution of theories
about human origins. Understanding the lives of
extinct forms represented only by bones would be
vastly more difficult in a world in which paleoanthropologists
had access only to the newly discovered
hominoid fossil and had never seen living
individuals. Live animals provide us with examples
of the range of possible adaptations for feeding,
ranging, territoriality, mating, offspring rearing,
and a variety of other behaviors without which
there would be no starting point for reconstructing
hominid lifeways. Not surprisingly, most self-proclaimed
conceptual models have settled on a very
chimpanzee-like creature as the presumed common
ancestor of all the hominids.7 Only by comparing
the ecological and anatomical features of the living
apes can we hope to distinguish the range of possible
adaptations from those that the extinct species
was likely to have had. The problem of extrapolating
from living to extinct forms is compounded by
the absence of fossil African apes that would provide
physical evidence of the evolution of the chimpanzee-
bonobo lineage after their divergence from
the shared ape ancestor.
Perhaps the greatest misconception about the use
of a chimpanzee model of human evolution is the
Man the Hunter 25
often repeated notion that using a chimpanzee analogy
limits our perspective of the unique traits that
early hominids would have possessed. Andrew Hill
speaks for many paleoanthropologists in stating
that “pretending that the very early hominids are
almost exactly like modern chimpanzees, or any
other particular animal, seems to me a dead end.
Work is then devoted to confirming this view of the
past, and this practice prevents the detection of differences.”
8 Hill is mistaken: if anything, using a
chimpanzee model keeps us from forming a cardboard
view of any extinct hominid species. The earliest
primate researchers used to speak of “the monkey”
to refer to all primates, in the days before we
appreciated the dramatic diversity of mating systems
and other behaviors among the primate order.
Likewise, William McGrew has chastised us for
thinking of “the chimpanzee” by pointing out that
the degree of cultural diversity among chimpanzee
populations across Africa prevents us from generalizing
about the species’ tool-use capabilities,
hunting styles, and so on. And in the same way,
paleoanthropologists err when they speak of “the
australopithecine” as though each species of early
human was monolithic. In all likelihood, there were
populations of Homo habilis that hunted avidly for
small mammals, and other populations living at the
same time 100 kilometers away that were scavengers.
This is the nature of the diversity of chim26
Chapter 2
panzee societies, and there is good reason to think it
would have characterized early human populations
as well.
Other theorists have used great ape societies as
referent points for reconstructing aspects of the earliest
hominid’s social behavior and ecology. These
models have focused on different aspects of species-
specific traits as critical features also present in
the common ancestor. Richard Wrangham compared
the social systems of the African apes to ascertain
the behavioral ecology of the common ancestor;
he concluded that polygynous, male-bonded
kin groups were the core of the common ancestor’s
society. He found inconclusive evidence about territoriality
and male philopatry.9 Michael Ghiglieri
conducted a similar analysis using only the chimpanzee
and bonobo and considered male reproductive
strategies and intense male kin group-enforced
territoriality to be the core features around which a
model of the stem hominid should be built.10 These
papers focused on male bonding in relation to male
reproductive strategies, because the emerging consensus
on the nature of both chimpanzee and bonobo
society is that they are composed of malebonded
kin groups. Only a few researchers have
considered female Pan behavior in such models of
early hominids. William McGrew posited the female
chimpanzee as a prototype of an early hominid
female; he saw active mate solicitation in a polyMan
the Hunter 27
gynous setting as evidence of females that were active
reproductive strategists rather than passive receptacles
for males’ reproductive ambitions.11
Models are of course only as well supported as
the information on wild apes that is used to build
them. In the 1970s, when a debate over whether
early humans were hunters and scavengers was
brewing, the archaeologist Glynn Isaac built a seminal
model of early human behavior based on the
sharing of food after butchering the carcasses of savannah
ungulates. Isaac rejected the behavior of
modern chimpanzees in building this model. He
did not know then that meat consumption by chimpanzees
is as frequent as we know it is today. Isaac
considered chimpanzee hunting to be more similar
to what we would call gathering in traditional human
societies because it involved only small packages
of protein.12 We know today that the cumulative
amount of meat can be quite high because of
the tendency for chimpanzees to capture several
monkeys in a single hunt. Chimpanzees share meat
nepotistically but they do not store food for later
consumption; hand to mouth is the rule. Isaac
would have been forced to frame the sharing hypothesis
differently had he known of the more recent
discoveries about chimpanzee behavior.
