Parasite Rex by Carl Zimmer

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Parasite Rex

 

Imagine a world where parasites control the minds of their hosts,

sending them to their destruction.

 

Imagine a world where parasites are masters of chemical warfare and

camouflage, able to cloak themselves with their hosts' own molecules.

 

Imagine a world where parasites steer the course of evolution, where

the majority of species are parasites.

 

Welcome to earth.

 

Parasites are among the world's most successful and sophisticated

organisms. They can transform the insides of other creatures into

hospitable homes. They can evade the onslaught of the immune system

and even make it serve them. They can even control the minds of their

hosts and force them to do their bidding. And thanks to these skills,

parasites may make up the majority of all species.

 

Parasite Rex offers a guided tour to the hidden, fascinating world of

parasites, from protozoans that turn rats into suicidal kamikazes to

wasps that turn their own DNA into viruses to help them parasitize

catepillars. It follows scientists who are beginning to appreciate how

parasites can control the fate of entire ecosystems and even steer the

course of evolution.

 

" One of the year's most fascinating works of popular

science " --Publisher's Weekly (starred review)

 

 

 

 

Chapter One: Nature's Criminals

 

Nature is not without a parallel strongly suggestive of our social

perversions of justice, and the comparison is not without its lessons.

The ichneumon fly is parasitic in the living bodies of caterpillars

and the larvae of other insects. With cruel cunning and ingenuity

surpassed only by man, this depraved and unprincipled insect

perforates the struggling caterpillar, and deposits her eggs in the

living, writhing body of her victim.

 

-- John Brown, in Parasitic Wealth or Money Reform: A Manifesto to the

People of the United States and to the Workers of the World (1898)

 

In the beginning there was fever. There was bloody urine. There were

long quivering strings of flesh that spooled out of the skin. There

was a sleepy death in the wake of biting flies.

 

Parasites made themselves, or at least their effects, known thousands

of years ago, long before the name parasite -- parasitos -- was

created by the Greeks. The word literally means " beside food, " and the

Greeks originally had something very different in mind when they used

it, referring to officials who served at temple feasts. At some point

the word slipped its etymological harness and came to mean a

hanger-on, someone who could get the occasional meal from a nobleman

by pleasing him with good conversation, delivering messages, or doing

some other job. Eventually the parasite became a standard character in

Greek comedy, with his own mask. It would be many centuries before the

word would cross over to biology, to define life that drains other

lives from within. But the Greeks already knew of biological

parasites. Aristotle, for instance, recognized creatures that lived on

the tongues of pigs, encased in cysts as tough as hailstones.

 

People knew about parasites elsewhere in the world. The ancient

Egpytians and Chinese prescribed different sorts of plants to destroy

worms that lived in the gut. The Koran tells its readers to stay away

from pigs and from stagnant water, both sources of parasites. For the

most part, though, this ancient knowledge has only left a shadow on

history. The quivering strings of flesh -- now known as guinea worms

-- may have been the fiery serpents that the Bible describes plaguing

the Israelites in the desert. They certainly plagued much of Asia and

Africa. They couldn't be yanked out at one go, since they would snap

in two and the remnant inside the body would die and cause a fatal

infection. The universal cure for guinea worm was to rest for a week,

slowly winding the worm turn by turn onto a stick to keep it alive

until it had crawled free. Someone figured out this cure, someone

forgotten now for perhaps thousands of years. But it may be that that

person's invention was remembered in the symbol of medicine, known as

the caduceus: two serpents wound around a staff.

 

As late as the Renaissance, European physicians generally thought that

parasites such as guinea worms didn't actually make people sick.

Diseases were the result of the body itself lurching out of balance as

a result of heat or cold or some other force. Breathing in bad air

could bring on a fever called malaria, for example. A disease came

with symptoms: it made people cough, put spots on their belly, gave

them parasites. Guinea worms were the product of too much acid in the

blood, and weren't actually worms at all -- they were something made

by a diseased body: perhaps corrupted nerves, black bile, elongated

veins. It was hard to believe, after all, that something as bizarre as

a guinea worm could be a living creature. Even as late as 1824, some

skeptics still held out: " The substance in question cannot be a worm, "

declared the superintending surgeon of Bombay, " because its

situtation, functions, and properties are those of a lymphatic vessel

and hence the idea of its being an animal is an absurdity. "

 

Other parasites were undeniably living creatures. In the intestines of

humans and animals, for instance, there were slender snake-shaped

worms later named Ascaris, and tapeworms -- flat, narrow ribbons that

could stretc h for sixty feet. In the livers of sick sheep were lodged

parasites in the shape of leaves, called flukes after their

resemblance to flounder (floc in Anglo-Saxon). Yet, even if a parasite

was truly a living creature, most scientists reasoned, it also had to

be a product of the body itself. People carrying tapeworms discovered

to their horror that strips of it would pass out with their bowel

movements, but no one had ever seen a tapeworm crawl, inch by inch,

into a victim's mouth. The cysts that Aristotle had seen in the

tongues of pigs had little wormlike creatures coiled up inside, but

these were helpless animals that didn't even have sex organs.

