by Nathan Wolfe
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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 toward 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 something we share with other animals. Of course, the objective of science is not to uncover 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 humans 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 fighting wars were unique have been overturned by discoveries that chimpanzees engage 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 bonobos, 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 chimpanzees 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 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 descendants 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 family 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 future 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)
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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 chimpanzee 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 evolution 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 monkeys, 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. Subsequent 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 infrastructure 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 levels 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 limited 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 human
s 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 acquiring new and potentially deadly microbes—risks that would continue to have an impact on their descendants for millions of years to come.
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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 another 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 between 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 spattering 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 ancestors around eight million years ago, forever changing the way that we would interact 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 knowledge 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 consumed 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.
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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 actually 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 monkey 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 probably lived with for millions of years. Another thing these monkeys have in common is that chimpanzees find them very tasty.
L: Lesser white nosed guenon; R: red-capped mangabey. (L: © Tier und Naturfotografie / SuperStock; R: Shutterstock / Nagel Photography)
The simian immunodefiency 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 begins 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 progeny.
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 conducted 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 chimpanzee 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 chimpanzees.
It is tempting to imagine a single chimpanzee hunter as patient zero—an individual, 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 chimpanzees 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 nineteenth or early twentieth century. And it all started because chimpanzees hunt.
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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 animals 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 remind me of the microbial significance of the event.