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It is often said that in the centuries after Columbus landed in the
New World on 12 October 1492, more native North Americans died each year
from infectious diseases brought by European settlers than were born. They
fell victim to epidemic waves of smallpox, measles, influenza, bubonic plague,
diphtheria, typhus, cholera, scarlet fever, chicken pox, yellow fever, and
whooping cough. Just how many died may never be known. For North America
alone, estimates of native populations in Columbus’s day range from 2 to
18 million. By the end of the 19th century the population had shrunk to
about 530 000.

Staggering losses. But why, asked a perplexed French missionary working
among the Mississippi Valley’s Natchez in the 1700s, should ‘distempers
that are not very fatal in other parts of the world make dreadful ravages
among them’? The answer seems obvious enough: because native Americans had
no immunity to the imported diseases. This begs a larger question, however:
why the lack of immunity? And why had native North Americans no deadly diseases
to infect Europeans with in return? Here the answers are not so obvious,
for they have little to do with events after 1492. Rather they are intimately
linked with the peopling of the Americas more than 11 500 years ago.

But let’s start with Columbus. His reports of the New World jolted Europeans:
here was a land of exotic plants, animals and people. The great savants
scrambled to explain who the native North Americans were, where they had
come from, and when they had arrived. One popular idea was that they were
descendants of the Ten Lost Tribes of Israel – a theory that explained where
the Israelites had been lost all those years but which offered no insights
into why native North Americans were dying in such fearful numbers.

Thomas Jefferson thought smallpox and other diseases were not entirely
to blame. ‘Spirituous liquors,’ war, and land snatching were also factors,
he said. Yet he had few illusions about disease. When, as president, he
sent Meriwether Lewis and William Clark West to explore the Pacific Coast,
he instructed them to ‘carry with you some matter of the kinepox’ and ‘inform
those (natives) with whom you may be, of its efficacy as a preservative
from the smallpox and instruct and encourage them in the use of it’. It
was usually too late, however, for disease inevitably got there first. In
April 1806, along the Columbia River in the Pacific Northwest, Clark came
upon the ruined village and badly scarred survivors of a smallpox epidemic
that roared through years earlier. This corner of America was about as remote
then as any place on earth.

The reasons why disease travelled farther and faster than those who
brought it to America are simple. Take smallpox. Over its incubation period
of 10 to 14 days, the smallpox virus readily spreads by human breath or
in a dried state on infected blankets and clothing. To make matters worse,
when the first symptoms appeared in a village, infected people who had yet
to develop symptoms would flee to neighbouring villages carrying the disease
with them.

But the spread was uneven. Hardest and often earliest hit were native
farmers living in densely populated permanent towns and villages along major
rivers like the Mississippi, Missouri, Ohio, Gila and Rio Grande, where
diseases spread quickly and widely, their devastation magnified by European
slave raids, warfare, and the forced neglect of crops and fields. Groups
like those on the Great Plains of the Midwest or the Great Basin farther
West, who were more mobile, who seasonally or annually dispersed into smaller
groups, and who lived far away from major population centres, were generally
hit later, and suffered relatively less. Individually, they were no more
immune to infections than their town-dwelling counterparts; they simply
encountered disease less often, and fewer of them died at once.

The appalling mortality astonished Europeans. Native American adults
and children were dying from childhood diseases which, at their worst in
Western Europe (in the 1700s), accounted for just lO to 15 per cent of all
deaths. And 70 per cent of those, according to the historian Alfred Crosby
of the University of Texas, were children under the age of two.

It used to be worse in the Old World, back when diseases such as smallpox
first evolved in humans. Our knowledge of their origins is meagre but two
main factors stand out: crowds of people and the domestication of animals.
Living in crowds began sometime after 9000 BC in Mesopotamia and elsewhere
in the Middle East, as people settled into villages and towns, cleared
land, piled up filth, fouled their water, and made breeding grounds for
rats, lice, worms, mosquitoes and other disease-causing or carrying organisms.
The growing populations of these settlements ensured a ready supply of victims
to sustain a chain of infections.

DISEASES COME HOME TO ROOST

Soon after people adopted village life in the Old World, they began
to domesticate wild animals, and in bringing these animals into their homes
they unwittingly shared with them the mix of pathogens from which potent
new diseases evolved. Just as historians such as William McNeill, of the
University of Chicago, and other researchers trace smallpox back to cowpox,
so measles probably evolved from rinderpest or canine distemper, and influenza
from hog diseases. In all, we inherited dozens of our most deadly diseases
from newly tamed animals.

When those diseases first appeared in the Old World, explains historian
William McNeill of the University of Chicago, neither old nor young were
spared. But over time, as the more susceptible individuals were eliminated
from the population, the hardier survivors came to dominate the gene pool
and diseases once deadly to all ages became childhood ills. Occasionally
they suffered spectacular epidemics, such as the Black Death of the 14th
century, but their numbers always rebounded.