Parasites, most scientists assumed, must have been spontaneously

generated in bodies, just as maggots appeared spontaneously on a

corpse, fungus on old hay, insects from within trees.

 

In 1673, the visible parasites were joined by a zoo of invisible ones.

A shopkeeper in the Dutch city of Delft put a few drops of old

rainwater under a microscope he had built himself, and he saw crawling

globules, some with thick tails, some with paws. His name was Anton

van Leeuwenhoek, and although in his day he was never considered

anything more than an amateur, he was the first person to lay eyes on

bacteria, to see cells. He put everything he could under his

microscope. Scraping his teeth, he discovered rod-shaped creatures

living on them, which he could kill with a sip of hot coffee. After a

disagreeable meal of hot smoked beef or ham, he would put his own

loose stool under his lenses. There he could see more creatures -- a

blob with leglike things that it used to crawl like a wood louse,

eel-shaped creatures that would swim like a fish in water. His body,

he realized, was a home to microscopic parasites.

 

Other biologists later found hundreds of different kinds of

microscopic creatures living inside other creatures, and for a couple

of centuries there was no divide between them and the bigger

parasites. The new little worms took many shapes -- of frogs, of

scorpions, of lizards. " Some shoot forth horns, " one biologist wrote

in 1699, " others acquire a forked Tail; some assume Bills, like Fowls,

others are covered with Hair, or become all over rough; and others

again are covered with Scales and resemble Serpents. " Meanwhile, other

biologists identified hundreds of different visible parasites, flukes,

worms, crustaceans, and other creatures living in fish, in birds, in

any animal they opened up. Most scientists still held on to the idea

that parasites large and small were spontaneously generated by their

hosts, that they were only passive expressions of disease. They held

on through the eighteenth century, even as some scientists tested the

idea of spontaneous generation and found it wanting. These skeptics

showed how the maggots that appeared on the corpse of a snake were

laid as eggs by flies, and themselves grew into flies.

 

Even if maggots weren't spontaneously generated, parasites were a

different matter. They simply had no way of getting inside a body and

so had to be created there. They had never been seen outside a body,

animal or human. They could be found in young animals, even in aborted

fetuses. Some species could be found in the gut, living happily

alongside other organisms that were being destroyed by digestive

juices. Others could be found clogging the heart and the liver,

without any conceivable way to get into those organs. They had hooks

and suckers and other equipment for making their way inside a body,

but they would be helpless in the outside world. In other words,

parasites were clearly designed to live their entire lives inside

other animals, even in particular organs.

 

Spontaneous generation was the best explanation for parasites, given

the evidence at hand. But it was also a profound heresy. The Bible

taught that life was created by God in the first week of creation, and

every creature was a reflection of His design and His beneficence.

Everything that lived today mustdescend from those primordial

creatures, in an unbroken chain of parents and children -- nothing

could later come squirting into existence thanks to some vital,

untamed force. If our own blood could spontaneously generate life,

what help did it need from God back in the days of Genesis?

 

The mysterious nature of parasites created a strange, disturbing

catechism of its own. Why did God create parasites? To keep us from

being too proud, by reminding us that we were merely dust. How did

parasites get into us? They must have been put there by God, since

there was no apparent way for them to get in by themselves. Perhaps

they were passed down through generations within our bodies to the

bodies of our children. Did that mean that Adam, who was created in

purest innocence, came into being already loaded with parasites? Maybe

the parasites were created inside him after his fall. But wouldn't

this be a second creation, an eighth day added on to that first week

-- " and on the following Monday God created parasites " ? Well, then,

maybe Adam was created with parasites after all, but in Eden parasites

were his helpmates. They ate the food he couldn't fully digest and

licked his wounds clean from within. But why should Adam, created not

only in innocence but in perfection, need any help at all? Here the

catechism seems to have finally fallen apart.

 

Parasites caused so much confusion because they have life cycles

unlike anything humans were used to seeing. We have the same sorts of

bodies as our parents did at our age, as do salmon or muskrats or

spiders. Parasites can break that rule. The first scientist to realize

this was a Danish zoologist, Johann Steenstrup. In the 1830s he

contemplated the mystery of flukes, whose leaf-shaped bodies could be

found in almost any animals a parasitologist cared to look at -- in

the livers of sheep, in the brains of fish, in the guts of birds.

Flukes laid eggs, and yet no one in Steenstrup's day had ever found a

baby fluke in its host.

 

They had, however, found other creatures that looked distinctly

flukish. Wherever certain species of snails lived, in ditches or ponds

or streams, parasitologists came across free-swimming animals that

looked like small versions of flukes except that they had great tails

attached to their rears. These animals, called cercariae, flicked

their tails madly through the water. Steenstrup scooped up some ditch

water, complete with snails and cercariae, and kept it in a warm room.

