The Viral Storm (6 page)

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 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 acquiring 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 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.

*   *   *

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.

*   *   *

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.

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 remember 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.