“Parasites May Rule the World”

[Guest guest]

" Parasites May Rule the World "

 

In the words of National Geographic,

" a sinister world of monstrous creatures that feed on living flesh:

parasites " . Discover magazine published a feature article in its

August 2000 issue:

 

" Every living thing has at least one parasite that lives inside or on

it, and many, including humans, have far more. …Scientists…are only

just beginning to discover exactly how powerful these hidden

inhabitants can be, but their research is pointing to a remarkable

possibility: Parasites may rule the world. The notion that tiny

creatures we've largely taken for granted are such a dominant force is

immensely disturbing. … We are collections of cells that work

together, kept harmonized by chemical signals. If an organism can

control those signals — an organism like a parasite — then it can

control us. And therein lies the peculiar and precise horror of

parasites. "

 

 

 

The combination of environmental toxins, an unhealthy diet and

parasites poses a grave danger to humans. " In fact, parasites have

killed more humans than all the wars in history " , reported National

Geographic in its award-winning documentary, The Body Snatchers.

 

 

Parasite Rex (book)

 

http://carlzimmer.com/parasite_2.html

 

 

 

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