He noticed that the cercariae would penetrate the mucus coating the

snail's body and shell, drop their tails, and form a hard cyst, which,

he said, " arches over them like a small, closely-shut watch glass. "

When Steenstrup pulled the cercariae out of these shelters, he found

that they had become flukes.

 

Biologists knew that the snails were home to other sorts of parasites

as well. There was a creature that looked like a shapeless bag. There

was also a little beast they called the King's yellow worm: a pulpy

animal that lived in the snail's digestive gland and carried within it

what looked like cercariae, all writhing like cats inside a burlap

sack. And Steenstrup even found another flukelike creature swimming

free, this one not using a missile-shaped tail but instead hundreds of

fine hairs that covered its body.

 

Looking at all these organisms swimming through the water and through

the snails -- organisms that in many cases had been given their own

Latin species names -- Steenstrup made an outrageous suggestion. All

these animals were different stages and generations of a single

animal. The adults laid eggs, which escaped out of their hosts and

landed in water, where they hatched into the form covered in fine

hairs. The hair-covered form swam through the water and sought out a

snail, and once it had penetrated a snail, the parasite transformed

itself into the shapeless bag. The shapeless bag began to swell with

the embryos of a new generation of flukes. But these new flukes were

nothing like the leaf-shaped forms inside a sheep's liver, or even the

finely haired form that entered the snail. These were the King's

yellow worms. They moved through the snail, feeding and rearing within

them yet another generation of flukes -- the missile-tailed cercariae.

The cercariae emerged from the snail, promptly forming cysts on the

snail. From there they somehow got into sheep or another final host,

and there they emerged from their cysts as mature flukes.

 

Here was a way that parasites could appear inside our bodies with no

precedent: " An animal bears young which are, and remain, dissimilar to

their parent, but bring forth a new generation, whose members either

themselves, or in their descendants, return to the original form of

the parent animal. " Scientists had already met the precedents,

Steenstrup was saying, but they couldn't believe that they all

belonged to the same species.

 

Steenstrup would eventually be proved right. Many parasites travel

from one host to another during their life cycles, and in many cases

they alternate between different forms from one generation to the

next. And thanks to his insight, one of the best cases for spontaneous

generation in parasites fell apart. Steenstrup turned his attention

from flukes to the worms that Aristotle had seen living in cysts

embedded in pig tongues. These parasites, called bladder worms at the

time, can live in any muscle in mammals. Steenstrup suggested that

bladder worms were actually an early stage in the development of some

other worm not yet found.

 

Other scientists noticed that bladder worms looked a bit like

tapeworms. All you had to do was cut off most of the tapeworm's long

ribbony body, and tuck its head and first few segments inside a shell,

and you had a bladder worm. Maybe the bladder worm and tapeworm were

one and the same. Maybe they were actually the product of tapeworm

eggs that had made their way into the wrong host. When the eggs

hatched in this hostile environment, the tapeworms couldn't take their

normal path of development but grew instead into stunted deformed

monsters that died before they could reach maturity.

 

In the 1840s, a devout German doctor heard about these ideas and was

outraged. Friedrich KŸchenmeister kept a little medical practice in

Dresden, and in his free time he wrote books on biblical zoology and

ran the local cremation club, called Die Urne. KŸchenmeister

recognized that the idea that bladder worms were actually tapeworms

certainly sidestepped the heresy of spontaneous generation. But it

then fell into another sinful trap -- the idea that God would let one

of his creatures wind up in a monstrous dead end. " It would be

contrary to the wise arrangement of Nature which undertakes nothing

without a purpose, " KŸchenmeister declared. " Such a theory of error

contradicts the wisdom of the Creator and the laws of harmony and

simplicity put into Nature " -- laws that even applied to tapeworms.

 

KŸchenmeister had a more pious explanation: the bladder worms were an

early stage in the natural life cycle of the tapeworm. After all, the

bladder worms tended to be found in prey -- animals such as mice,

pigs, and cows -- and the tapeworms were found in predators: cats,

dogs, humans. Perhaps when a predator ate prey, the bladder worm

emerged from its cyst and grew into a full tapeworm. In 1851,

KŸchenmeister began a series of experiments to rescue the bladder worm

from its dead end. He plucked out forty of them from rabbit meat and

fed them to foxes. After a few weeks, he found thirty-five tapeworms

inside the foxes. He did the same with another species of tapeworm and

bladder worm in mice and cats. In 1853, he fed bladder worms from a

sick sheep to a dog, which soon was shedding the segments of an adult

tapeworm in its feces. He fed these to a healthy sheep, which began to

stumble sixteen days later. When the sheep was killed and

KŸchenmeister looked in its skull, he found bladder worms sitting on

top of its brain.

