THE HUNTING APE
I wiped the sweat out of my eyes and swatted away the prickly branches in my path
as I tried to listen for the screeches and hollers of the wild chimpanzees my col-
leagues and I had been trailing through Uganda’s Kibale Forest for the past five
hours. The sudden silence of the three large male chimpanzees could only mean
trouble. At times, such silence can foreshadow a sudden murderous rush into a
neighboring territory to kill competing males. Or perhaps scientists. Chimpanzee
warfare was not, thankfully, in the air that day. When our group emerged into a
small clearing, we observed the chimpanzees seeming to quietly confer with one
another as a crew of red colobus monkeys ate and played in the fig trees above, un-
aware of any danger. As two of the males inched up two nearby trees, the third—
the apparent leader—created a diversion by screaming and scrambling up the tree
toward the monkeys. Commotion ensued as the monkeys scrambled out of the tree
and landed in the path of the other two hunters, waiting. One of the chimpanzees
grabbed a young monkey and made his way to the ground to share his catch with
his teammates.
As the chimpanzees feasted on the monkey’s raw flesh, a rush of thoughts ran
through my brain: teamwork, strategy, flexibility. All in this close relative to hu-
mans. Truly, this was why people studied chimpanzees. While the rigors of scien-
tific literature would never allow us to state this in technical journal articles, the
reality seemed clear enough—these chimpanzees had worked collectively and
strategically to mount a coordinated attack. The leader had diminished his chances
of landing a kill by making a noisy attack, but the knowledge that his actions would
increase the chances of success for his partners made this a strategic approach. In
the end, they’d share the meat no matter who made the kill, exactly the sort of
behavior that humans display every day. As the chimpanzees tore through the ani-
mal, it also occurred to me that the contact with the monkey’s blood and guts pro-
vided the ideal opportunity for our carnivorous kin to contract microbes.
Studying our closest living primate relatives affords us the opportunity to better
understand ourselves, genetically, socially, and otherwise. However imperfect the
conclusions we draw about ourselves from studying wild primates, we’re lucky to
have them since the fossil record only offers its gems sporadically. Humans love
the idea that we’re the chosen species—unique among the members of the animal
kingdom—yet such claims should meet a high standard of proof. If our ape
cousins share our supposedly unique traits, then perhaps they’re not unique traits
after all. If, for example, we’d like to know if humans evolved the capacity to hunt
or share food independently, we can look to chimpanzees and bonobos and ask if
they exhibit the same behaviors. If they do, then Occam’s razor should push us to-
ward concluding that we all share these traits because of shared descent: evolving
the ability to hunt collectively twice or thrice within the very same close lineage is a
less parsimonious explanation than simply concluding that hunting emerged in
our joint ancestors before we split with them.¹ That a human trait is interesting
does not mean it is unique to us. Many undoubtedly have ancient origins.
Some people have an almost instinctually negative response to the discovery
that a treasured aspect of humanity is in fact not unique—that it’s actually some-
thing we share with other animals. Of course, the objective of science is not to un-
cover the things that make us comfortable but rather the things as they are.
Another perspective on these shared traits is that they can help us feel less alone
and more connected to the rest of life on our planet.
The parsimony rule of thumb applies not only to our behaviors. Each organ,
each cell type, each infectious disease presents a new point of comparison with
our kin. Are they found in us alone, or are they found in multiple other species
along our same branch of the evolutionary tree? Through careful studies of hu-
mans and our closest living relatives we have the potential to at least begin to sort
through historical mysteries and solidify which elements of humanity are unique
and which are not. Already, earlier ideas that human traits like using tools or fight-
ing wars were unique have been overturned by discoveries that chimpanzees en-
gage in the same behaviors. What other supposedly unique human traits will fall
next remains to be seen.
Fortunately, we have close living relatives that we can observe. The apes, our
own branch of the primate lineage, include humans, chimpanzees, bonobos, as
well as gorillas, orangutans, and the least studied apes, the gibbons. Studies of ape
skeletons during the past hundred years provide a rough guide to the historical
relationships among all of us. Over the last decade, a mass of genetic data from
these animals has further refined the picture, providing a clear pattern of primate
relationships. The information, commonly represented by the geneticists who
study these data in phylogenetic trees such as the one below, helps to graphically
describe how the relationships shake out.
