鈥淚T RAGED amongst us, killing vast numbers of people,鈥� recalled the
survivors, 16th-century Aztec elders whose memories of the great sickness
lingered with bitter clarity even 30 years later. 鈥淚t covered many all over with
sores: on the face, on the head, on the chest, everywhere. It was devastating.鈥�
In just two months, tens of thousands fell victim to the smallpox virus which
had been carried to tropical Mexico in 1520 aboard Spanish ships. And in the
weeks and months that followed, the disease swept south, rampaging through the
small Mayan kingdoms of Guatemala. By 1527 it had spread down the mountainous
spine of South America, claiming the lives of Incan farmers, nobles, generals
and emperors.
For years, some scholars have argued that the smallpox epidemic of 1520
stalked the length and breadth of the Americas鈥擭orth as well as South. But
did it? Over the last year this view has been challenged by those who say
smallpox and other viral plagues spread much more slowly in the north. And this
is no idle debate. The outcome promises to shape researchers鈥� ideas about the
political sophistication of prehistoric native cultures in North America.
According to earlier theories, the Miami, Mandan, Cheyenne and Cree who greeted
Europeans penetrating the North American interior in the 17th and 18th centuries
were the survivors of a hideous viral holocaust. Liberal historians have talked
of genocide and the destruction of entire, unknown civilisations by
disease-carrying European sailors. Some native American writers have envisaged a
lost golden age in North America, before European chroniclers arrived, when
flourishing native nations occupied a host of sprawling cities.
Wholesale slaughter
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Henry Dobyns, a retired ethnohistorian who has been professor of anthropology
at Cornell University and the University of Oklahoma, believes that the smallpox
epidemic of 1520 was just one of many. In his influential book, Their Number
Become Thinned: Native American Population Dynamics in Eastern North
America, published in 1983, he put forward the hypothesis that European
microbes launched their pandemic assaults on North America almost as soon as the
Spanish landed in the Caribbean. In the century and a half that followed, Dobyns
suggests, Old World diseases swept the entire Western Hemisphere, slaughtering
as many as 100 million native Americans. 鈥淚t was,鈥� wrote Dobyns recently, 鈥渢he
world鈥檚 most catastrophic epidemic.鈥�
But convincing archaeological evidence for this 16th-century tragedy is hard
to unearth. Viruses such as smallpox leave few if any traces on human skeletal
remains, which are all that is preserved in most archaeological sites. And most
prehistoric villages and campsites yield only ambiguous clues to the size of
their population.
Dean Snow, professor of anthropology at Pennsylvania State University, now
claims to have succeeded where other researchers have failed. Having amassed
evidence from one corner of North America, Snow has dealt a stunning blow to
Dobyns鈥檚 pandemic hypothesis (Science, vol 268, p 1601). The swift
spread of disease throughout the Incan empire of South America in the early 16th
century remains undisputed, says Snow, but far from winnowing millions of lives
in North America during that same period, Snow says that smallpox and other Old
World plagues lagged decades behind early European immigrants in some parts of
the continent.
In the US, his work has rekindled controversy over prehistoric populations.
Dobyns estimates that just before Old World diseases began to take their toll,
some 18 million people were living north of Mesoamerica, a region that includes
Central America and the southern two-thirds of Mexico. He arrives at this figure
by assuming that every outbreak reported in historial literature was part of a
pandemic. Snow, on the other hand, now pegs the number at just under 2 million.
From these figures flow radically different ideas about native American society.
Social scientists say that political complexity arises directly from population
density: the higher the density, the more sophisticated political affairs
become. If Dobyns鈥檚 estimates are right, North America鈥檚 native populations
should have founded many large cities. But Snow discounts these possibilities,
noting that archaeologists have turned up only one centre that reached city
proportions. 鈥淲e鈥檝e looked at a lot of areas and aren鈥檛 able to find
补苍测迟丑颈苍驳.鈥�
Snow鈥檚 findings have spawned a host of puzzling new questions. If he is
correct, why did European diseases infect the aboriginal inhabitants of Mexico
and Peru so swiftly in the 16th century, but take root only slowly in North
American regions colonised by Dutch, English and French settlers? What prevented
smallpox and other European microbes from stealing throughout North America in
the 16th century?