 

When KŸchenmeister reported his findings, he stunned the university

professors who made parasites their life's work. Here was an amateur

out on his own, sorting out a mystery the experts had failed to solve

for decades. They tried to poke holes in KŸchenmeister's work wherever

possible, to try to keep their own ideas about dead-end bladder worms

alive. One problem with KŸchenmeister's work was that he sometimes fed

the bladder worms to the wrong host species and the parasites all

died. He knew, for example, that pork carried a species of bladder

worm, and he knew that the butchers of Dresden and their families

often suffered from tapeworms called Taenia solium. He suspected that

the two parasites were one and the same. He fed Taenia eggs to pigs

and got the bladder worms, but when he fed the bladder worms to dogs,

he couldn't get adult Taenia. The only way to prove the cycle was to

look inside its one true host -- humans.

 

KŸchenmeister was so determined to prove God's benevolent harmony that

he set up a gruesome experiment. He got permission to feed bladder

worms to a prisoner about to be executed, and in 1854 he was notified

of a murderer to be decapitated in a few days. His wife happened to

notice that the warm roast pork they were eating for dinner had a few

bladder worms in it. KŸchenmeister rushed to the restaurant where they

had bought the pork. He begged for a pound of the raw meat, even

though the pig had been slaughtered two days earlier and was beginning

to go bad. The restaurant owners gave him some, and the next day

KŸchenmeister picked out the bladder worms and put them in a noodle

soup cooled to body temperature.

 

The prisoner didn't know what he was eating and enjoyed it so much he

asked for seconds. KŸchenmeister gave him more soup, as well as blood

sausage into which he had slipped bladder worms. Three days later the

murderer was executed, and KŸchenmeister searched his intestines.

There he found young Taenia tapeworms. They were still only a quarter

of an inch long, but they had already developed their distinctive

double crown of twenty-two hooks.

 

Five years later, KŸchenmeister repeated the experiment, this time

feeding a convict four months before his execution. Afterward he found

tapeworms as long as five feet in the man's intestines. He felt

triumphant, but the scientists of his day were disgusted. The

experiments were " debasing to our common nature, " said one reviewer.

Another compared him to some doctors of the day who cut the

still-beating heart out of a just-executed man, merely to satisfy

their curiosity. One quoted Wordsworth: " One that would peep and

botanise/Upon his mother's grave? " But no doubt was left that

parasites were among the strangest things alive. Parasites were not

spontaneously generated; they arrived from other hosts. KŸchenmeister

also helped discover another important thing about parasites that

Steenstrup hadn't observed: they didn't always have to wander through

the outside world to get from one host to another. They could grow

inside one animal and wait for it to be eaten by another.

 

The last possibility still left for spontaneous generation was

represented by the microbes. That was shortly put to rest by the

French scientist Louis Pasteur. To make his classic demonstration, he

put broth in a flask. Given enough time the broth would go bad,

filling with microbes. Some scientists claimed that the microbes were

spontaneously generated in the broth itself, but Pasteur showed that

the microbes were actually carried in the air to the flask and settled

into it. He went on to prove that microbes weren't just a symptom of

diseases but often their cause -- what came to be known as the germ

theory of infection. And out of that realization came the great

triumphs of Western medicine. Pasteur and other scientists began to

isolate the particular bacteria that caused diseases such as anthrax,

tuberculosis, and cholera and to make vaccines for some of them. They

proved that doctors spread disease with their dirty hands and scalpels

and could stop it with some soap and hot water.

 

With Pasteur's work, a peculiar transformation came over the concept

of the parasite. By 1900, bacteria were rarely called parasites

anymore, even though, like tapeworms, they lived in and at the expense

of another organism. It was less important to doctors that bacteria

were organisms than that they had the power to cause diseases and that

they could now be erased with vaccines, drugs, and good hygiene.

Medical schools focused their students on infectious diseases, and

generally on those caused by bacteria (or later, by the much smaller

viruses). Part of their bias had to do with how scientists recognize

causes of diseases. They generally follow a set of rules proposed by

the German scientist Robert Koch. To begin with, a pathogen had to be

shown to be associated with a particular disease. It also had to be

isolated and grown in pure culture, the cultured organism had to be

inoculated into a host and produce the disease again, and the organism

in the second host had to be shown to be the same as that inoculated.

Bacteria fit these rules without much trouble. But there were many

other parasites that didn't.

 

Living alongside bacteria -- in water, soil, and bodies -- were much

larger (but still microscopic) single-celled organisms known as

protozoa. When Leeuwenhoek had looked at his own feces, he had seen a

protozoan now called Giardia lamblia, which had made him sick in the

first place. Protozoa are much more like the cells that make up our

own bodies, or plants or fungi, than they are like bacteria. Bacteria

are essentially bags of loose DNA and scattered proteins. But protozoa

keep their DNA carefully coiled up on molecular spools within a shell

called the nucleus, just as we do. They also have other compartments

dedicated to generating energy, and their entire contents are

surrounded by skeleton-like scaffolding, as with our cells. These were

only a few of many clues biologists discovered that showed the

protozoa to be more closely related to multicellular life than to the

bacteria. They went so far as to divide life into two groups. There

were the prokaryotes -- the bacteria -- and the eukaryotes: protozoa,

animals, plants, and fungi.