The research reveals that for humans, two key species, chimpanzees and bono-
bos, lie closest to us. The other apes (gorillas, orangutans, and gibbons) differ
substantially more and thus represent distant cousins of our human-chimpanzee-
bonobo group. This relationship has led to the notion that humans are best seen
as the third chimpanzee species, described in great detail in Jared Diamond’s book
of the same title.
Once referred to as pygmy chimpanzees, scientists now recognize bonobos as
an entirely separate species, yet one closely related to chimpanzees. Bonobos live
only south of the Congo River in central Africa, while chimpanzees live only north
of it. And while they look very similar, bonobos and chimpanzees have evolved to
exhibit significant differences in their behavior and physiology during the time
they’ve been separated by the great river. Current estimates suggest that the chim-
panzees and bonobo lineages diverged roughly one to two million years ago. This
divergence occurred some time after our own lineage separated from these
cousins, around five to seven million years ago.
Phylogenetic tree, representing the evolution of apes. (Dusty Deyo)
This research helps point us to a very pivotal and informative character in the
evolution of our own species, a character referred to by anthropologists as the
Phylogenetic tree, representing the evolution of apes. (Dusty Deyo)
This research helps point us to a very pivotal and informative character in the
evolution of our own species, a character referred to by anthropologists as the
most recent common ancestor, which I’ll refer to simply as the common ancestor.
Around eight million years ago in central Africa lived an ape species whose descen-
dants would go on to include humans as well as the chimpanzees and bonobos.
We can use our parsimony rule of thumb and simple common sense to imagine
the common ancestor in a bit more detail. It had extensive body hair and likely
spent much of its time in the trees as do chimpanzees and bonobos. It lived in
central Africa and consumed a diet dominated by fruit, tropical fruit in the fig fam-
ily probably making up the major staple. Had we been able to study this ape, it
would certainly have told us important things about what would come for us in the
future, what changes were brewing. One thing that would end up affecting the fu-
ture of our relationship with infectious diseases was a new tendency present in this
animal: the urge and ability to hunt and eat meat.
An artist’s conception of “Ardi,” a female Ardipithecus ramidus, 4.4 million years
old, representative of the most recent common ancestor between humans and
chimpanzees. (Science Magazine / Jay Matternes)
That humans share with chimpanzees the trait of hunting animals has been known
for some time. It first emerged in the early 1960s when the British primatologist
Jane Goodall documented wild chimpanzees hunting and eating meat at Gombe
National Park in Tanzania during her pioneering efforts to study wild chimpanzee
behavior. Before the Goodall studies and a related set of studies conducted by
Japanese colleagues in the Mahale region of Tanzania, our understanding of chim-
panzee behavior in the wild was largely nonexistent. The finding that chimpanzees
hunted came as a shock to anthropologists, many of whom had come to believe
that hunting had emerged after our split with chimpanzees and shaped our evolu-
tion in a way that distinguished us from them.
Since then, detailed studies in Gombe and Mahale as well as in some of the
half-dozen more recently studied wild chimpanzee communities have solidified
our understanding of the important role of meat in the chimpanzee diet. While
chimpanzees hunt opportunistically, it is by no means sporadic. Chimpanzees can
hunt forest antelopes and other apes (even humans), but they tend to specialize in
a few critical species of monkey as prey. Their hunting is not only cooperative and
strategic; it is also very effective.
In the 1990s the primatologist Craig Stanford set out to study red colobus mon-
keys, but because so many of them died at the hands of chimpanzees, he ended up
switching his study to just that: how and why chimpanzees hunt these red colobus
monkeys. He found that chimpanzees were so successful in the hunting of red
colobus that the entire social structure of these monkeys was swayed by the annual
patterns of chimpanzee hunting. He calculated that some of the most successful
communities can bring down nearly a ton of monkey meat in a single year. Subse-
quent work among some groups of chimpanzees living in west Africa has shown
that they even employ tools for hunting, using a specially modified branch spear to
kill prey that nest within the holes of tree trunks.
