Such is the case with David Roos: the only way he can understand what Toxoplasma is today, and how it is that malaria is a green disease, is to plunge back hundreds of millions of years. These sorts of histories are just as fascinating as those of free-living animals. They are tangled up with the evolution of the rest of life, going back 4 billion years. In fact, the history of parasites is, to a great extent, the history of life itself.
Reconstructing that history isn’t easy. Parasites tend to be squishy or crunchy—two conditions that don’t augur well for fossils. Every few million years, a parasitic wasp may stumble into a blob of amber, or a male crab feminized by a parasitic barnacle may leave behind its transgendered fossil, but for the most part parasites vanish in the rotting tissues of their hosts. Rocks don’t have a monopoly on clues to life’s history, though. Evolution has formed a vast tree, and biologists today can inspect its leafy tips. By comparing the biological features they find there, they can work their way back to the crooks of branches, to the tree’s base.
Biologists draw the branches of this tree by figuring out which species are most closely related to one another. Their close heritage shows that they must have diverged from a common ancestor more recently than from other species. To see this kinship, biologists look at the similarities and differences among organisms, judging which ones are the result of common descent or the illusions of evolution. A duck, an eagle, and a bat all have wings, but the duck and the eagle are much more closely related. The evidence is in their wings: on birds they consist of feathers hanging from a fused hand; a bat has membranes stretched over long fingers. The fact that bats are hairy, give birth to live young, and nurse them with milk helps show that despite their wings, they’re actually more closely related to us and other mammals than to a bird.
Flesh and bone can say only so much, though. They do not say definitively whether bats are closer cousins to primates or to tree shrews, for instance. And for organisms that don’t have flesh or bone, they say nothing at all. That silence has pushed biologists in the past twenty-five years to compare the protein and DNA of organisms rather than wings or antlers. They have learned how to sequence the genes and compare them with the help of computers. This approach brings its own pitfalls—genes can sometimes create trees as confusing as flesh and bone—but while they may be provisional, they have allowed biologists to look for the first time with one grand sweep of the eye at all of life.
The base of the tree represents the origin of life. Many of the organisms that occupy the branches closest to the base live today in scalding water, often around hydrothermal vents. That suggests that life may have gotten its start in such a place 4 billion years ago. Gene-like molecules may have assembled inside little fatty capsules or perhaps in oily films coating the sides of the vents. After untold millions of years, the first true organisms formed, bacteria-like things that carried genes floating loose inside their walls. Out of these bacterial beginnings, life began to diverge into separate lineages. The Archaea continued a basically bacteria-like kind of life, while a third branch—the eukaryotes with their DNA balled up tight in a nucleus and their power coming from mitochondria—took on a drastically different form.
Parasites, according to the traditional definition of the word (the creatures that cause malaria and sleeping sickness, that cram into guts and livers, that burst out of caterpillars as if their hosts were giant birthday cakes), all sit on branches on the eukaryote part of the tree. They have abandoned a life in the sea or on land for one inside other eukaryotes. They include organisms separated by vast evolutionary gulfs from ourselves—trypanosomes and Giardia branched off on their own separate destinies at the dawn of the age of eukaryotes, over two billion years ago. Among the parasites there are also much closer relatives, such as fungi and plants. Parasitic animals, such as blood flukes and wasps, are practically our kissing cousins. Parasitism is scattered across the eukaryote domain, a way of life that lineages have independently adopted and have found to be immensely profitable for many hundreds of millions of years.
Yet, this tree also makes it clear just how shallow the conventional definition of parasite is. Why should the name be restricted to organisms that are found on one of the three great branches of life? Nineteenth-century biologists were right to call infectious bacteria parasites. Just as some eukaryotes abandoned the free-living life, so did certain bacteria such as Salmonella and Escherichia coli, while other bacteria have kept up their independence in oceans, swamps, and deserts—even under Antarctic ice. The difference is only in genealogy, not lifestyle.
And even this definition of parasites is too parochial. Nowhere on this tree, for instance, can you find a flu virus. That’s because viruses aren’t, strictly speaking, living things. They have no inner metabolism and can’t reproduce on their own. They are nothing more than protein shells, which carry in them the equipment necessary to get into cells and then use the cell’s own machinery to make copies of themselves. Yet, viruses have the same sorts of parasitic hallmarks you could find in creatures like blood flukes—they thrive at their host’s expense, they use some of the same tricks to evade the immune system, and they can sometimes even change their hosts’ behavior to increase their spread.
In the 1970s, the English biologist Richard Dawkins made viruses less of a paradox. Viruses may not be alive in the traditional sense, but they get the basic job of life done: they replicate their genes. Animals and microbes exist, Dawkins argued, to do the same thing. We should think of their bodies, their metabolism, their behavior all as vehicles that genes build in order to get themselves replicated. In that sense, a human brain is no different from the protein coat that allows a virus to slip inside a cell. This view of life is a controversial one, and many biologists believe it downplays the importance of life’s complexity. But it works very well when it comes to parasitism. For Dawkins, parasitism is not what some particular flea or thorny-headed worm does. Parasitism is any arrangement in which one set of DNA is replicated with the help of—and at the expense of—another set of DNA.
That DNA can even be part of your own genes. Huge swaths of human genetic material do nothing for the good of the body they’re in. They don’t make hair, they don’t make hemoglobin, they don’t even help other genes do their job. They consist of little more than the instructions for getting themselves replicated faster than the rest of the genome. Some of them produce enzymes that slice them free and then insert them at another point in your genes. Soon the gap they leave behind is visited by proteins that search for damaged DNA. Because human genes come in pairs, these proteins can use the undamaged copy as a guide, and rebuild the stretch that disappeared. In the end, there are two copies of the jumping DNA.
These chunks of wandering genetic material are sometimes called selfish DNA or genetic parasites. They use their host—their fellow genes—to get themselves replicated. Like more conventional parasites, genetic parasites can harm their host. As they insert themselves at random places in the genome, they can cause diseases. Because genetic parasites can replicate at a faster rate than their fellow genes, they have swamped the genome of many hosts, including humans.
Parents pass their genetic parasites down to their children, and it’s possible therefore to sort selfish DNA into families, descendants of common ancestors that lived within the common ancestors of their hosts. Genetic parasites have their own dynasties that rise and fall. When a founder first turns up in a new host’s DNA, it starts copying itself at an explosive pace, packing its host gene with parasites. (I speak here of an explosion over evolutionary time—perhaps thousands of years.) Genetic parasites are sloppy duplicators, though, and they often make defective copies of themselves. These misfits can’t replicate themselves and simply clog up their host’s DNA. Genetic parasites are thus always risking self-inflicted extinction.