Analogic models do not necessarily have to be
built upon the biology of our very closest relatives.
The first modern use of a referent species for early
28 Chapter 2
humans was in fact a monkey, the savannah baboon.
Irven DeVore and Sherwood Washburn published
a series of papers in the early 1960s that
focused on the role of the male baboon and its statusseeking,
mate-guarding, and sometimes meateating
nature. These findings held sway for many
years in modeling our view of what the most
important factors driving primate societies were.
Eventually it was pointed out that female baboon
behavior is as interesting and important as that
of males, but male behavior continued to dominate
the research agenda on baboons until recent
decades. We saw ourselves as baboon-like, and
so our models of our ancestors suggested baboon
traits.
Hunting and meat eating by baboons was studied
by Shirley Strum in a troop nicknamed the
Pumphouse Gang at Gilgil in Kenya. Strum has
now studied these animals for more than twentyfive
years, but in the early 1970s she observed their
hunting behavior in particular. Her study of meat
eating showed that the Gilgil baboons ate meat
more frequently—once per day—than any other
known population of nonhuman primates. The exact
pattern of both hunting and sharing varied depending
on the individuals involved in each hunt,
but the baboons at Gilgil were more sophisticated in
their hunting behavior than any other baboons that
Man the Hunter 29
have been studied. Strum reported cooperation,
particularly when a particular adult male was involved
in the hunt. This male appeared to be a catalyst
who promoted hunting by other males, females,
and even juveniles. The Gilgil baboons thus
provide a model for a cultural basis of hunting; they
suggest that aside from any energetic concerns
about the rate of return in hunting by baboons,
meat eating is a learned tradition that may be exterminated
or initiated depending on the composition
of the group. Past experience, observation, and imitation
of others dictated much of the meat-eating
behavior of the Pumphouse Gang. Individuals did
not scavenge carcasses unless they had some prior
experience with the carcass or could watch another
group member eating from it.13 Why one population
of baboons scavenges while another does not may
be due to experience and learning opportunities.
This hallmark of complex social behaviors, including
hunting, certainly applies to chimpanzees as
well. In the Ta¨ı forest of the Ivory Coast, the majority
of adult males who did the hunting in Christophe
Boesch’s long-term study of chimpanzees recently
died from an outbreak of the ebola virus.
Learned traditions of hunting may have died with
them. If there can be a silver lining in such a tragedy,
it will be to observe whether and how the hunting
tradition reemerge as a new crop of young male
30 Chapter 2
hunters matures without the benefit of observing
their elders.
Meanwhile, in the forests of Latin America there is
another, much smaller primate that also hunts and
eats meat voraciously whenever it has the chance.
This primate is a monkey much smaller than a baboon,
and in addition to its carnivorous habits it is
also the most adept tool user among the New World
monkeys. The capuchins of the genus Cebus, found
across South and Central America, hunt as avidly
and as successfully as chimpanzees. They engage in
relay chase hunts that resemble those reported for
both baboons and chimps. They also employ tools
more than any other monkey. Their brain-to-body
size ratio is very high, as is true of chimpanzees.
Studies of captives have shown capuchins to be active
and strategic food sharers as well. It has only
been in the last several years that any attention has
been focused on their hunting prowess and sharing
behaviors. This lack of attention is in turn due to the
general lack of interest anthropologists have shown
in the smaller-bodied Neotropical monkeys as models
of human evolution.
Meat eating, tool use, and large relative brain size
therefore occur in two distantly related primate
groups—apes and New World monkeys—and
meat eating and related behaviors are also known
in an Old World monkey, the baboon. Is this a ranMan
the Hunter 31
dom evolutionary convergence, or has natural selection
driven the coevolution of these traits? These
animals are exemplars of how effective nonhuman
primates can be as hunters. They are just three of
two hundred primate species, so one might argue
that meat eating was not a fundamental factor in
the rise of the human species. The coincidence of
traits among these species, however, is striking.
Other Origins
Finally, there is no good reason to limit ourselves
to primates when using living animals to reconstruct
the origins of intelligence and complex social
behavior. Anthropologists study prosimians such as
bushbabies and lorises because they are our primate
kin, but they are so distantly related that what
we are likely to learn from them about the origins of
human behavior is extremely limited. If an anthropologist
were to turn to the cetaceans—the dolphins
and whales—for answers about the origins of
humanness, he would risk ridicule. However, if we
are interested in how complex social systems arose,
then comparing how the Darwinian process has
molded unrelated highly intelligent species may be
more informative than comparing two more related
but fundamentally different animals.