And hunting is by no means restricted to chimpanzees. Related studies among
bonobos have been hampered by ongoing (human) wars and the lack of infra-
structure in the Democratic Republic of Congo (DRC), the only country in the
world with wild bonobo populations. Nevertheless, recent studies have begun to
detail the lives of these important relations. Evidence from research conducted
over the last ten years or so shows that bonobos, like their chimpanzee (and
human) cousins, actively hunt. Some bonobo sites show meat consumption at lev-
els similar to those that have been documented among chimpanzees.
In contrast to humans, chimpanzees, and bonobos, studies of our more distant
ape relatives—the gorillas, orangutans, and gibbons—have shown strikingly lim-
ited evidence of meat consumption and no evidence to date of hunting. It appears
that some of these apes may occasionally scavenge, but even that seems to be
quite limited. Taken together the evidence shows that hunting emerged sometime
before the split between humans and the lineage that would include chimpanzees
and bonobos. Our early common ancestor, living around eight million years ago,
probably hunted whatever it could get its hands on but almost certainly hunted the
monkeys in the forest habitats in which it lived.
The advent of hunting in these early ancestors surely had many advantages. The
increased caloric intake from hunted animals must have played well in a primarily
fruit- and leaf-eating species. The regular supply of monkeys must have increased
food stability in a constantly fluctuating food environment. It would have also
opened the door for future migration to regions with different kinds of food, a topic
to which we will return in chapter 3. Hunting, while undoubtedly beneficial for the
first of our ancestors who engaged in it, presents certain undeniable risks for ac-
quiring new and potentially deadly microbes—risks that would continue to have an
impact on their descendants for millions of years to come.
Hunting, with all of its messy, bloody activity provides everything infectious agents
require to move from one species to another. The minor skirmishes that our early
ancestors likely had with other species probably resulted in minor cuts, scratches,
and bites—insignificant compared to the intense exposure of one species to an-
other that is a direct result of hunting and butchering.
The chimpanzees who were devouring their feast of red colobus monkey in
Kibale forest that day were an instant, visual example of the blurring of lines be-
tween species. The manner in which they were ingesting and spreading fresh blood
and organs was creating the ideal environment for any infectious agents present in
the monkeys to spread to the chimpanzees. The blood, saliva, and feces were spat-
tering into the orifices of their bodies (eyes, noses, mouths, as well as any open
sores or cuts on their bodies)—providing the perfect opportunity for direct entry of
a virus into their bodies. And since they hunted a range of animals, their exposure
to new microbes would have been broad. Those conditions emerged in our ances-
tors around eight million years ago, forever changing the way that we would inter-
act with the microbes in our world.
While we still only understand the basics of how microbes move through
ecosystems, extensive research on toxins gives us an idea of how it works.
Microbes, like toxins, have the potential to negotiate their way up through different
levels of a food web, a process referred to as biological magnification.
Many pregnant woman are aware that there are risks associated with consuming
certain kinds of fish during pregnancy. This health suggestion follows from knowl-
edge of how certain chemicals move through food webs. In the complex food webs
of the oceans, small crustaceans are consumed by larger fish that are in turn con-
sumed by larger fish and so on. This goes on until we reach the top predator—a
hunter who is never hunted—the top of the food chain. Crustaceans have some
levels of toxins, such as mercury, that they’ve accumulated from the environment.
The fish that prey on crustaceans accumulate many of these toxins, and the fish
that consume these second-order predators accumulate even more. The higher in
the food chain we go, the higher the concentrations of such chemicals. So top
predator fish like tuna have high enough concentrations of toxins to represent a
potential threat to the fetus.
In the same way, animals higher in the food chain should generally be expected
to maintain a wider diversity of microbes than those lower on the food chain. They
have accumulated microbes like the mercury among fish, in a process we can think
of as microbial magnification. When our ancestors some eight million years ago
took up hunting, they changed the way they would interface with other animals in
their environment. And this would mean not only increased interaction with their
prey animals. It also meant increased contact with their prey’s microbes.