Snow entered the pandemic debate almost by chance in the early 1980s. He is
an expert on the Mohawk Nation鈥攐ne of the five tribal groups that lived
south and east of Lake Ontario and which joined forces in a political alliance
known as the Iroquois Confederacy. While gathering data on the origins of the
Mohawk, Snow began to see a way of tracking changes in the size of their
populations.
The Mohawk had practised slash-and-burn agriculture, cultivating fields of
corn in what is now New York State. Every decade or two, villagers packed up
their belongings and moved nearer to virgin or mature forest. 鈥淚t meant the
Mohawk villages were lived in and abandoned very quickly, and this leaves a
sequence of village sites for archaeologists to find,鈥� says Snow. This makes
them unusually easy to date and analyse. 鈥淲e don鈥檛 have them sort of piled one
on top of each other for hundreds of years at a crack.鈥�
Snow鈥檚 curiosity was raised and he began searching for ways to estimate
village populations accurately from meagre clues left in the ground. As luck
would have it, in the 18th century a pair of European visitors had sketched
floor plans of three Mohawk villages and counted inhabitants. Poring over these
documents, Snow discovered that two Mohawk families, averaging five people each,
once gathered around each hearth. It was just the clue he needed to transform
village rubble into population figures. He began counting hearths in the floor
plans of all excavated Mohawk villages and sending kernels of corn for
radiocarbon dating.
Unearthing clues
But what about known sites of villages which had not yet been excavated?
Their hearths remained hidden, buried beneath layers of debris and sediment.
Undaunted, Snow sought other lines of evidence. Did the Mohawk, for example,
favour some standard residential density, as many cultures do today? Snow and
his colleagues set to work measuring the area of each excavated village and
calculating its population density. 鈥淭his figure was very consistent, much to
our surprise,鈥� he says. For the most part, each Mohawk required 20 square metres
of village space.
All that was missing from the population equation were the areas covered by
unexcavated villages. And there modern farmers came to Snow鈥檚 aid. Centuries of
ploughing had churned up bits of Mohawk pottery and other objects from buried
villages: Snow鈥檚 team mapped the boundaries of around 50 villages by searching
for these remains on the surface. And the presence of characteristic shards and
tiny trade beads allowed them to date the settlements.
Patchy populations
Snow then had the data he needed to plot changes in the total Mohawk
population over four centuries, from 1400 to 1776. The results amazed him. Far
from collapsing in the 16th century, as Dobyns had confidently predicted, the
Mohawk had prospered, doubling in number in the course of the century. Snow
could find no sign of a pandemic. At least, not until 1634 鈥攁lmost a
century after the first recorded contact between Europeans and an Iroquoian
group along the Gulf of St Lawrence. Both archaeological evidence and
eye-witness accounts suggest that in a single year, 63 per cent of Mohawks had
been wiped out, and the culprit was almost certainly foreign disease. 鈥淚鈥檓 99
per cent certain it was smallpox,鈥� says Snow, 鈥渂ecause there was a
well-documented smallpox epidemic just prior to this one over in the Connecticut
River valley.鈥�
But why hadn鈥檛 this virulent disease spread north from Mexico a century or so
earlier? Snow says the answer lies in the patchiness of aboriginal populations
in North America. 鈥淲e鈥檝e had this tendency for years to draw maps of North
American tribal distributions where we show boundaries and territories that are
contiguous with each other.鈥� In fact, he says, the population distribution was
far from even. 鈥淧opulations were concentrated in clusters of towns or villages
in parts of North America where agriculture was practised. And there were huge
areas between them that were not permanently occupied by anyone.鈥� He believes
that these empty regions served as natural barriers to the spread of
infection.
A more tantalising question, he suggests, is why smallpox and other diseases
lagged so far behind the arrival of European fishing and trading ships in the
region. In an attempt to answer this question, Snow began scouring historical
records on crews and crossing times. He noted that in the late 15th and 16th
centuries, when Europeans first landed on North American coasts, small crews
made the crossing in six weeks. By the 17th century, however, larger crews
crossed in just four weeks, greatly increasing the chances of infected people
aboard surviving long enough to transmit their viruses to the native population.