It is hard to imagine mammals living in more dif32
Chapter 2
ferent worlds than a dolphin and a chimpanzee.
And yet these two creatures are representatives of
the only two evolutionary lines of the billions that
have ever lived to produce big-brained, socially sophisticated,
and highly intelligent beings. Their social
convergence may be due to ecological factors.
Dietary and foraging constraints on the two species
may be similar. For dolphins, the food resource is
fish, which are unpredictable in occurrence and
widely dispersed. This patchy resource promotes
foraging in small, fluid social units, paralleling
chimpanzee society, which is structured around the
patchy distribution of their favored fruits in African
forests. Male chimpanzees form long-lasting bonds
based on kinship, and these coalitions attempt to
control females in order to obtain matings. Male
bottlenose dolphins in a landmark study in Shark
Bay, Western Australia, have also been reported to
form long-lasting alliances that cooperatively coerce
females to mate with them. This cooperative
behavior is also valuable in driving off predatory
sharks, and possibly in finding schools of fish.14
Both species are highly communicative, with vocalizations
that show regional dialectical variation and
a strongly learned basis. These parallel social adaptations
suggest that the social complexity and brain
size increase that we see in ourselves have their
roots in the social and ecological environment in
which our ancestors found themselves.
Man the Hunter 33
The Missing Link Is Neither
The idea of a missing link in human evolution is
deeply instilled in all of us. No matter how complete
the evolutionary sequence of human fossils
becomes, there will always be those who demand a
missing link. By definition, the missing link is the
most recent common ancestor of both humans and
great apes, which must have lived immediately before
the split of these two lineages in the late Miocene
or earliest Pliocene era. But in reality, there is
no such thing as a missing link or even a most recent
common ancestor. The concept of a common
ancestor is just a metaphor for the enormously complex
process of speciation that preceded the emergence
of the hominids.
Consider the process by which two modern primate
species might form. A vast river changes its
course in the Amazon basin, splitting what was
once a population of monkeys into two smaller
populations. Over millennia, each population goes
its own evolutionary way due to an accumulation
of mutations that do not penetrate into the other,
now disjunct population. The result will be multiple
new populations that are genetically, morphologically,
and behaviorally different enough that we
consider them separate species. The criteria we use
to recognize the populations as distinct species depends
on which concept of species we use. Initially,
34 Chapter 2
there was not a single different population that
gave rise to other, daughter populations. Rather,
there were a number of populations, each with its
own set of slight genetic and morphological and
behavioral differences. We see this variation in geographically
distinct populations today in many animal
species. It is a gross oversimplification to
imagine that there was one or even just a few populations
of ape ancestors that gave rise to the hominids.
In reality, there may have been numerous
ones, and they may have differed substantially in
morphology and behavior from one another, and
had periodic contact during which their genes
mixed. To invoke the missing link by calling any
one of these populations the founder of our gene
pool is almost certainly wrong. But because we are
so limited in our knowledge of the fossil record, and
even more in understanding what the fossil record
can inform us about behavior, we employ the common
ancestor logic in trying to model early hominid
behavior.
Early Hominids as Weaklings
One of the most indelible images in our depiction
of early humans is that they were weaklings. As humans
became human, they lost two adaptations
that characterize the great apes: climbing ability
and long canine teeth. The usual depiction of these
Man the Hunter 35
nascent humans suggests that they were not particularly
well adapted to either their new grassland
environment or to the forest habitat they are said to
have left. We see them traveling warily across the
savannah, armed with only their brains against the
carnivores that eagerly preyed on them. Often, their
gait is depicted as a shuffling, inefficient form of
semibipedalism, as though the creatures had some
half-formed ability to walk. This caricature is pervasive
even among human evolutionary scientists—
indeed, it was present in both a recent widely
watched documentary on human evolution and a
recent book about human nature. The image of
early humans taking up bipedalism after a long ancestry
of ape quadrupedalism is implicitly that of
people who take up a sport late in life—that, because
it is not their lifelong habit, they will take
time to do it well and may never be really proficient.