 

Many protozoa, such as the amoebae grazing through forest floors, for

instance, or the phytoplankton that turn the oceans green, are

harmless. But there are thousands of species of parasitic protozoa,

and they include some of the most vicious parasites of all. By the

turn of the century, scientists had figured out that the brutal fevers

of malaria weren't caused by bad air but by several species of a

protozoan called Plasmodium, a parasite that lived inside mosquitoes

and got into humans when the insects pierced the skin to suck blood.

Tsetse flies carried trypanosomes that caused sleeping sickness. Yet,

despite their power to cause disease, most protozoa couldn't live up

to Koch's rigorous demands. They were creatures after Steenstrup's

heart, passing through alternating generations.

 

Plasmodium, for example, enters a human body through a mosquito bite

as a zucchini-shaped form known as a sporozoite. It travels to the

liver, where it invades a cell and there multiplies into forty

thousand offspring, called merozoites -- these are now shaped like a

grape. Merozoites pour out of the liver and seek out red blood cells,

where they make more merozoites. The new generations burst out of the

cells and seek out more blood cells. After a while, some of the

merozoites produce a different form -- a sexual one, called a

macrogamont. If a mosquito should take a drink of the host's blood and

swallow a blood cell with macrogamonts in it, they will mate inside

the insect. The male macrogamont fertilizes the female one, and they

produce a round little offspring called an ookinete. The ookinete

divides in the mosquito's gut into thousands of sporozoites, which

travel to the mosquito's salivary glands, there to be injected into

some new human host.

 

With so many generations and so many different forms, you can't raise

Plasmodium organisms simply by throwing them in a petri dish and

hoping they'll multiply. You have to get male and female macrogamonts

to believe that they're living in the gut of a mosquito, and once

they've bred, you have to make their offspring believe they've been

shot out of the mosquito's mouth and into human blood. It's not

impossible to do, but it took until the 1970s, a century after Koch

set up his rules, for a scientist to figure out how to culture

Plasmodium in a lab.

 

Parasitic eukaryotes and parasitic bacteria were pushed further apart

by geography. In Europe, bacteria and viruses caused the worst

diseases, such as tuberculosis and polio. In the tropics, protozoa and

parasitic animals were just as bad. The scientists who studied them

were generally colonial physicians, and their specialty became known

as tropical medicine. Europeans came to look upon parasites as robbing

them of native labor, of slowing down the building of their canals and

dams, of preventing the white race from living happily at the Equator.

When Napoleon took his army to Egypt, the soldiers began to complain

that they were menstruating like women. Actually they had been

infected with flukes. Like the flukes Steenstrup had studied, these

were shed by snails and swam through water looking for human skin.

They ended up in the veins in the abdomens of the soldiers and pushed

their eggs into their bladders. Blood flukes attacked people from the

western shores of Africa to the rivers of Japan; the slave trade even

brought them to the New World, where they thrived in Brazil and the

Caribbean. The disease they caused, known as bilharzia or

schistosomiasis, drained the energy of hundreds of millions of people

who were supposed to build European empires.

 

As bacteria and viruses occupied the center of medicine, parasites (in

other words, everything else) were spun out to the periphery.

Specialists in tropical medicine went on struggling against their own

parasites, often with a staggering lack of success. Vaccines against

parasites failed miserably. There were a few old cures -- quinine for

malaria, antimony for blood flukes -- but they did only a little good.

Sometimes they were so toxic that they caused as much harm as the

disease itself. Meanwhile, veterinarians studied the things living

inside cows and dogs and other domesticated animals. Entomologists

looked at the insects dug into trees, the nematodes that sucked on

their roots. All these different disciplines became known as

parasitology -- more of a loose federation than an actual science. If

anything held together its factions, it was that parasitologists were

keenly aware of their subjects as living things rather than just

agents of disease, each subject with a natural history of its own --

in the words of one scientist at the time, " medical zoology. "

 

Some actual zoologists studied this medical zoology. But just as the

germ theory of disease was changing the world of medicine, they were

reckoning with a revolution of their own. In 1859, Charles Darwin

offered a new explanation for life. Life, he argued, hadn't existed

unchanged since Earth's creation but had evolved from one form to

another. That evolution had been driven by what he named natural

selection. Every generation of a species was made up of variants, and

some variants fared better than others -- they could catch more food

or avoid becoming food for someone else. Their descendants inherited

their characteristics, and with the passing of thousands of

generations, this unplanned breeding produced the diversity of life on

Earth today. To Darwin, life was not a ladder rising up to the angels

or a cabinet filled with shells and stuffed animals. It was a tree,

bursting upward with all the diversity of the species on Earth alive

today and long past, all rooted in a common ancestry.

