New analyses of chimpanzees and humans reveal them to be far more different than suspected, perhaps as much as 95% different.
Sometimes it takes time to see something clearly. This is especially true in writing a book. Book writing, even non-fiction book writing, is voodoo magic. It is a pot of incantations out of which emerges the animal with which one must wrestle in mornings, afternoons, evenings and dreams.
Book writing begins under reasonable control. A proposal emerges. The proposal includes a table of contents; the table of contents is pushed this way and that by, if you are lucky, an agent and an editor. As is often the case these days, it might even be pushed this way and that by more than one editor, one who starts with the book and another who jumps in when the first editor takes a more prestigious job at a different publishing house, or burns out.
But once the bones of a proposal are knocked together with mallet, red pen, and track changes, the business of writing begins. Once writing starts, several things happen. Characters emerge around which stories develop. Some of those characters threaten to take over the book. Others prove too thin and are vanquished. Then, if the book deals with science, new truths emerge that were missed in pulling together the table of contents. Some old truths are revealed, upon closer inspection, to be less clear than they seem. Then there are mysteries. One of the wondrous pleasures my unusual set of career choices has given me is the ability to write enough about topics in fields adjacent to my own to meet up not only with the mysteries of my people but also those of other people. The wax and wane of characters, truths and mysteries leads a book in directions different from where it began.
The net result is that the current of a book is often similar to that which was originally intended, but the undercurrents, those understories, can be far more complex. On the surface the story goes in one direction, but deeper down forces tug in ways of which the writer him or herself, may be only partially aware.
When the writer finishes the book, turns it in to be edited, proofed, copy-edited and turned into the strange form that appears on bookshelves, a form that feels relative to the living animal with which the writer has lived, frozen, dead even, the understories continue to live in the writers mind. Sometimes they take over. Every so often the writer will be at a dinner party and will be asked a totally ordinary question and will be unable to respond, caught somewhere between the question being asked and some inner conversation with the understories.
The understories, in my experience, lead to some mix of modest madness and next books. My first book was Every Living Thing in which I wrote about how much remains to be discovered and how consistently scientists underestimate their/our ignorance. I focused the book on poorly studied places on Earth: rain forest canopies, deep cores into the Earth, space, clouds, and the bottom of the sea. But the undercurrent of the book was the realization that the book’s premise extended not only to these realms but also to our daily lives, including our bodies, the biology and ecology of which seemed far less well explored than imagined by most scientists or the public. Building on this undercurrent, I then wrote the book, The Wild Life of Our Bodies, in which I considered the many consequences of our shifting relationship with other species and our ignorance about our own bodies.
Many undercurrents, swirling vertices, emerged in writing The Wild Life of Our Bodies. Those vertices changed the research I do in my lab, altered, in some ways, how I live, and led to a new book, about the ecology, evolution and history of one of the parts of our bodies that we seem to have most ignored (the heart, resulting in the forthcoming book, The Man Who Touched His Own Heart). But two of the undercurrents from The Wild Life of Our Bodies still twist in my mind in ways that wake me up. One is the extent to which studies of one aspect of the interactions between humans and other species—our microbiomes (a hip new term for a very, very old idea that makes the idea seem as though it is new) have ballooned in the last years, turning a neglected aspect of human biology into an aggressive, take-no prisoners, sexy field. This transformation has been remarkable and will be one historians will write about for generations (When they do, I hope they are able to track down some of the scandalous emails and phone calls exchanged by the current kings of the field. They will make for good narrative that is still too fresh for me to touch.).
But the other undercurrent is one that has crystallized recently, it is the question of how much humans have changed relative to other species and how to quantify that change. The closest living relatives of humans are chimpanzees and their kin bonobos. That these animals are similar to us in many ways is no surprise to anyone who has stood near a chimp. Their hands are like ours, their eyes like ours, their entire bodies like ours. This sense of deep similarity was confirmed in early genetic studies that revealed that chimpanzees and humans were nearly identical genetically. One can make such a comparison in multiple ways (focusing on coding and non-coding DNA, for instance), but almost no matter how one makes the comparison, humans and chimpanzees are nearly identical, genetically, 95 to 99.5% similar. Rightly, this similarity has led to a huge focus on the differences, that 0.5 to 5%.