In the twenty years since its discovery, HIV-1 has caused death and illness on a
previously unimaginable scale. The AIDS pandemic has affected people in every
country in the world. Even today with antiviral drugs that can control HIV, the virus
that causes AIDS, it continues to spread, infecting over 33.3 million people at last
count. The spread of HIV in contemporary society has a range of determinants,
from poverty and access to condoms to cultural practices that dictate whether or
not a child is circumcised. The pandemic now has economic and religious
significance—and it invites commentary and discussion from philosophers and
social activists. Yet it was not always that way.
The history of HIV begins with a relatively simple ecological interaction—the
hunting of monkeys by chimpanzees in central Africa. While people normally think
about the origins of HIV as occurring sometime during the 1980s, the story actu-
ally begins about eight million years ago when our ape ancestors began to hunt.
More precisely, the story of HIV begins with two species of monkey, the red-
capped mangabey and the greater spot-nosed guenon of central Africa. They hardly
seem the villains at the center of the global AIDS pandemic, yet without them this
pandemic would have never occurred. The red-capped mangabey is a small mon-
key with white cheeks and a shocking splash of red fur on its head. It is a social
species living in groups of around ten individuals and eating a diet primarily of
fruit. It is listed as vulnerable, meaning its population numbers are threatened. The
greater spot-nosed guenon is a tiny monkey, one of the most diminutive of the Old
World monkeys. It lives in small groups consisting of one male and multiple
females and is able to communicate alarm calls that vary depending on the kind of
predator it encounters. One of the things these monkeys share is that they are
naturally infected with SIV, the simian immunodeficiency virus. Each monkey has
its own particular variant of this virus, something it and its ancestors have prob-
ably lived with for millions of years. Another thing these monkeys have in common
is that chimpanzees find them very tasty.
The simian immunodeficiency virus is a retrovirus. That means that unlike most
forms of life on the planet that use DNA as their code, which translates into RNA
and then into the protein building blocks that make up the meat of us all, SIV
works in reverse—hence the name “retro” virus. The retrovirus class of viruses be-
gins with RNA genetic code, which is translated into DNA before it can insert itself
into the DNA of its host. It then proceeds with its life cycle, creating its viral prog-
eny.
Many African monkeys are infected with SIV, and the red-capped mangabey and
greater spot-nosed guenon are among them. While few studies have been con-
ducted on the impact of these viruses on wild monkeys, it is suspected that they do
the monkeys no substantial harm. Yet when the viruses move from one host
species to the next, they can kill. This would become their destiny.
The work that deciphered the evolutionary history of the chimpanzee SIV was
reported in 2003 by my collaborators Beatrice Hahn and Martine Peeters and their
colleagues. Over the past decade, Hahn and Peeters have worked tirelessly to chart
the evolution of SIV—and they’ve succeeded. In 2003 they showed that the chim-
panzee SIV was in fact a mosaic virus consisting of bits of the red-capped
mangabey SIV and bits of the greater spot-nosed guenon SIV. Since SIV has the
potential to recombine, or swap, genetic parts, the findings showed that rather than
coming from an early chimpanzee ancestor, the virus had jumped into chim-
panzees.
It is tempting to imagine a single chimpanzee hunter as patient zero—an indi-
vidual, the first of its species to harbor the novel virus—acquiring these viruses in
short order from the monkeys it hunted, possibly on the same day. Alternatively,
the mangabey virus may have crossed sometime earlier and gained the ability to
spread among chimpanzees sexually, with patient zero acquiring it from another
chimpanzee and only subsequently acquiring the guenon virus through hunting. Or
perhaps both the guenon and mangabey viruses circulated for some time in chim-
panzees after they were acquired through hunting, with the final moment of genetic
mixing coming in a chimpanzee already infected by the two viruses. No matter
what the particular order of cross-species jumps, at some moment a chimpanzee
became infected with both the guenon virus and the mangabey virus. The two
viruses recombined, swapping genetic material to create an entirely new mosaic
variant—neither mangabey virus nor guenon virus.
