But this is actually only one way of many that parasites may be able to help turn their hosts into new species. Genetic parasites can speed up the evolution of their hosts, for instance. In order for evolution to take place, genes have to take on new sequences. That can happen with ordinary mutations—the occasional cosmic ray from outer space slamming into DNA or the sloppy crossing of genes as cells divide. But it can happen faster with the help of a genetic parasite. As it hops from chromosome to chromosome within a cell, or as it leaps from species to species, it can wedge itself into the middle of a new gene. This sort of rude arrival usually causes trouble, in the same way throwing a random string of commands into the middle of a computer program does. But every now and then, the disruption turns out to be a good thing, evolutionarily speaking. An interrupted gene may suddenly become able to make a new kind of protein that does a new sort of job. The blind jump of one genetic parasite seems to have made us able to fight parasites more effectively. The genes that make the receptors on T and B cells show signs of having been created out of the blue by genetic parasites.
And once a genetic parasite has established itself in a new host, it can disrupt the unity of the entire species. The typical fate of a genetic parasite is to explode through its host’s genome during the succeeding generations, wedging itself into thousands of sites. As time passes, the hosts that carry it will diverge on their own into separate populations—not distinct species, but groups that tend to breed among themselves. As they do, the genetic parasite continues to hop from place to place in their DNA. Its hopping will be different in each population, and it will make their genes more and more different from one another. Eventually, when a Romeo and Juliet from the two populations meet and try to mate, their distinct collections of genetic parasites may make them incompatible. By making it harder for different populations of their hosts to mix their genes, the genetic parasites encourage them to split into new species.
Another way parasites might be able to create a new species is by mucking up the sex lives of their hosts. A bacterium called Wolbachia lives in 15 percent of all insects on Earth as well as many other invertebrates. It lives within its host’s cells, and the only way it can infect a new host is by colonizing a female’s eggs. When the egg that Wolbachia lives inside becomes fertilized and grows into an adult, it grows up with a case of Wolbachia infection.
There’s a downside to this way of life: if Wolbachia should grow up in a male it faces a dead end, because there are no eggs for it to infect. As a result, Wolbachia has taken control of its hosts’ sex lives. In many of its host species, it tampers with the sperm of infected males so that they can successfully mate only with Wolbachia-carrying females. If one of these infected males should try to mate with a healthy female, all of their offspring will die. Wolbachia uses a different strategy in some species of wasps: normally these insects are born as males and females, which reproduce sexually, but when Wolbachia infects them, the wasps become female-only, able to mother only more females. By turning its hosts all female, the bacteria gives itself that many more hosts.
In both these cases, Wolbachia genetically isolates the infected hosts from the uninfected ones. A newly born host will be the offspring of either Wolbachia-carrying parents or two healthy ones. It won’t be a healthy-unhealthy hybrid. By setting up this reproductive wall, the parasite may be able to set the stage for a new species to form. Wolbachia is only the best-known parasite out of many that tamper with their hosts’ sex lives, so this may turn out to be a common way new species form.
Darwin always had a sharp sense of irony, but this one might have been too much for him to bear. To understand how life changes its form, how evolution is driven forward, and how new species come to be, he could have found inspiration in his dying children. When it comes to the tapestry of life, parasites are a hand at the loom.
7
The Two-Legged Host
Humanity has but three great enemies: fever, famine and war; of these by far the greatest, by far the most terrible, is fever.
The beauty of parasites is an inhuman one. It’s inhuman not because parasites have come from another planet to enslave us but because they have been on this planet so much longer than we have. I sometimes think about Justin Kalesto, the Sudanese boy who was so racked by sleeping sickness that he could only whimper in his bed. He was twelve years old, and on his own he’d be no match against a dynasty of parasites that have lived in almost every sort of mammal—in reptiles, birds, dinosaurs, amphibians—everything backboned since fish came ashore, that have lived inside fish before anything walked on land, that have evolved their way into the guts of insects as well as vertebrates, that even thrive inside trees. The entire human race is a child like Justin: a young species perhaps only a few hundred thousand years old, a tender new host for trypanosomes and other parasites to make their own.
Of course parasites have never encountered a host quite like us. We can fight against them with inventions such as medicines and sewers as no animal has before. And we’ve changed the planet around us as well. After billions of years of glorious success, parasites now must live in the world we’ve made: a world of shrinking forests and swelling shanty towns, of vanishing snow leopards and multiplying chickens. But thanks to their adaptability, they’re doing well overall. We should worry about the disappearance of condors and lemurs; their extinction will show us how badly we’re stewarding the planet. But we shouldn’t worry about the extinction of parasites. The tick species that live on black rhinos will probably disappear with their hosts in the next century. But there is no danger of parasites in general disappearing from the planet during the lifetime of our species; just about all of them will probably still be here when we’re gone.
While parasites must live in the world we’ve made, the opposite is true as well. They have structured the ecosystems that we depend on, and they have sculpted the genes of their hosts for billions of years, our own included.
It is surprising just how precisely they’ve shaped us. When immunologists began studying antibodies, they found that they could sort them into categories. Some had hinged branches; some were built like five-rayed stars. Each group of antibodies has evolved to work against particular sorts of parasites. Immunoglobulin A works against the influenza virus and little else. The star-shaped immunoglobulin M staples its rays to bacteria like Streptococcus and Staphylococcus.
And then there was a strange little antibody called immunoglobulin E (IgE). When scientists first found this antibody, they couldn’t figure out what it was for. It would remain at barely detectable levels in most people, except during a bout of hay fever or asthma or some other allergic reaction, when it would suddenly surge through the body. Immunologists have worked out how IgE helps trigger these reactions. When certain harmless substances get into the body—ragweed pollen, for example, or cat dander, or cotton fibers—B cells make IgE antibodies tailored to their shape. These antibodies then are anchored to special immune cells called mast cells that are found in the skin, the lungs, and the gut. Later, the harmless substance for which the IgE was made enters the body again. If it latches onto a single IgE antibody on a mast cell, nothing happens. But if it should latch onto two of them sitting side by side on the mast cell, the harmless substance switches it into action. Suddenly the the mast cell blasts out a flood of chemicals that make muscles contract, fluids pour in, and other immune cells flood the site. Hence the sneezing of hay fever, the wheezing of asthma, the red hives of a bee sting.