Divine Worms by Carl Zimmer

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http://loom.corante.com/archives/2003/10/02/divine_worms.php

 

 

Divine Worms

 

 

Posted by Carl Zimmer

 

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.

 

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