Other tapeworms may have survived the asteroid by abandoning their old hosts. The tetrabothriid tapeworms live only in marine birds like puffins and grebes, and marine mammals like whales and seals. On the face of it, this sort of combination of hosts doesn’t make sense. These animals are too distantly related to share the tapeworms as an heirloom from some common ancestor. Birds evolved from reptiles—probably ground-running dinosaurs over 150 million years ago. Marine mammals invaded the oceans much later. Whales arose from coyote-like mammals about 50 million years ago, and seals from bear-like mammals about 25 million years ago. You have to reach back over 300 million years to find a common ancestor for birds and mammals, and that same ancestor gave rise to many other lineages of vertebrates, ranging from crocodiles to tortoises to cobras to wallabies to humans—none which is a host for tetrabothriids.
The birds and the whales had to get their tapeworms from somewhere. They probably didn’t get them from fish, because the closest relatives of tetrabothriids live in reptiles on land, which aren’t closely related to the birds and the whales. So tetrabothriids must descend from a tapeworm that lived in some group of ancient reptilian hosts. It just so happens that before whales and sea birds existed, there were reptiles in the oceans that played the same ecological roles. If you had sailed across an ocean 200 million years ago, you wouldn’t have seen birds flying overhead but pterosaurs: narrow-headed reptiles that soared on wings of hairy skin, plucking fish to bring back to their rookeries on shore. And breaching the water around you would not have been whales but monstrous reptiles of many pedigrees, such as long-necked plesiosaurs and swordfish-shaped ichthyosaurs.
Between 200 and 65 million years ago, these reptiles dominated the marine food chain. Pterosaurs began sharing the sky with birds, and Hoberg thinks that as a sort of welcoming present, they gave them their tapeworms as the birds ate the fish that served as the parasite’s intermediate host. The extinction 65 million years ago that claimed the big dinosaurs also wiped out the marine reptiles and the pterosaurs. No one knows why birds survived the impact, but it seems that they carried on the cycle of the tetrabothriid. Whales and seals later took up the roles left vacant by the marine reptiles, and the tapeworms colonized them as well. As long as an ecosystem remains intact—even if the animals that constitute it change—parasites will survive.
In the past 65 million years, tapeworms have continued to thrive, and their travels continue to mark the history of their hosts. The tapeworms that live in stingrays in the Amazon, for example, show how the river once flowed backward. If stingrays had colonized the Amazon from the Atlantic, where it flows today, their tapeworms would be most closely related to tapeworms in living Atlantic rays. But the tapeworms are actually more closely related to those in the Pacific. And making matters more puzzling, there are still other tapeworms in the Atlantic and Pacific stingrays that are more closely related to one another than either is to the Amazon tapeworms.
The scenario that reconciles these facts best has stingrays coming upriver 10 million years ago. At that time, the Andes hadn’t yet formed, and the Amazon flowed out of Brazil to the northwest coast of South America. Another big difference in the geography of that time was that the isthmus of Panama hadn’t yet formed, so that the Atlantic and Pacific were joined by a broad channel. Groups of stingrays from the Pacific swam into the Amazon when it flowed in the opposite direction. As the Amazon stingrays adapted to fresh water and became isolated from their ocean-going cousins, the marine stingrays still mingled between the two oceans. By the time Panama had risen out of the ocean, they had shared some new species of tapeworms that the freshwater rays couldn’t pick up.
In the last few million years, tapeworms have discovered yet another host, one that walks on two legs. Hoberg has been studying tapeworms that live in humans. Over the years parasitologists have come up with many ideas for how tapeworms came to live inside us. One has it that ten thousand years ago, when humans domesticated livestock, they picked up the tapeworms that cycled between wild relatives of cattle and their predators. But looking at evolutionary trees, Hoberg doesn’t think that’s the case. He and his colleagues have compared the genes of human tapeworms with their closest relatives and have found they branched off on their own a million years ago, not a few thousand. At that point, our ancestors were hominids who were a long way from farming. The closest thing to a cow or a pig they would have eaten then would have been the scavenged carcasses of wild game that had been killed by lions. Which would explain something else Hoberg discovered: the closest relatives to human tapeworms make lions and hyenas their final host. Hoberg pictures hominids following after lions, scavenging their kills and picking up their tapeworms.
There is more than one way to look back at the dawn of humanity. You can go to Ethiopia and sift the dust for stone tools and scoured bones, but you can also go to the National Parasite Collection, find the right jar, and stare at a fellow traveler.
As tapeworms moved into new hosts they had to evolve new ways to live inside them. They had to adapt to new geographies of intestines; the tapeworms that began living inside rats stumbled across new ways to get flour beetles into their final host’s jaws. Reconstructing the rise of these adaptations is treacherous work because sensible-sounding stories about evolution are easy to make up. You see long tails on a swallow and decree that they must have evolved to let the bird maneuver more precisely, but someone else looks at them and decrees that they have evolved that way because female swallows find them attractive on male ones. Or maybe no adapation is involved at all—maybe most of the swallows that happened to establish this species just happened to have long tails, and it’s been that way ever since.
Consider the journeys of the nematode Strongylus. In one species, for instance, Strongylus vulgaris, the larva crawls to the top of blades of grass and lies in wait for a horse to graze by. Once swallowed, the worm takes a long, seemingly pointless journey. It travels down the horse’s throat to its stomach and then passes on into the gut. From there it chews out into the horse’s abdominal cavity and wanders the arteries of the horse for weeks until it has matured. Thereupon it returns to the intestines, burrows its way back in, and spends the rest of its life there.
Why should a parasite leave the intestines only to return for the rest of its life? Suzanne Sukhdeo has sorted through the close relatives of Strongylus and she has come to a working hypothesis for how this pilgrimage came to be. The ancestor of these nematodes lived in the soil more than 400 million years ago, spending its days burrowing and feeding on bacteria, amoebae, and other microscopic game (as many thousands of species of nematodes still do today). About 350 million years ago, it began to encounter something new—soft-skinned amphibians slithering around in the muck. The nematodes used their burrowing abilities to plow into these hosts and make their way to the gut, where they lived happily on the food that the amphibians ate.