This is nonsense, since at each stage of the
evolutionary process natural selection molded a
means of moving about that was efficient enough to
be favored and perpetuated. There was no way to
predict, nor any a priori reason to think, that evolutionary
change in the way apes moved would ultimately
produce an upright walker. We do not know
why hominids became bipedal, but we can explain
many of the consequences, such as greatly enhanced
energetic efficiency over the ape’s knuckle
walking.15 Nevertheless, the shuffling protohominid
36 Chapter 2
is a mainstay of popular and scientific accounts of
early humans. The depiction of early humans as
weaklings is also odd considering that we do not
see apes in this light; what could be more gracefully
agile than a chimpanzee? Even when chimpanzees
travel on the ground, they are hardly defenseless,
mobbing dangerous animals such as leopards and
driving them away through joint effort.
Our ancestors may not have had huge canines,
but they certainly may have been highly efficient
killers and predators. During the years I conducted
research at Gombe there was one elderly male
chimpanzee, Evered, who was an accomplished
hunter even though in his last years he had lost the
muscle tone needed for treetop agility as well as
nearly all of his teeth! Even modern people who
lack anatomical adaptations to tree living but who
live in forested environments have a tree-climbing
ability far beyond that of anyone raised in Western
society. Neither chimpanzees nor humans have any
anatomical traits that specifically adapt them to
a predatory way of life. Instead, both use their
ability to hunt socially and cooperatively to compensate
for a lack of such adaptations. One of the
arguments against a hunting ancestry for early
hominids has been that they lacked such adaptation
and therefore were forced to scavenge for carcasses
as their sole source of meat. This is a highly
implausible scenario, in part because early homiMan
the Hunter 37
nids could make a living on the numerous small
and medium-sized mammals with which they
shared their habitat.
Our deep preconceptions influence and constrain
the ways in which we theorize about the early nature
of humanity. These preconceptions change, but
they are always constrained by the limitations of
our evidence and by the prevailing biases of the
day. The extent to which these models reflect reality
versus our own reflections of ourselves is embodied
in some of the influential models of human origins
that follow.
Man the Hunter
In 1966 about fifty anthropologists who studied
the life ways of traditional foraging people gathered
in Chicago for a conference to examine the status
of the world’s hunter-gatherers.16 Perhaps the
foremost scientific conclusion that came out of
the meeting was that the importance of meat in the
diets of foraging people had been exaggerated. This
was deeply ironic, since the most influential and ultimately
notorious perspective to emerge from the
meeting came to be known as “Man the Hunter.”
Sherwood Washburn, the most prominent and listened-
to biological anthropologist of his day, and
Chet Lancaster contributed a paper called “The
Evolution of Hunting.” It set out to explain how
38 Chapter 2
and why the human brain had experienced a 3.5-
fold increase in size and complexity since the dawn
of humanity. Washburn and Lancaster claimed that
“our intellect, interests, emotions, and basic social
life—all are evolutionary products of the success of
the hunting adaptation”17 (p. 293). They were referring
mainly to our more recent human ancestors
during the Pleistocene. In their view, hunting for
game animals was at the soul of the human experience.
However, they noted that the sexes had different
roles in many traditional societies when it came
to acquiring meat. Men, according to Washburn and
Lancaster, hunt, while women gather. This view put
men in the important role of obtaining the highestquality
nutrients and the calories that their households
would use. Hunting requires communication
and coordination of action among the hunters. This
placed an evolutionary premium on intelligence
and communicative ability in order to successfully
track and hunt down potentially dangerous prey.
Men did this, and women did not. Moreover, Washburn
and Lancaster linked the deep human love of
hunting to the equally deep human love of going to
war and to acts of aggression in general. The fact
that it is almost always males who carry out these
acts served to reinforce the idea that men had a natural
right to occupy the glamor role of cleverminded
forager, meat provider, and conqueror in
Man the Hunter 39
human societies. Ever since, theories of human evolution
have focused on male activities rather than
female as the core human adaptations.
In principle, the evolutionary logic on which Man
the Hunter was based was sound. It was an analogy
based on the behavior of traditional foragers that
employed a model of cognitive evolution equating
a fundamental change in human anatomy and behavior
that was carried out by one sex only—men.
However, we know of many cases in which, due to
genetic linkages called “pleiotropic effects,” one sex
exhibits traits that natural selection clearly produced
in the other sex only. Male nipples are an obvious
example. The serpentine neck of the giraffe,
long thought to be the product of natural selection
favoring those giraffes that could reach high
branches to forage more effectively than shortnecked
neighbors, was probably not the result of
natural selection for neck length. Male giraffes use
their necks to combat for females, twisting and
thumping each other in a dominance struggle.
