One thing is clear, though: that ancestor of Plasmodium and Toxoplasma didn’t live inside animals. A billion years ago, there weren’t any animals yet to parasitize. At the time, single-celled creatures were only just beginning combining into colonies and collectives. Many of the first multicellular creatures were like nothing alive today. Some of them looked like inflatable mattresses or the ornate coins of some ancient kingdom. It wasn’t until about 700 million years ago that the first kinds of animals we see today arose: corals, jellyfishes, arthropods. Meanwhile, algae began organizing into more complicated forms, giving rise to plants, and about 500 million years ago they moved on shore, forming a mossy carpet and later evolving into low-stalked plants, and finally trees. Soon afterward, animals came on shore as well—centipedes and insects and other invertebrates by 450 million years ago, and the first lumbering vertebrates 360 million years ago.
Multicellular organisms created a seductive new world for parasites to explore. They concentrated food into big, dense bodies that were stable homes for weeks or years at a time. The animals of the Cambrian oceans attracted protozoa like Plasmodium as well as bacteria and viruses and fungi. And once again, a new kind of parasite came into existence: animals themselves evolved to live inside other animals. Flatworms made their way into crustaceans, where they diversified into flukes, tapeworms, and other parasites. Crabs, insects, arachnids—at least fifty times other lineages of animals followed suit.
The parasites evolved quickly within their hosts into forms quite unlike their ancestors. Relatives of jellyfish began to parasitize fish, and stripped themselves down into little sporelike shapes, which today plague the trout of American rivers with whirling disease. As their hosts became bigger and more widespread—growing to towering trees, ant colonies millions strong, marine reptiles eighty feet long—parasites enjoyed an ever-expanding habitat. After the first flush of success at the dawn of life, after the brutal clamp-down as hosts became better organized, now came a new golden age for parasites.
Our own lineage, the vertebrates, hasn’t done a very good job at becoming parasites. Among the few that have are some species of catfish in the rivers of Latin America. The most famous one of them is the candiru, a pencil-thin fish. It earns its fame by attacking people who urinate in rivers. It follows the odor of their urine and rams itself into their urethra. Once it sinks its teeth into a penis or a vagina, it’s almost impossible to get out. Attacking people is not how the candiru makes a living, though; it usually feeds on other fish, working its way under their gill flaps and sucking blood from the delicate vessels underneath. After a few minutes it drops off and looks for another fish to make its host. Other species have an even more parasitic way of life. When fish are caught in Latin America, they’re sometimes found with inch-long catfish lodged in their gills. Those little fish may spend most of their lives there, feeding on blood or mucus from their hosts.
No one knows why there aren’t more candirus in the world, but there may be some things about being a vertebrate that make a parasitic life hard. Vertebrates have high metabolisms compared with invertebrates, so they may not be able to get enough food within another animal. To be a parasite, an animal needs to produce a lot of young, because getting into the next host is so difficult and so essential. Vertebrates need to put a lot of energy into each offspring, so they may not be able to meet the challenge. But parasitism, as Richard Dawkins pointed out, doesn’t have to take a conventional form like a tapeworm. Imagine an animal that could somehow trick another animal into raising its young. The tricker would be more likely to pass on its genes, while the trickee would have less time to tend to its own offspring and to its own genetic legacy. In fact, there are many species—both invertebrates and vertebrates—that practice just this sort of social parasitism.
Among the invertebrates, one of the most extreme cases can be found in the Swiss Alps. There you find nests of the ant Tetramorium. If you look for the queen, chances are good you’ll find some pale, strangely shaped ants sitting on her back. They are not a special caste of Tetramorium ant but a different species altogether: Teleutomyrmex schneideri. Teleutomyrmex spends most of its life on a Tetramorium queen’s back, hugging her with specially designed gripping legs. Instead of attacking these aliens, the Tetramorium workers let them eat the food they regurgitate for their queen. The Teleutomyrmex parasites mate inside their host’s nest, and the new queens leave to find a new colony where they can hop on a new host.
The secret to parasitizing ants this way is creating illusions of smell. Ants depend mainly on smells to perceive the world, and they’ve evolved a complicated vocabulary of airborne chemicals to communicate with each other—to lay down food trails, to set off a colony-wide alarm, to recognize each other as nestmates. Teleutomyrmex can fool their hosts into caring for them rather than eating them because they can produce signals that make their hosts perceive them as queens themselves. The reason why Teleutomyrmex can cast these spells is probably that they evolved from their own host, turning their common language against their kin.
But many animals are social parasites of ants that aren’t ants themselves. Some butterflies, for example, can trick ants into rearing their caterpillars. The butterflies lay their eggs on flowers, and when the caterpillars hatch, they drop to the ground, where ants come across them. Normally, ants look at a caterpillar as a gigantic lunch. But if they come across a social parasite, they act as if the caterpillar is a lost larva from own colony. Deceived by the caterpillar’s odors, the ants drag it back to their nest, where they feed it and groom it the way they would any of their own larvae. Sometimes the ants even prefer the parasite to their own young. The caterpillar spends the winter growing in this luxury, after which it forms a cocoon. The ants go on caring for it as it metamorphoses into a winged butterfly. Only when it emerges from its cocoon does it finally occur to the ants that a huge intruder is in their midst and they try to attack it. But the butterfly bolts out of the nest and flies away.
All these social parasites essentially do what any conventional parasite does: they find the weaknesses in their hosts’ defenses and turn them to their own advantage. There are vertebrates that do the same thing. The cuckoo, for instance, lays its eggs in the nests of other birds such as reed warblers. When a young cuckoo hatches, it proceeds to hurl its host’s eggs and nestlings to the ground. The reed warbler feeds the cuckoo anyway, even as it grows so large that it dwarfs its stepparent. Once it is fully grown, the cuckoo flies off to find a mate, leaving the childless reed warbler behind.
Ants perceive their world mainly by smells, but birds depend much more on their eyes and ears. So cuckoos and other parasitic birds don’t create fake smells but fake sights and sounds. The cuckoo egg mimics those of its host species, so the host is unlikely to get the urge to throw it out of the nest. After the cuckoo is born, it tricks the reed warbler into feeding by playing on the signals it uses to feed its young. To figure out how much food to catch, reed warblers look down in their nest, where their babies are holding open their mouths. If they see a lot of pink—the inside of bird mouths—they automatically hunt for more food. At the same time they rely on the sound of their crying babies as a second signal. If the babies are still hungry and are crying, the warbler will find more food.