As someone who writes a lot about evolutionary biology, I’ve often had people say to me, “I just can’t believe that evolved.” Originally, that referred to the lovely side of nature–the beauty of flowers, for example, or the grace of birds in flight. The implication was that these things were so beautiful and intricate that they had to be created for a purpose–a beautiful purpose, obviously.
But after I started writing about parasites, that underwent a fascinating change. Parasites may be deadly and gross, but they also have some mind-boggling adaptations. Most mind-boggling of all is the way many species travel through two or more hosts during their life cycle. Some flukes live first in snails, which cough them up in slime balls, then in the ants that eat the slime balls. Then the flukes drive the ants up a blade of grass, so that they can be eaten by sheep and cows, their final host. There they mate and lay eggs, which then get passed out with the host’s dung. Tapeworms live in cows and pigs, and then in humans.
When people find out about these creepy life-cycles, they emphatically say, “I just can’t believe that evolved.”
That that fascinates me. It reminds me of the way tapeworms were used to prove God’s wisdom in the 1800s. At the time people didn’t realize that tapeworms lived first in cows and pigs, and then in humans. They had some similarities in both hosts, but in us, they’re long and skinny, while in cows and pigs they look like little buttons with fringes of hooks. So some scientists claimed that the tapeworms in cows and pigs were deformed dead-enders in the wrong host. This outraged a devout German doctor named Friedrich Kuchenmeister (a name that invites repeated utterances, I can tell you). Kuchenmeister declared that dead-end tapeworms would be “contrary to the wise arrangement of Nature.” He had the brilliant idea that tapeworms went through two hosts. To prove his case, he plucked the button-shaped tapeworms out of a roast pork and fed them in a soup to a criminal about to be executed. After the man was hanged, Kuchenmeister slit open his intestines and discovered the tapeworms maturing into their long, skinny form. Kuchenmeister found a gruesome vindication of his faith (and made a major biological discovery).
It is certainly hard–at first–to imagine how a parasite could evolve from a single host to two or more–including ones as different as snails, ants, and sheep. After all, parasites are exquisitely adapted to their hosts, able to hijack their metabolism, evade their immune system, and sometimes manipulate their brains. So how could a parasite so well adapted to one host evolve to live inside a completely different one. It seems too complex a pattern. It invites the idea that it must have been designed. But what does this mean about the designer of these parasites? That it/he/she gets personally involved in making parasites into exquisitely sophisticated killers, that it/he/she revels in the baroque sadism of these creatures?
Fortunately, the evolution of parasite life cycle is not incomprehensible. This week biologists from the University of Liverpool mapped out some interesting new ideas about how parasites find new hosts. There’s a lot of mathematical modeling and parasitological minutae in the paper, but the key is that in most cases, multiple hosts are linked together in food webs. In other words, one eats the other.
Here’s a simplified scenario that gives you the gist of their argument. Imagine millions of years ago there’s a species of tapeworm that lives only in wildebeest. Fairly often, its hosts get killed by lions. The tapeworms that die with their wildebeest host can no longer reproduce. So now there’s an automatic edge for any parasite that can manage to survive a lion attack. Perhaps the first mutations allowed a tiny fraction of the tapeworms eaten by lions to escape with the lion’s droppings. Over time, these mutations would become more common because they boosted the tapeworm’s chances of reproducing.
As the tapeworms evolved better strategies for surviving in the lions, evolution would begin to favor the ones that could feed on the lion as well as the wildebeest. After all, here’s a big, long-lived host who can provide a massive food supply to any parasite that can survive in its gut. So the tapeworms start adapting to the lions as well. After a while, the tapeworms hold off on developing into their adult form until they get into the lion, so that they can take full advantage of their new host. Over time, the wildebeest stage of the parasite comes to look almost completely different from the adult.
Not all parasites can jump into new hosts this way. They have to be able to navigate that intermediate stage, when they can complete their lifecycle in either wildebeest and lions, for example. But as the Liverpool team points out, there are plenty of examples of parasites today that can switch-hit this way.
Ultimately, this research is important not for debates about the wisdom vs the sadism of God’s creation, but as medicine. We humans, are the final host not only for tapeworms, but for lots of other parasites, including blood flukes (also carried by snails) and Plasmodium, the mosquito-borne source of malaria. These diseases each have histories of their own, as parasites built up their life cycles and then modified them by switching from one host to another. And new ones will keep coming into existence, so we have to be prepared. You can believe that that didn’t evolve, but do so at your peril.