This hybrid virus would go on to succeed in a way that neither the mangabey
nor guenon virus alone could, spreading throughout the range of chimpanzees and
infecting individual chimpanzees from as far west as the Ivory Coast to the sites in
East Africa where Jane Goodall began her work in the 1960s. The virus, now known
to harm chimpanzees,² would persist in chimpanzee populations for many years
before it would jump from chimpanzees to humans some time in the late nine-
teenth or early twentieth century. And it all started because chimpanzees hunt.
For a large and growing part of humanity, the meat we consume arrives clean and
prepackaged, and goes straight to our refrigerators. The killing and butchering of
the animals occurs far away on a farm or in a factory that we have never seen and
can scarcely imagine. Rarely do we witness blood or body fluids from these ani-
mals that were living and breathing beings even a few days earlier. This is because
the hunting and butchering of animals is a messy process. We don’t want to see it
or even think about it; we just want the steak.
During the years I’ve spent working with people hunting and butchering wild
game in places like the DRC and rural Malaysia, I’ve never become completely
accustomed to exactly what is required to prepare meat for consumption. We take
for granted what it means to remove hair and skin from a dead animal, the effort
needed to separate meat from the many bones distributed in an animal to support
its movement. We forget how many parts of an animal must be negotiated to get to
the prime cuts: the lungs, the spleen, the cartilage. Watching the process on the
dirt floor of a hut or on leaves spread out on the ground in a hunting camp, seeing
the blood-covered hands that separate the various parts of the animal and hearing
the bits of discarded meat and bone hit the floor still shocks me. It also helps to re-
mind me of the microbial significance of the event.
We tend to think of events like sex or childbirth as intimate, and they certainly
bring together individuals in ways that normal interactions cannot. But from the
perspective of a microbe, hunting and butchering represent the ultimate intimacy, a
connection between one species and all of the various tissues of another, along
with the particular microbes that inhabit each one of them.
The butchering in our own kitchen bears little resemblance to the hunting and
butchering that our common ancestor would have engaged in eight million years
ago. While these first hunting events are now lost, they probably held much in
common with the chimpanzees I saw sharing their red colobus meal in Kibale—the
dominant male holding down the animal with one hand and using its other hand
and teeth to pull apart the skin of the gut while seeking a preferred organ. I remem-
ber seeing the chimpanzee holding the organ in its hand, its fur slicked down with
blood, and thinking to myself that it would be nearly impossible to imagine a better
situation for the movement of a new microbe from one species to the next.
While we still hunt and butcher, the ways that we do so and the methods we use
to prepare meat differ radically from the methods of the past. The early ancestors
of humans and chimpanzees lacked the ability to cook, they lacked tools for
butchering, and they certainly lacked dental hygiene! Whether through a wound
from a broken monkey bone, an open sore in the mouth, or a cut on the arm, the
microbes of hunted animals infected these animals in ways that had not occurred
prior to the advent of hunting. Hunting fundamentally changed how they were ex-
posed to the microbes in their worlds, many of which had remained relatively iso-
lated in the animals that shared the forests with them. As much as hunting repre-
sented a milestone for our eight-million-year-old ancestors, it had equal impor-
tance for the world of our microbes.
There are many methods for comparing animals within an ecosystem. We can
chart the diversity of foods they consume, the diversity of habitats they utilize, the
range of space that they cover within an average year. We can also consider them
based on the diversity of microbes they possess, what I call their microbial
repertoire. Each species has a particular microbial repertoire. It includes viruses,
bacteria, parasites—all of the various microbes that can call that species home.
And while no single animal within a species will likely have all of the various pieces
of the microbial repertoire at any one time, it acts as a conceptual tool for mea-
suring that species’ microbial diversity—the range of microbes that infect it.