But the undercurrent of The Wild Life of Our Bodies was that in addition to these differences, human populations also differ from each other and from chimpanzees in far more substantive ways when one considers the species we interact with. Some human populations farm (which makes us more similar, in some ways, to other societies that farm, such as leaf-cutter ants). Other human populations face crowd diseases such as colds and flus. These differences were the stories of The Wild Life of Our Bodies, but they were and are complex, a shifting set of webs that we both engineer and unintentionally create.
What only crystalized recently was the idea that one could measure these differences and put a number on them akin to that used to mark the genetic differences between humans and chimps. We have started to do this in the context of skin microbes, trying to measure how different the microbes of human and ape skin are (hint, far more than 5%). A recent study has considered gut microbes, also finding large differences among human populations and between humans and those of other primates.
But the most concrete measure of our difference, and this is the undercurrent that just swirled into my mornings and dreams (and, I suppose, my colon) is in the context of our parasites and pathogens, those beasts that kill and/or eat at us. The data necessary to make such a comparison have been floating around and, simply, not directly, considered. Several large databases of the parasites and pathogens of humans exist. One of these databases, the one we have worked on most in my lab, GIDEON, focuses on diseases rather than the pathogens themselves. This database considers which diseases are found in each political region and in as much is the best dataset for considering spatial patterns in human diseases and their parasites and pathogens. We recently used it to discern the basic biogeographic regions of human pathogens, producing a set of maps that I’ll hazard say more about how the world works than nearly any other (that isn’t because we are so good, it is just because pathogens and parasites and their presence or absence are really that important). But this dataset isn’t useful for comparing humans to non-humans because nothing similar exists for non-humans. What does exist, however, is a list, compiled by my friend Charles Nunn and his collaborators, of all of the pathogens and parasites known from non-human primates. A separate, similar list exists for humans. What happens if we compare these lists? How many parasites and pathogens do we share with non-human primates? How complete has this transition that I have written a whole long book about been?
Well, we can now produce an answer. The answer is that non-human apes appear to have about 40 to 50 species of parasites and pathogens that have ever been recorded. Some of these are rare. Some are probably found on most apes but have only been well-studied in one. More will be discovered, but let’s say fifty per species. While on the one hand one might argue that apes have been poorly studied, it is also true that zoo and lab apes have provided many opportunities for parasites and pathogens to be noted. The number for humans? Roughly TWO THOUSAND species of parasites and pathogens have been found in humans.
So for starters, as a species, we are FAR more diverse in terms of our parasites and pathogens than are the other apes. But what is more, although we have the potential to be infected by any of 2000 parasites and pathogens, we DO NOT GET INFECTED by most of the species found in non-human apes. The proportion of parasites and pathogens shared between humans and non-human apes is incredibly low. Non-human apes are infected by about 10 parasites and pathogens also found in humans and half or so of those are things that we have given them. In other words, while we may be very similar genetically to chimpanzees, bonobos, gorillas and orangutans, we are not similar at all in terms of our interactions. The same is obviously true with regard to the species we eat. The same will be shown to be complexly true with the bacterial symbionts we depend on. Humans and chimpanzees may be 95% similar genetically, but we are, at best, 5% similar in terms of the species we interact with (If we compare humans who live in apartments in New Jersey and hunter gatherers in Africa, we also find enormous differences in our interactions).
In this light, what makes us human, what most separates us from the chimpanzees in the zoo is not our brains. It is not some measure of our emotions. It is instead the change in the species we interact with. This was the understory of The Wild Life of Our Bodies. As for the stories of these transitions, which species we have lost and which we have gained, well, that was the overstory. That you can find in the book.