 

Parasites fared as badly in the evolutionary revolution as they had in

the medical one. Darwin contemplated them only in passing, usually

when he was trying to argue that nature was a bad place to try to

prove God's benevolent design. " It is derogatory that the Creator of

countless systems of worlds should have created each of the myriads of

creeping parasites, " he once wrote. He found that parasitic wasps are

a particularly good antidote to sentimental ideas about God. The way

that the larvae devoured their host from the inside was so awful that

Darwin once wrote of them, " I cannot persuade myself that a

beneficient and omnipotent God would have designedly created the

Ichneumonidae [one group of parasitic wasps] with the express

intention of their feeding within the living bodies of Caterpillars. "

 

Yet, Darwin was downright kind to parasites compared with the later

generations of biologists who carried on his work. Instead of benign

neglect, or even mild disgust, they felt outright scorn for parasites.

These late Victorian scientists were drawn to a peculiar, now debunked

form of evolution. They accepted the concept that life evolved, but

Darwin's generation-by-generation filter of natural selection seemed

too random to account for the trends they saw in the fossil record

that had lasted millions of years. They saw life as having an inner

force driving it toward greater and greater complexity. To their mind,

this force brought a purpose to evolution: to produce the higher

organisms -- vertebrates such as us -- from the lower beings.

 

One influential voice for these ideas belonged to the British

zoologist Ray Lankester. Lankester grew up with evolution. When he was

a boy, Darwin came to his family's house and told him stories about

riding a giant tortoise on a Pacific island. When Lankester became a

man, he had a giant frame and a puffy, vaguely Charles Laughton-like

face. As an Oxford professor and the director of the British Museum he

carried Darwin's theory forward with what seemed at times like sheer

bodily power. He made the people around him feel small in both size

and mind; he reminded one man who met him of a winged Assyrian beast.

Once King Edward VII offered him some tidbit of scientific knowledge

while paying him a royal visit, and Lankester bluntly replied, " Sir,

the facts are not so; you have been misinformed. "

 

To Lankester, Darwin's theory had brought a unity to biology as

impressive as that in any other science. He had no patience for

doddering dons who looked at his science as a quaint hobby. " We are no

longer content to see biology scoffed at as inexact or gently dropped

as natural history or praised for her relation to medicine. On the

contrary, biology is the science whose development belongs to the

day, " he declared. And its understanding would help free future

generations from stupid orthodoxies of all sorts: " the jack-in-office,

the pompous official, the petulant commander, the ignorant pedagogue. "

It would help carry human civilization upward, as life itself had been

striving for millions of years. He laid out this view of the

biological and political order of things in an essay he wrote in 1879,

titled " Degeneration: A Chapter in Darwinism. "

 

The tree of life you find described in that essay isn't the wild bush

of Darwin. It's shaped like a plastic Christmas tree, with branches

sticking out to the side from a main shaft, which rises to higher and

higher glories until it reaches humans at the top. At each stage in

the rise of life, some species abandoned the struggle, comfortable

with the level of complexity they had achieved -- a mere amoeba,

sponge, or worm -- while others kept striving upward.

 

But there were some drooping branches on Lankester's tree. Some

species not only stopped rising but actually surrendered some of their

accomplishments. They degenerated, their bodies simplifying as they

accommodated themselves to an easier life. For biologists of

Lankester's day, parasites were the sine qua non of degenerates,

whether they were animals or single-celled protozoa that had given up

a free life. To Lankester, the quintessential parasite was a miserable

barnacle named Sacculina carcini. When it first hatched from its egg,

it had a head, a mouth, a tail, a body divided into segments, and

legs, which is exactly what you'd expect from a barnacle or any other

crustacean. But rather than growing into an animal that searchedand

struggled for its own food, Sacculina instead found itself a crab and

wiggled into its shell. Once inside, Sacculina quickly degenerated,

losing its segments, its legs, its tail, even its mouth. Instead, it

grew a set of rootlike tendrils, which spread throughout the crab's

body. It then used these roots to absorb food from the crab's body,

having degenerated to the state of a mere plant. " Let the parasitic

life once be secured, " Lankester warned, " and away go legs, jaws,

eyes, and ears; the active, highly gifted crab may become a mere sac,

absorbing nourishment and laying eggs. "

 

Since there was no divide between the ascent of life and the history

of civilization, Lankester saw in parasites a grave warning for

humans. Parasites degenerated " just as an active healthy man sometimes

degenerates when he becomes suddenly possessed of a fortune; or as

Rome degenerated when possessed of the riches of the ancient world.