Longer-necked males have greater mating success,
though at the same time they suffer greater mortality
from predators than smaller males.18 All else being
equal, natural selection would favor shorter
necks. But not only male giraffes have long necks;
females do, too, because of pleiotropic effects. In the
same way, the early hominid brain could have
40 Chapter 2
increased in both males and females even if only
males were hunting for meat.
The intellectual stakes were higher in Man the
Hunter, however, since it was about the roots of human
gender relations, not giraffe necks. The response
to Man the Hunter was angry and its impact
long lasting. Many anthropologists were angered
by the suggestion that a hallmark in our ancestry
was effected by natural selection for male cognitive
abilities, implying that women were merely carried
along in some genetic linkage. Anthropologists Adrienne
Zihlmann and Nancy Tanner pointed out
that in some of the traditional societies that are most
vaunted for the man’s role in hunting, up to 85 percent
of the animal protein obtained by a household
came not from men at all, but from the less glamorous
role of women gathering foods such as nuts,
tubers, and small animals.19 The role of human females
had been neglected in Man the Hunter, according
to the anthropological community, due to
the gender politics of scientific advances and partly
due to the failure to appreciate the role of women in
foraging.
In the largest cross-cultural database that exists—
a survey of 179 societies that examines how labor is
divided in human groups—men alone hunt in 166,
both men and women hunt in 13, and in not one do
women alone do the hunting. Women, on the other
Man the Hunter 41
hand, are the main gatherers of plant foods in about
two-thirds of societies in the same survey.20 So the
reality of male predominance as hunters and of
women as gatherers is not in dispute. Instead, anthropologists
began to realize that although men
hunt, they often fail to catch enough prey to sustain
the family, and this task falls to women. Men might
kill one giraffe and talk about it around the fire at
night for a year until another is killed. In the reaction
to Man the Hunter, the fact was lost that while
meat may not be the valuable food resource it had
been assumed to be, it is nevertheless the most valued
food resource in most human groups, including
among foraging people.
The backlash to Man the Hunter permeated all
fields of anthropology. In the study of nonhuman
primates, it contributed to a reappraisal of the way
in which the field was practiced. Observers had always
tended to focus more on the behavior of males
than females, because they are often bolder and
therefore more visible.21 The practice of observational
primatology was made more systematic
when it was recognized that females also played a
central role in the primate group. More recent theoretical
advances made it clear that females rather
than males are often the central players around
which the mating system is structured.22 Man the
Hunter’s backlash eventually led to an engendering
42 Chapter 2
of the field of archaeology; it was recognized that
the role of women in early human societies had
long been ignored in favor of the often more visible
role of men. Stone tools made for butchering carcasses
will preserve in the fossil record, while the
implements of gathering made and used by women
might not. Women nevertheless performed critical
tasks and occupied spheres of influence in antiquity
that went far beyond where the largely male archaeological
community had considered them for
decades.23 Archaeologists today refer to the failure
of an earlier generation of scholars to consider the
role of women as a “Paleolithic glass ceiling.”
A Good Story
A good model of human origins should provide a
number of hypotheses about our ancestors’ behavior
and anatomy that interweave in a sensible way.
That is, the model must be both internally and externally
consistent. It must explain the origins of
those traits that are uniquely human above all else,
since other traits that we share with the great apes
are likely primitive ones that we possess simply
due to a common ancestry. What is left after these
are stripped away is those traits that define our humanness.
To retrospectively build a human ancestor
requires that we consider the roots of the key components.
Some models make wonderful stories; like
Man the Hunter 43
a novel they may be internally consistent even
though they do not overall accord well with physical
evidence. For example, the idea that humans
passed through an aquatic phase in prehistory has
been advocated and accepted in some popular accounts,
24 even though the specific lines of evidence
in the model have no support.25 It is simply an attractive,
internally consistent story about who we
are and how we came to be human.
The key adaptations that we must consider will
vary depending on what stage of human evolutionary
history we are thinking about. For instance,
when imagining the common ancestor of all hominids,
the key character is bipedalism, arising at least
five million years ago and exhibited by no other primate.
Our very large and complex brains, our toolusing
capabilities, the increased amounts of meat in
our diet, and our unusual social system all evolved
at much later dates—2.5 million years for stone tool
use and less than 200,000 years ago for a modern
level of brain-size increase. The theories of hominid
origins that have gained the most attention and notoriety
have been those that have woven the greatest
number of human traits together in an internally
consistent way, even though we will see that these
often become houses of cards by virtue of the number
of variables they seek to link. In the following
sections I discuss some key adaptations and how
they fit into a portrait of early hominid behavior.