Species vary considerably in terms of their microbial repertoires. And hunting
and butchering do not provide the only avenue for microbes to jump from one
species to the next. Species that don’t hunt or butcher still have regular exposure
to the microbes of other species. Blood-feeding insects provide an important route
for microbes to move around. Mosquitoes, for example, often feed on a range of
different animals, in effect acting as physical carriers on which microbes can hitch
a ride to move from species to species within ecosystems. Similarly, contact with
waste from other animals, either through direct contact or indirect contact through
water, also provides critical connections in the networks that permit microbes to
negotiate the otherwise largely separated worlds of different host species.
Nevertheless, mosquitoes and water provide narrow paths from one host to the
next. Mosquitoes, for example, are not syringes. They are fully functional animals
that have their own immune systems, and even those microbes that can manage to
evade the mosquitoes’ defenses will be limited to those in the blood. Similarly,
water generally passes on those microbes that live in the digestive tract. Hunting
and butchering, in contrast, provide superhighways connecting a hunting species
directly with the microbes in every tissue of their prey.
When our ancestors began to hunt and butcher animals, they put themselves at
the center of the vast web of microbes living in the full range of tissues of their
various prey animals. Whether in the form of a virus in the brain of a bat, a parasite
in the liver of a rodent, or a bacterium living on the skin of a primate, the microbial
worlds of these various species suddenly converged on the common ancestor,
changing for them (and ultimately us) the range of microbes that they would carry.
The impact that the advent of hunting had on the microbial repertoires of the
common ancestor and its descendants would continue to play itself out over mil-
lions of years. As the lineage of the common ancestor diverged, multiple species
(chimpanzees, bonobos, and humans) would emerge, each with the capacity to
hunt. These species would go on to accumulate their own sets of novel microbes
from the animals on which they preyed. At times, these species would collide when
their habitats overlapped, allowing them to exchange microbes, with serious
consequences for both species.
Because humans are focused largely on our own health, we often forget that
cross-species transmission is not a one-way street. This was brought home for me
in vivid detail during my time working with chimpanzees in the Kibale Forest in
Uganda. On one afternoon, people from a local village came into our research
camp asking for assistance. The distraught villagers explained that a chimpanzee
had grabbed an infant child and severely bitten his brother, who had tried to pro-
tect it. The infant had not been seen again and was presumably eaten by the chim-
panzee. Upon a visit to the village, an eyewitness confirmed the young boy’s story.
The nasty bite wound on his upper arm was a reminder that would stay with him
forever.
The events made me think more carefully about chimpanzee predation, and a
subsequent analysis with my colleagues revealed that the event was not unique. Re-
ports from as early as the 1960s had documented similar events. Although it was
not a common activity, chimpanzees had hunted humans, usually infants, espe-
cially those who were left close to the edge of the forest while their mothers worked
on their farms. While disturbing, the idea that chimpanzees occasionally hunt
people should not surprise us. From the perspective of a chimpanzee, a red
colobus monkey, a forest antelope, and a human infant would all represent logical
potential prey. In the same way, humans, while occasionally observing food
taboos, hunt opportunistically and generally consume the full variety of local ani-
mals in their environment. Whether a closely related ape or a more distantly related
antelope, they all present opportunities for vital calories, and both chimpanzees
and humans exploit every one of them.
The fact that chimpanzees hunt humans and humans hunt chimpanzees would
come to have significance for the two species’ microbial repertoires. In the years
that followed the advent of hunting by the common ancestor, these two closely re-
lated but ecologically distinct species would each accumulate substantive micro-
bial diversity through hunting and other routes. And then, critically, from time to
time they would exchange microbes. We’ll explore the range of implications that
this exchange has in the coming chapters.
As the human lineage broke off and diverged, going through a near extinction
event, but then coming back full force with agriculture, animal domestication, and,
later, global travel and practices such as blood transfusions, the connections with
our ape cousins would continue to have importance for our microbial repertoires
in sometimes surprising ways. As we’ll explore, the role of this close connection
continues now with chimpanzees and other apes acting as the missing piece of the
puzzle in some of our most important diseases. Two close primate relatives—
chimpanzees that live and hunt diverse animal species in central Africa, and hu-
mans with rapidly expanding territory and globally interconnected relationships—
would prove to be an important combination. A recipe for pandemics.