The longer a population lives with diseases, the less likely its members
are to die from them. By the time Columbus arrived in America, European
populations had been living with diseases for a very long time. Not so the
native Americans. When a group with adult immunity transmits the disease
to one without, whole groups are invariably wiped out.

While the New World had its native infections, including Chagas and
Carrion’s diseases, trichinosis, tapeworm, and perhaps syphilis, few were
deadly, and none (with the possible exception of syphilis), seriously threatened
whole communities of European colonists. Time and again, Crosby relates,
Europeans in America showed their robust health. Statistics certainly bear
him out: the first settlers of New England averaged an astonishing 71.8
years at death. So why were Native Americans virgin populations for Old
World diseases, and why did they have so few diseases of their own?

Back to last Ice Age and the peopling of the Americas. Native North
Americans are descendants of northeast Asian populations. This can be seen
in their teeth, their genes (particularly in mitochondrial DNA sequences)
and their languages, which all retain vestiges of an Asian ancestry. Their
port of entry to America was the Bering Sea region. When they arrived, glaciers
up to 3 kilometres thick shrouded much of the northern hemisphere, extending
from the Arctic to mid-Ohio, around 40 degrees North, and nearly continuously
from the Atlantic to the Pacific. Those glaciers froze over 5 per cent
of the world’s water, reducing global sea levels to 100 or 150 metres below
their present position.

Lowered sea levels exposed the shallow continental shelf beneath the
Bering Sea and the Chukchi Sea, farther north, creating the Beringia land
bridge between Siberia and Alaska across which the first Americans came.
And what a bridge it was: some 1500 kilometres wide, Beringia was covered
by a vast expanse of grassland on which herds of mammoth, horse and bison
grazed. While archaeologists haggle about the timing of the first Americans’
arrival – some believe that they arrived between 30 000 and 40 000 years
ago – it is certain they had arrived by 11 500 years ago, in time to share
the Americas with an extraordinary zoo of large animals. There were mammoths,
ground sloths, and glyptodonts among the herbivores, and huge lions, sabre-toothed
tigers and giant bears filling the ranks of the carnivores.

The great beasts lumbered into extinction 10 800 years ago, for reasons
still unknown. Ecologist Paul Martin of the University of Arizona charges
Paleolithic Indian hunters with killing (or, more properly, overkilling)
the megafauna. More likely, the animals succumbed to climatic changes at
the end of the Pleistocene, when it got warmer and more seasonal, and summers
grew longer. Halfway around the world, these same changes set the Neolithic
stage for the taming of wild cattle, sheep, goats, pigs, and a dozen other
species. Yet few animals were domesticated in the New World: just three
species of South American camel, the turkey and the guinea pig.

There are two main reasons for that imbalance. First, the New World
fauna was impoverished. Whatever their cause, Pleistocene extinctions wiped
out 80 per cent of America’s large mammal fauna. All that remained in North
America were bison, moose, elk, deer, musk ox, mountain goat and pronghorn
antelope – a very narrow pool of mammals from which to draw potential domestic
animals. Secondly, not all animals are easily domesticated. The biologist
Jared Diamond argues that suitable candidates must possess particular attributes.
They must be social and have a physiology, ecology and behaviour compatible
with efficient food conversion. They must not compete with humans for food
nor have an instant flight reflex. Yet they must breed well in captivity
and be adaptable to a wide range of climates.

Of the surviving prehistoric New World large mammals, only bison come
close to meeting those requirements. But bison are unruly animals and, besides,
there was little incentive to domesticate them. Domestication is partly
a response to food shortage and with the prehistoric bison population of
the North American Plains standing at an abundant 75 million, what was the
point in corralling them?

FILTERING GERMS

New World peoples did domesticate a cornucopia of plants (far more than
in the Old World, in fact). Throughout the Americas, the list extends from
avocado to yams, but most important were maize, beans and squash. These
plants fed the great Pre-Columbian towns of North America and the civilisations
of Central America, while potatoes, peanuts and yams did the same for South
America. So successful was food production in the New World that by 1492
it had cities rivalling in size and population any in the Old. Tenochtitlan,
the Aztec capital (on the site now occupied by Mexico City), had a population
estimated at over 200 000, with perhaps 1.5 million in the surrounding basin;
that one extended city had a population larger than that of all the North
American Colonies combined on the eve of the American Revolution.

So there were crowds before Columbus arrived, but no crowd diseases.
Obviously many people tightly packed need not hatch a disease, though they
will surely transmit a newly arrived infection with deadly speed. But until
the arrival of Europeans, native Americans had none to pass along. The ancestors
of the native North Americans had left the Old World – Siberia, not Mesopotamia
– thousands of years before the first cow or sheep was ever corralled and
well before the first epidemic diseases evolved. And in the unlikely event
that they were carrying diseases from the Old World, those would have been
checked at their North American gateway. Beringia has been described by
T. Dale Stewart, an anthropologist at the Smithsonian Institute, as a ‘germ
filter’, where harsh Arctic climates killed off bacteria or the carriers
of disease such as mosquitoes and worms. Beringia supported so few people
in any one place that any infected groups would have found themselves naturally
quarantined, before being wiped out by their disease.