But as Snow contemplated the early passenger lists and studied the epidemiology
of smallpox and other early plagues like measles, he began to spy another key
factor.
Baby carriers
In 17th-century Europe, smallpox and measles were childhood illnesses:
infants and toddlers who overcame them acquired lifelong immunity. As a
consequence, European children were the likely carriers of these plagues. As
Snow studied colonial history, he found that the European powers had very
different ideas about the desirability of bringing children to their overseas
empires. Spanish authorities encouraged swift colonisation, conveying entire
families to the Caribbean, Florida and Mexico in the early 1500s. The Dutch,
English and French had other objectives in mind. 鈥淚t was a commercial venture
for the Dutch and it was more a military venture for the English,鈥� says Snow.
鈥淎nd among the French, it was mainly the men who immigrated and then they would
marry native women.鈥� The French and Dutch immigrants only started to bring their
families鈥攁nd the diseases that the children carried鈥攊n the
1630s.
As far as Snow is concerned, the disparate pieces of the puzzle now fit into
a coherent whole. But Dobyns remains unconvinced. Archaeology, he argues, has
little to contribute to studies of these ancient epidemics, because only
fragmentary evidence remains in the ground. Dobyns believes Snow鈥檚 calculations
of past populations are highly suspect. In all likelihood, he says, many Mohawk
villages remain undiscovered in the now heavily urbanised Mohawk River valley.
鈥淚t鈥檚 a miracle that Snow can find any sites at all to excavate,鈥� he says.
And there are other sceptics. One, Ann Ramenofsky, an archaeologist at the
University of New Mexico, concedes that Snow is probably right about the timing
of the first European epidemic in the Mohawk Valley, but she remains unconvinced
about the reliability of his dating techniques. Radiocarbon dating of samples
becomes unreliable from 1600 onwards, she notes, due to shifts in the
hydrocarbon reservoir. 鈥淗e needs more ways to verify that post-contact record
other than carbon-14 dates and documents,鈥� she says.
Snow, however, stands by his findings. Dismissing charges of faulty
methodology, he points out that the Mohawk chose to live in areas very different
from those favoured by 20th-century urban dwellers. 鈥淢odern towns and cities are
all being built down next to the river, and the Iroquois villages are all on the
hilltops.鈥� The Mohawks settled in the regions that modern farmers now cultivate,
says Snow, because they too were looking for good soil. 鈥淟ocal collectors have
been scouring those hills for a long time,鈥� he says. 鈥淪o we are pretty much
convinced that we haven鈥檛 missed very many villages鈥攊f any.鈥�
Another recent study by Gary Warrick, an archaeologist at the Ministry of
Transportation of Ontario, lends further weight to Snow鈥檚 findings. Warrick
unearthed data on another Iroquoian group, the Huron. He tracked population size
from the 14th to the mid-17th century using a variety of techniques, including a
statistical analysis of the populations represented in four Huron burial
grounds. Like Snow, Warrick found no hint of devastating disease until
1634鈥攖he year that a French ship carrying 30 children dropped anchor in
what is now Quebec.
鈥淚t鈥檚 the first time you have a boatload of colonists with their families in
New France,鈥� says Warrick. 鈥淎nd it was that summer that diseases were
transmitted to the Montaignais and the trading Huron.鈥� The Montaignais were
hunter-gatherers who lived along the northern shores of the St Lawrence
River.
Many researchers now believe that these new data lay to rest the notion that
Old World microbes emptied inland North America before Europeans so much as laid
eyes on it. And they are increasingly confident that native populations were
much sparser than Dobyns and others have envisaged.
What remains far from clear is why populations were so much thinner in North
than in South and Central America. Snow suggests that this was because farming
only took off in North America around a thousand years ago, once people had
developed strains of corn able to survive the extremes of climate there. 鈥淏y the
time Columbus arrived, people in North America had had only five hundred years
to take a crack at this horticulture business with all of its implications, the
big cities and so on,鈥� concludes Snow. 鈥淭he Mexicans and the Peruvians had a
much earlier start.鈥