Mary Claire King, as much as any individual scholar, has changed how we think about what it means to be human. She did so using genetics as a lens through which to see what was otherwise invisible. In her hands this lens offered many insights. It was King who first identified a key gene in breast cancer. It was King who helped to identity the missing dead in Argentina in the 1980s. It would also be King who, in 1975, first compare the genetic similarity of humans and chimpanzees. It was known chimpanzees and humans were similar, kin, but just how similar? One could only really guess. And to compare a chimpanzee and a human, one hunched, one upright, one furry, one relatively bald, one able to build cities, write, read and make music, one not, it seemed clear that a fair number of differences needed to be accounted for.
King compared the genes of chimps and humans. When she did, she was in for a surprise. The humans and chimpanzees were not 50% similar genetically, or 60%, or even 80%, they were 98 to 99% similar, nearly identical. All of the differences between us and them, must relate to the 2%.
What has followed has been a rich and detailed consideration, a consideration that is still very much underway, of the 2%. We now know many of the genes that lead human chimpanzee immune systems to be so different, and have some suspicion as to which genes are associated with chimpanzees never get the kind of heart attack we get (for more on that, see the last three chapters of The Man Who Touched His Own Heart). But there was a catch.
While geneticists were comparing humans and chimps, another tribe of biologists, parasitologists, were doing something very different, tallying lists of the species that live in chimps or live in humans (and every other species for that matter). For parasitologists these included both big things (worms and mites, for instance), little things, such as protists, and truly tiny things including viruses. But there was a barrier to progress. The parasitologists studying humans and those studying chimpanzees, they weren’t friends. And so while each group had a tally of the organisms found in their study animals, the tallies were not compared, EVER. But such a comparison is important. A large proportion of the genes (and their products, including proteins, but also our behaviors and much more) of any organism are those of its parasites and pathogens.Someone with more than one worm species, for instance (many people have more than one worm species), may actually be dragging around more worm genes than human genes! We are what eats us.
When I pulled together the lists of pathogens and parasites of chimpanzees (derived from a list provided by Charlie Nunn and his collaborators) and a list of those of humans, I was in for a surprise like that of Mary Claire King, but in the opposite direction. Most of the parasites and pathogens of humans are NOT found in chimpanzees. We have many of the same parasites and pathogens as them (though far less than 98%) but they lack nearly all of those found in us. From the perspective of the genes of parasites and pathogens, humans and chimpanzees are not similar at all.