The habit of parasitism clearly acts upon animal organization in this

way. " To Lankester, the Maya, living in the shadows of the abandoned

temples of their ancestors, were degenerates, just as Victorian

Europeans were pale imitations of the glorious ancient Greeks.

" Possibly we are all drifting, " he fretted, " tending to the condition

of intellectual Barnacles. "

 

An uninterrupted flow from nature to civilization meant that biology

and morality were interchangeable. People of Lankester's day took to

condemning nature and then using nature in turn as an authority to

condemn other people. His essay inspired a writer named Henry Drummond

to publish a best-selling screed, Natural Law in the Spiritual World,

in 1883. Drummond declared that parasitism " is one of the gravest

crimes in nature. It is a breach of the law of Evolution. Thou shalt

evolve, thou shalt develop all thy faculties to the full, thou shalt

attain to the highest conceivable perfection of thy race -- and so

perfect thy race -- this is the first and greatest commandment of

Nature. But the parasite has no thought for its race, or for its

perfection in any shape or form. It wants two things -- food and

shelter. How it gets them is of no moment. Each member lives

exclusively on its own account, an isolated, indolent, selfish, and

backsliding life. " People were no different: " All those individuals

who have secured a hasty wealth by the chances of speculation; all

children of fortune; all victims of inheritance; all social sponges;

all satellites of the court; all beggards of the market-place -- all

these are living and unlying witness to the unalterable retributions

of the law of parasitism. "

 

People had been referred to as parasites before the late 1800s, but

Lankester and other scientists gave the metaphor a precision, a

transparency, that it never had before. And it's a short walk from

Drummond's rhetoric to genocide. Listen to how closely his line about

the highest conceivable perfection of a race meshes with these words:

" In the struggle for daily bread all those who are weak and sickly or

less determined succumb, while the struggle of the males for the

females grants the right or opportunity to propagate only to the

healthiest. And struggle is always a means for improving a species'

health and power of resistance and therefore, a cause of its higher

development. " The author of these words wasn't an evolutionary

biologist but a petty Austrian politician who would go on to

exterminate six million Jews.

 

Adolf Hitler relied on a confused, third-rate version of evolution. He

imagined that Jews and other " degenerate " races were parasites, and he

took the metaphor even further, seeing them as a threat to the health

of their host, the Aryan race. It was the function of a nation to

preserve the evolutionary health of its race, and so it had to rid the

parasite from its host. Hitler probed every hidden turn of the

parasite metaphor. He charted the course of the Jewish " infestation, "

as it spread to labor unions, the stock exchange, the economy, and

cultural life. The Jew, he claimed, was " only and always a parasite in

the body of other peoples. That he sometimes left his previous living

space has nothing to do with his own purpose, but results from the

fact that from time to time he was thrown out by the host nations he

had misused. His spreading is a typical phenomenon for all parasites;

he always seeks a new feeding ground for his race. "

 

Nazis weren't the only ones to burn the brand of parasite on their

enemies. To Marx and Lenin, the bourgeoisie and the bureaucrats were

parasites that society had to get rid of. An exquisitely biological

take on socialism appeared in 1898, when a pamphleteer named John

Brown wrote a book called Parasitic Wealth or Money Reform: A

Manifesto to the People of the United States and to the Workers of the

World. He complained of how three-quarters of the country's money was

concentrated in the hands of 3 percent of the population, that the

rich sucked the wealth of the nation away, that their protected

industries flourished at the people's expense. And, like Drummond or

Hitler, he saw his enemies precisely reflected in nature, in the way

parasitic wasps live in caterpillars. " With the refinement of innate

cruelty, " he wrote, " these parasites eat their way into the living

substance of their unwilling but helpless host, avoiding all the vital

parts to prolong the agony of a lingering death. "

 

Parasitologists themselves sometimes helped consecrate the human

parasite. As late as 1955, a leading American parasitologist, Horace

Stunkard, was carrying on Lankester's conceit in an essay published in

the journal Science, titled " Freedom, bondage, and the welfare state. "

" Since zoology is concerned with the facts and principles of animal

life, information obtained from the study of other animals is

applicable to the human species, " he wrote. All animals were driven by

the need for food, shelter, and the chance to reproduce. In many

cases, fear drove them to give up their freedom for some measure of

security, only to be trapped in permanent dependency. Conspicuous

among security-seeking animals were creatures such as clams, corals,

and sea squirts, which anchored themselves to the ocean floor in order

to filter the passing sea water for food. But none could compare with

the parasites. Time after time in the history of life, free-living

organisms had surrendered their liberty to become parasites in

exchange for an escape from the dangers of life. Evolution then took

them down a degenerate path. " When other food sources were

insufficient, what would be easier than to feed upon the tissues of

the host? The dependent animal is proverbially looking for the easy way. "

 

Stunkard was only a little coy about how this rule of parasites could

apply to humans. " It may be applied to any group of organisms, and is

not intended to refer merely to political entities, although certain

implications may be in order. " With its complete surrender of its

liberty, the parasite had entered the " welfare state, " as Stunkard put

it -- with hardly a tissue of metaphor dividing the tapeworm and the

New Deal. Once parasites gave up their freedom, they rarely managed to

regain it; instead, they channeled their energies into making new

generations of parasites. Their only innovations were weird kinds of

reproduction. Flukes alternated their forms between generations,

reproducing sexually in humans and asexually in snails. Tapeworms

could produced a million eggs a day. How could Stunkard have had

anything but fast-breeding welfare families in mind? " Such a welfare

state exists only for those lucky individuals, the favored few, who

are able to cajole or compel others to provide the welfare, " he wrote.