44 Chapter 2
Key Adaptations
bipedalism
No aspect of modern humans has been speculated
about more than our unique bipedal posture and
locomotion. Whatever the protohominid’s mode of
travel, bipedalism would probably have evolved
only if it had increased the efficiency of movement
in the emerging new species. For instance, Karen
Steudel of the University of Wisconsin has analyzed
the energetic efficiency of bipedalism in order to assess
the hypothesis that early humans became more
efficient travelers when they became bipedal; she
disputes this widely held notion.26 In the 1970s,
studies of chimpanzee walking patterns led researchers
to claim that bipedalism required no less
an output of energy per unit of distance than quadrupedalism.
27 Later, anthropologists Peter Rodman
and Henry McHenry found the opposite result;
they reported that bipedalism must have arisen due
to its far greater efficiency over the knuckle walking
of apes.28 Steudel argues that while bipedal walking
is more efficient than knuckle walking, it is not
likely to have been more efficient in its earliest incarnations
than the quadrupedal alternatives available
at that time. In other words, natural selection
probably did not favor the continued evolution of
bipedal locomotion in emerging hominids due to its
efficiency unless our common ancestor with chimMan
the Hunter 45
panzees was a knuckle-walker. Ruling out improved
energetics as the primary stimulus for bipedalism
leaves a number of other reasons that
have been promulgated. For example, here are
some published explanations, along with at least
one piece of contradictory information:
1. Being upright gives a height advantage to intimidate
predators and other hominids.29 Problem: Why
is it important to be permanently upright? Standing
upright for just a few seconds would achieve the
same result.
2. Being upright allows an early grassland hominid to
see over tall grass.30 Problem: Same as above, plus the
doubt over whether early hominid evolution really
occurred in grassland versus woodland habitats.
3. Being upright reduces one’s exposure to intense
tropical sun and heat, thereby reducing heat stress
on the savannah.31 Problem: Again, the evidence that
this key period of evolution took place on the savannah
is now considered shaky.
4. Being upright is not about walking, but rather about
posture when foraging. The bipedal posture may
have evolved to allow apes to pull down low-hanging,
fruit-laden branches,32 or to allow for better treeclimbing
ability on vertical trunks.33 Problem: Neither
of these receives strong support from the behavior
of modern quadrupedal chimpanzees.
5. An upright walker has its hands freed for carrying
food, offspring,34 or tools.35 I will deal with this last
scenario below, for it incorporates some of the most
widely held assumptions that have recently dogged
models of human origins.
46 Chapter 2
bipedalism and meat eating
In 1981 physical anthropologist Owen Lovejoy published
a paper that proved an influential but controversial
model of human origins. He suggested that
we should look at both bipedalism and the unusual
reproductive system of humans to establish the
likely social behavior of the earliest hominids as
well as why they became bipedal. Unlike chimpanzees
and bonobos, in which females possess
large fluid-filled swellings when ovulating, human
females conceal their ovulation. Lovejoy saw the
roots of female manipulation of male behavior in
the concealment of ovulation; unable to time the exact
period of ovulation, male protohominids would
have had to remain near the female in order to mateguard
against the possibility of cuckoldry. At the
same time, increased use of savannah habitat led to
efficient bipedal walking. Lovejoy hypothesized
that male provisioning of stay-at-home females resulted.
Males used their newfound freedom of the
hands to carry meat back to females. They thereby
enhanced females’ nutritional status, enabling an
increased reproductive rate that was good for both
sexes.
Unfortunately for Lovejoy’s model, there is no
reason to think that advertisement of ovulation has
ever been a part of hominid biology. Chimpanzees
and bonobos had a common ancestor with other
Man the Hunter 47
great apes, and probably evolved their swellings after
their lineage had split with other apes. This is
the most parsimonious explanation and would
explain why none of the other apes or humans
possess swellings while both the chimpanzees and
bonobos do. The concealment of ovulation in women
is simply the retention of a widespread primitive
primate feature. The swelling is a chimpanzeebonobo
feature that evolved sometime between 5
and 2.5 million years ago. Moreover, we have little
reason to assume that early hominids would have
been monogamous, any more than all modern human
societies are (the majority are polygynous).
