Hoberg’s favorite part of the collection was the slides. He grabbed a few cases and took them up with us to his office, which is dominated by a compound microscope. He focused slides for me to look at, showing sections of tapeworms from puffins, from bearded seals, from killer whales. It’s hard to tell tapeworm species apart. Sometimes the only visual difference is the shape of the chamber that houses their sexual organs. Sometimes only their genes will tell you that two tapeworms are separate species. Yet, by studying their relationships, Hoberg re-creates 400 million years of parasite history without a single fossil to guide him. He does so by finding strange patterns in parasites and their hosts. Why, Hoberg wonders, do these kinds of tapeworms—called tetrabothriids—live only in sea birds and marine mammals? Why do none of them live in humans or sharks? Why does another kind of tapeworm turn up in only two places in the world: in Australia and the thorn forests of Bolivia? The answers to these questions add up to a history of tapeworms, an epic that also carries secrets about the history of their vertebrate hosts, about drifting continents and pulsing glaciers.
A century ago, biologists thought this history was simple and drab. Once parasites surrendered to their inner life, they had reached an evolutionary dead end, since they could live nowhere else. What little evolution they experienced came only when their host dragged them in their wake. Their hosts might divide into new species when a population became isolated on an island or a mountain range, and the parasite, similarly cut off from the rest of its species, formed a new species of its own.
If that were true, you’d expect to see a certain pattern when you compared an evolutionary tree of closely related hosts to the parasites they carried: they would form mirror reflections of each other. Say you dissected four closely related bird species and found tapeworms inside. The lineage of birds that had branched off earliest on their own would have carried away the tapeworms that branch off first among the parasites. Each subsequent branch of host would have carried along its own branch of parasite as well.
It wasn’t until the late 1970s that biologists such as Daniel Brooks of the University of Toronto started actually lining up host and parasite trees in this way. Before long they realized that these twinned histories were actually far more complicated than they had thought. Sometimes the trees looked like perfect mirrors, like the tree above. But other times they looked like the tree on the next page.
Parasites did sometimes follow their hosts into new species, but they could also leap to entirely new hosts (as did tapeworms B, C, and E in this example). Sometimes they split into two new species on a single host without the host splitting as well. And sometimes they vanished from their hosts altogether. Parasites, in other words, have evolutionary stories as stormy and complex as their free-living cousins.
The most important clues to the early history of tapeworms come from the deepest roots in their tree. These primitive tapeworms all live in fish. Two main groups of fishes are alive today: the cartilaginous fishes, such as sharks and rays, and the bony fishes. They branched apart about 420 million years ago. About 400 million years ago, the bony fish lineage split into two branches of its own. One lineage led to ray-finned bony fish: salmon, trout, gar, and thousands of other species. The other led to bony fish with fleshy lobe fins, such as lungfish and coelacanths. It was this lobe-finned branch that eventually produced vertebrates with legs, able to climb on shore—in other words, that became our ancestors.
Tapeworms probably first evolved in the earliest ray-finned fish. That history is reflected in the fact that the most primitive tapeworms alive today live in the most primitive ray fins, such as sturgeon and bowfin. It was in these hosts that tapeworms evolved from a leafy shape to their distinctively long, segmented bodies. From this origin, the tapeworms later colonized sharks and other cartilaginous fish. But apparently they didn’t approach lobe fins. Neither lungfish nor coelacanths are known to carry the parasites.
Yet, tapeworms live inside their closest relatives—the terrestrial vertebrates. In fact, they live in just about every sort of amphibian, bird, mammal, and reptile. Life on land didn’t inherit tapeworms from their aquatic ancestors. The parasites must have invaded them, coming out of the water in some ray-finned fish. Perhaps 50 million years after vertebrates had come ashore, some reptilian creature eating a fish picked up a tapeworm inside its meal, and a new lineage was born. Since then, tapeworms on land have evolved with their hosts as they diverged into new forms, and they’ve continued to hop from branch to branch, shuttling, for instance, from mammals to amphibians and from mammals to birds.
Vertebrates on land had split into reptiles and the forerunners of mammals by about 300 million years ago. By 200 million years ago, the reptile branch had produced dinosaurs, which rapidly became the dominant land animal. Did tapeworms live in dinosaurs? No one can say for sure, but it’s hard to imagine they didn’t, given that their closest relatives, birds and crocodiles, both carry them. And it’s hard to imagine that they wouldn’t have taken advantage of the space inside these giants, growing to lengths of one hundred feet or more. That’s a thought that makes a parasitologist smile. The Santa Barbara parasitologist Armand Kuris has mused about what kind of ecology such a monster would have. The biggest dinosaurs were long-necked plant-eaters called sauropods, which could grow to weigh over one hundred tons. It’s hard to fathom how any predator, even one as big as Tyrannosaurus rex, could have brought them down. Perhaps it only scavenged the big dinosaurs, or perhaps it got some help. Perhaps, Kuris has suggested, the tapeworms turned the sauropods and Tyrannosurus rex into foreshadowings of moose and wolf. The sauropods picked up tapeworm eggs on the plants they ate, and the parasites developed into giant cysts inside them. As they tore up their hosts’ lungs or brains, they might have slowed down the sauropods enough to let Tyrannosaurus rex catch them, and let the tapeworm get into its final host. A dinosaur tapeworm might even have left its mark on the fossil record. The cysts of some tapeworms today get so big, and grow with such force, that they can split open a human skull. If dinosaurs carried cysts so big you’d need a forklift to carry them, paleontologists might be able to recognize their traces.
Over the 400 million years that tapeworms have been alive, Earth has been blasted by four major mass extinctions. The most recent one took place 65 million years ago and was most likely triggered by a ten-mile-wide asteroid that crashed into the Gulf of Mexico. It was powerful enough to kill the dinosaurs as well as 50 percent of all species on Earth. But tapeworms survived. It’s even possible in some parts of the world to find tapeworms still living the way they did when dinosaurs walked the Earth. The thorn forests of Bolivia are home to marsupials such as mouse opossums. They are hosts to a rare group of tapeworms called linstowiids, which need an arthropod as an intermediate host. The only other place on Earth where linstowiid tapeworms live is Australia, where they also live in similar marsupials. Today these parasites are split by thousands of miles of Pacific water, but 70 million years ago, Australia, South America, and Antartica were all joined in a single continental mass. The ancestor of the Australian and Bolivian tapeworms originated in a marsupial on that vanished continent, and host and parasite gradually split apart as the land mass was split by continental drift. But over the 70 million years that have since passed, the ecosystem that supported the tapeworm’s cycle through the mammals has remained intact.