" The well-worn attempt to obtain comfort without effort, to get

something for nothing, persists as one of the illusions that in all

ages has intrigued and misled the unwary. "

 

Writing in 1955, Stunkard represented a dying gasp of the old take on

evolution. As he was attacking food-stamp parasites, his fellow

biologists were unceremoniously dumping the whole foundation of his

scientific view. They discovered that every living thing on Earth

carries genetic information in its cells in the form of DNA, a

molecule in the shape of a double helix. Genes (particular stretches

of DNA) carried the instructions for making proteins, and these

proteins could build eyes, digest food, regulate the creation of other

proteins, and do thousands of other things. Each generation passed its

DNA to the next, and along the way the genes got shuffled into new

combinations. Sometimes mutations to the genes turned up, creating new

codes altogether. Evolution, these biologists realized, was built on

these genes and the way they rose and fell as time passed -- not on

some mysterious inner force. The genes offered up rich variety, and

natural selection preserved certain kinds. From these genetic ebbs and

flows new species could be created, new body plans. And since

evolution was grounded on the short-term effects of natural selection,

biologists no longer had any need for an inner drive for evolution, no

longer saw life as a plastic Christmas tree.

 

Parasites should have benefited from this change of scientific heart.

They were no longer the backward pariahs of biology. Yet, well into

the twentieth century, parasites still couldn't escape Lankester's

stigma. The contempt survived both in science and beyond it. Hitler's

racial myths have collapsed, and the only people who still believe in

eradicating social parasites are at the fringes, among the Aryan

skinheads and the minor dictators. Yet, the word parasite still

carries the same insulting charge. Likewise, for much of the twentieth

century, biologists thought of parasites as minor degenerates, mildly

amusing but insignificant to the pageant of life. When ecologists

looked at how the sun's energy streamed through plants and into

animals, parasites were nothing more than grotesque footnotes. What

little evolution parasites experienced was the result of being dragged

along by their hosts.

 

Even in 1989, Konrad Lorenz, the great pioneer in animal behavior, was

writing about the " retrograde evolution " of parasites. He didn't want

to call it degeneration -- that word was perhaps too loaded by Nazi

rhetoric -- and so he replaced it with " sacculinasation, " after

Sacculina, Lankester's backsliding barnacle. " When we use the terms

'higher and lower' in reference to living creatures and to cultures

alike, " he wrote, " our evaluation refers directly to the amount of

information, of knowledge, conscious or unconscious, inherent in these

living systems. " And according to this scale, Lorenz despised

parasites: " If one judges the adapted forms of the parasites according

to the amounts of retrogressed information, one finds a loss of

information that coincides with and completely confirms the low

estimation we have of them and how we feel about them. The mature

Sacculina carcini has no information about any of the particularities

and singularities of its habitat; the only thing it knows anything

about is its host. " Much like Lankester 110 years earlier, Lorenz saw

the only virtue of parasites as a warning to humans. " A retrogression

of specific human characteristics and capacities conjures up the

terrifying specter of the less than human, even of the inhuman. "

 

From Lankester to Lorenz, scientists have gotten it wrong. Parasites

are complex, highly adapted creatures that are at the heart of the

story of life. If there hadn't been such high walls dividing

scientists who study life -- the zoologists, the immunologists, the

mathematical biologists, the ecologists -- parasites might have been

recognized sooner as not disgusting, or at least not merely

disgusting. If parasites were so feeble, so lazy, how was it that they

could manage to live inside every free-living species and infect

billions of people? How could they change with time so that medicines

that could once treat them became useless? How could parasites defy

vaccines, which could corral brutal killers like smallpox and polio?

The problem comes down to the fact that scientists at the beginning of

this century thought they had everything figured out. They knew how

diseases were caused and how to treat some of them; they knew how life

evolved. They didn't respect the depth of their ignorance. They should

have borne in mind the words of Steenstrup, the biologist who had

first shown that parasites were unlike anything else on Earth.

Steenstrup had it right in 1845 when he wrote, " I believe that I have

given only the first rough outlines of a province of a great terra

incognita which lies unexplored before us and the exploration of which

promises a return such as we can at present scarcely appreciate. "

 

2000 by Carl Zimmer