The rising Bering Sea would have been just as effective a barrier against
a backwash of diseases from the New World to the Old World, were there any
New World epidemic diseases to wash back. But there weren’t. The combination
of Pleistocene extinctions, and the difficulty or disincentive to tame the
survivors, left native Americans no animals with which to share and hybridise
deadly new pathogens. Only in South America, where camels were herded, was
there a potential animal source for new disease strains. But camels and
their herders lived high in the Andes, in small and dispersed groups, and
were too few and too isolated, McNeill explains, to sustain infections in
the wild.

In contrast to the Americas, the Old World saw no major mammalian extinctions
at the end of the Pleistocene, and had plenty of animal candidates for domestication.
Neolithic farmers and town dwellers seized the advantage, and along with
it began to foster new diseases they could not avoid. No germ filter here,
and though its absence caused repeated epidemics, Old World populations
developed an immunity over the long haul of history which helped them to
colonise the Americas.

What happened in Columbus’s wake is tragically clear. The first Americans
left the Old World early, and far from the scene of disease coevolution.
They crossed a disease barrier to the New World, its large fauna nearly
wiped out and with it any potential animals for domestication – and the
diseases that come with it. The following 11 000 or more years rendered
them utterly defenceless against European disease. After 1492, it would
take nearly 500 years of exposure to repeated epidemics and the advent of
modern medicine, before their populations would begin to rebound. Only in
the 1970s did native North Americans finally regain the million mark.

David J. Meltzer is in the Department of Anthropology at the Southern
Methodist University, Dallas, Texas. This essay is adapted from his forthcoming
book, The Search for the First Americans (Smithsonian Institution Press
1993).

* * *

Replaying the tape of American history

What would America be now if the native Americans hadn’t been devastated
by disease? If, for example, the first Americans hadn’t left the Old World
when they did? Or if they had brought diseases with them? Or if they had
not come via Beringia? Or if 80 per cent of America’s megafauna had not
gone extinct at the end of the Pleistocene? Or if the remainder had been
more readily domesticated, or there had been strong incentive to do so?

Consider this. In November 1519, conquistador Hernan Cortes and his
army marched into the Aztec capital of Tenochtitlan, and quietly seized
its ruler, Moctezuma. Cortes commanded the uneasy city for six months, then
marched off to meet a force under Panfilo de Narvaez, who had been sent
by Spanish authorities to arrest Cortes. Tenochtitlan, left under the watch
of an unsavoury lieutenant, erupted in battle, sparked by the slaughter
of 600 unarmed Aztec worshippers. Led by Moctezuma’s brother Cuitlahuac,
the Aztecs pinned down the resident Spaniards and were on the verge of annihilating
them.

Cortes defeated Narvaez and then, receiving desperate bulletins from
his troops in Tenochtitlan, hurried back to recapture the Aztec capital.
He re-entered the city on 24 June, 1520 with 1250 Spaniards (including many
of Narvaez’s men), and 8000 allied Tlaxcallan warriors. Seven days later
he and his forces were decisively beaten. They suffered dreadful casualties
– perhaps all but a third of his army lost.

Even so, within the year Cortes returned to lay siege to Tenochtitlan,
and after another round of bloody battles conquered the Aztec state. It
was an astonishing victory against overwhelming numbers. Some credit this
to Cortes’s superior weaponry and horses. Others speak of his missionary
zeal, courage and military genius.

Cortes may have been a great general, but what gave him victory at Tenochtitlan
was neither strategy nor tactics but smallpox. One of Narvaez’s men had
it, and once the disease took root among the natives it devastated the city’s
population, young and old alike. Cortes had lost the city on 1 July 1520,
and neither his horses, nor his pikes, nor his native Tlaxcallan reserves,
nor his strategies could have retaken Moctezuma’s Aztec kingdom with its
1.5 million souls. But with smallpox they could. While Cortes and his beaten
troops were off licking their wounds, smallpox felled Cuitlahuac and a
host of Aztec leaders and warriors. They never knew what hit them. By the
time Cortes regrouped and returned, Tenochtitlan had been wracked by the
pestilence. The Spaniards’ 75-day siege of the city in the summer of 1521
was the final blow. Tenochtitlan fell.

The impact of European disease was rarely militarily so decisive. Still,
the triumphs of the conquistadores and the conquest of America were, as
Crosby bluntly put it, the victory of smallpox. Had it not been for introduced
epidemic diseases, American history would have been quite different. Instead
of celebrating the Columbus quincentennial in 1992, Americans might now
be anticipating the quincentennial of the great Aztec victory over the invading
hordes from the east.

In reality the victory of smallpox and all the other imported epidemic
diseases became foreordained more than 11 000 years ago, when the first
Americans stepped on to the western edge of the vast grassy plain of Beringia,
then headed west.