Some form of polygyny, the social system of three of
the four great apes and of the majority of human
societies, is the probable social organization of our
common ancestor with the apes. Lovejoy’s model is
thus internally consistent but fails in the face of the
evidence.
Karen Steudel argues that bipedalism is not likely
to have replaced quadrupedalism on energetic
grounds alone.36 So we might consider the existence
of an early bipedalism and a later bipedalism. This
is speculative since we as yet have no fossil that
exhibits a clearly different, unknown form of bipedal
walking than ourselves.37 However, it must have
existed, since bipedalism arose over thousands of
generations with each intermediate form molded
by natural selection serving some useful purpose
48 Chapter 2
of its own. We do not know if the common ancestor
of humans and great apes was a knuckle walker.
Since all of the four great apes walk either on their
knuckles or on the sides of their fists (orangutans),
it is possible that there was a knuckle-walking stage
in human evolution, as proposed many years ago
by Sherwood Washburn and others.38 If bipedalism
arose for reasons other than enhancing walking efficiency,
however, then the concern that it is more efficient
than knuckle-walking but not more than
quadrupedalism is unnecessary. But an early type
of bipedalism, whether it arose from knuckle walking
or not, could certainly have characterized
the earliest hominids. The cause of this bipedalism
could have been any of the hypotheses I presented
earlier, or one that has yet to be proposed. The more
modern form of bipedalism that we see in both ourselves
and our known ancestors could have arisen
later and been enhanced for energetic reasons.
One of the most striking differences between the
foraging behavior of chimpanzees and that of
humans is that chimpanzees crave the meat of other
animals but do not search for it. Instead, they forage
for plant foods and eat prey animals opportunistically
in the course of looking for fruits and leaves.
Certainly they are skilled hunters, particularly
when in large groups. They could presumably obtain
much larger amounts of meat if they actively
Man the Hunter 49
searched for it. Yet there is little evidence that chimpanzees
search for meat.39 The only factor that could
reasonably account for the chimpanzees’ failure to
search for food is that the return rate on their energy
expenditure is not enough to do so. A biped
can walk longer distances in search of desired food
compared to a knuckle walker. Thus, if there was
a knuckle-walking stage of hominid evolution
through which early hominids passed, it would
have precluded searches for unpredictable, moving
sources of meat. Once bipedalism had evolved to a
point of energetic efficiency, active searching could
become justifiable, and meat would increase as a
percentage of the diet.
Any model that seeks to explain human evolution
must explain the enormous increase in the size
of the neocortex of the brain in humans. I have already
argued that, based on the behavior of chimpanzees,
hominids ate meat much more frequently
and at an earlier stage in their evolution than is
commonly thought. This meat eating probably increased
as early hominids began to use larger-sized
animals as prey and as they began to use tools to
make use of the carcasses. As the size of the brain
increases during such a trend, one of the body’s
metabolically most expensive organs to maintain
must be nourished. Where does the energy come
from to nourish such a large brain? Leslie Aiello
50 Chapter 2
and Peter Wheeler have hypothesized that the energy
to nourish an increasingly larger brain came
from a metabolic trade-off in which the size and
scope of the metabolic investment in the gut were
reduced.40 They suggest that in the course of primate
evolution, the size of the brain has coevolved
in inverse relation to the size of the digestive tract.
This would partially explain why fruit-eating and
meat-eating primates have shorter guts; these foods
are more easily digested, allowing more energy to
be devoted to increasing brain size. We know that
leaf-eating animals sometimes have smaller brains
and lower basal metabolic rates than fruit eaters of
comparable body size. This relationship holds true
among primates as well. Aiello and Wheeler suggest
that the australopithecines improved the quality
of food in their diet over that of their forerunners
and thereby enabled a continued degree of encephalization.
Likewise, among living primates the capuchin
monkeys have among the highest brain-tobody-
size ratio and also one of the highest-quality
diets. This diet includes a high percentage of vertebrate
animal protein. The implication of Aiello and
Wheeler’s “expensive tissue hypothesis” is that in a
wide range of animal groups, those species that eat
a high-quality diet should show greater brain development
than those with a poor-quality diet. This
has yet to be shown. But the notion that a highMan
the Hunter 51
quality diet frees the metabolism of an evolving
hominid to develop a larger and larger brain is extremely
appealing because it would explain both
the trend toward greater encephalization and toward
more meat in the diet of the evolution of the
human lineage.
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