At the end of the day at the National Parasite Collection, after Eric Hoberg and I had spent an afternoon in his office talking and searching slides, I asked if I could go back down to the collection. “Sure. Just let me unlock it for you,” he said. We walked back downstairs, and he opened the door. It was empty now; Donald Poling had finished his slide-scraping for the day and had gone home. As I walked in, Hoberg stood by the door and told me to ask if I needed anything, and then he shut me in. The heavy door closed with more finality than I would have liked. Now I was trapped with the parasites. But after I got used to being shut in with them, the place became meditative. This was the closest thing to a proper museum I could think of for parasites, even though a grand diaspora of parasites was missing—the parasitic wasps and gall makers scattered in entomological collections, the protozoa hidden in schools of tropical medicine, Sacculina in the hands of some Danish expert on barnacles. Someday, I thought, you’ll all be reunited, and maybe in something classier than an old guinea pig barn.
6
Evolution from Within
The wise learn many things from their enemies.
The Origin of Species is a mournful book. God did not put species here on Earth balanced in perfect harmony, Darwin was saying. They are born out of a vast, ongoing death. “We behold the face of nature bright with gladness, we often see superabundance of food,” he wrote. “We do not see, or we forget, that the birds which are idly singing round us mostly live on insects or seeds, and are thus constantly destroying life; or we forget how largely these songsters, or their eggs, or their nestlings, are destroyed by birds and beasts of prey.” Most plants and animals never get a chance to reproduce, he argued, because they are killed by some predator or grazer, are outcompeted by members of their own species for sunlight or water, or just starve to death. The few that survive all these menaces and reproduce pass on their secret to success to the next generation. And out of all this death comes natural selection, which can transform it into the songs of birds, the leap of a flying fish—into a world that looks, at least on its surface, bright with gladness.
Yet, Darwin said little about one particularly powerful evolutionary menace, one that brought him a lot of personal sadness. His ten children struggled against diseases such as influenza, typhoid, and scarlet fever, and by the time The Origin of Species came out in 1859, three of them had died. Darwin himself suffered for much of his adult life with fatigue, dizzy spells, vomiting, and heart trouble. He once described his health this way: “Good, when young, bad for the past 33 years.” Although no one is sure what made him suffer, some have suggested that he had Chagas disease. Chagas disease is caused by Trypanosoma cruzi, a species of trypanosomes related to Trypanosoma brucei, the cause of sleeping sickness. T. cruzi slowly wrecks parts of the nervous system, and the ways to die of Chagas are horrible in their variety: your misfiring heart may stop beating, for example, or your intestines may stop getting the proper commands for peristalsis and let food pile up in the colon until you die of blood poisoning. T. cruzi is spread by the benchuca, a biting insect of South America, and Darwin was bitten by one as he was traveling the world on the H.M.S. Beagle; many of his symptoms arose only when he returned to England. The Darwins didn’t have to worry about getting eaten by wolves or starving to death, but infectious diseases—in other words, parasites—could still ravage them.
The toll that parasites take on the rest of life is far heavier—a toll that in terms of evolution is on par with predators and starvation. Viruses and bacteria tend to do their work quickly, multiplying madly and causing diseases that either kill or are defeated by the immune system. Eukaryotic parasites can be swiftly fatal as well—witness the brutality of sleeping sickness and malaria—but they can also do other kinds of damage. Ticks and lice may only live on the skin, but they can leave their host gaunt and emaciated. Intestinal worms can let their hosts live for years, but they stunt their growth and cut down their litters. The flukes that Kevin Lafferty studied in the Carpinteria salt marsh don’t destroy their killifish hosts themselves, but they turn them into dancing bird food. A crab infected with Sacculina may live a long life, but because it has been castrated by its parasite, it cannot pass on its genes. Evolutionarily speaking, it’s a walking corpse.
By keeping their hosts from passing on their genes, parasites create an intense natural selection. Perhaps parasites caused Darwin too much misery for him to recognize that they can be a creative evolutionary force in their hosts. A lot of the evolution that results takes place where you’d expect it: in the immune system, which defends animals from invaders. But it also brings out things that seem at first to have nothing to do with diseases. There’s growing evidence that parasites are responsible for the fact that we, and many other animals, have sex. The tail of a peacock, and other devices that males use to attract females, may be brought to us thanks to parasites. Parasites may have shaped societies of animals ranging from ants to monkeys.
Parasites have probably been driving the evolution of their hosts since the dawn of life itself. Four billion years ago, when genes formed loose confederations, parasitic genes could take advantage of them and get themselves replicated faster than the rest. In response, these early organisms probably evolved ways to police their genes. This sort of monitoring still goes on today in our own cells, which carry genes that do nothing but search for genetic parasites and try to suppress them.
When multicellular organisms evolved, they became a particularly choice target for parasites, since each one offered a big, stable habitat rich with food. And multicellular organisms had to fight a new sort of parasitism as well, in which some of their own cells tried to replicate at the expense of the rest of the organism (a problem we still face with cancer). All these pressures led to the evolution of the first immune systems. But for every step that a host takes against parasites, parasites are at liberty to evolve a step in response. Say an immune system evolves a tag it can put on parasites to make them more recognizable and easier to kill. The parasite can then evolve the tools it needs to rip that tag off. Immune systems became increasingly sophisticated in response; about 500 million years ago, for example, vertebrates evolved the ability to recognize specific kinds of parasites with T and B cells, and make antibodies to them.
This evolutionary back-and-forth didn’t just happen back in the depths of time. It happens today, and biologists can watch it in action if they run the right sort of experiment. A. R. Kraaijeveld of the Imperial College in England performed one such experiment with fruit flies and the wasps that parasitize them. For his experiment, he chose a wasp and two of its host species: the fruit flies Drosophila subobscura and Drosophila melanogaster. He raised the wasps on D. subobscura flies, and then put a few dozen of the parasites in a chamber with D. melanogaster. The wasps parasitized these new hosts, and they killed nineteen out of every twenty D. melanogaster. But one out of twenty D. melanogaster managed to marshal its immune system and kill the wasp larvae. Kraaijeveld took these resistant fruit flies and used them to breed the next generation of D. melanogaster.