Latest Issue of Science News

cover 3/7


Today's information revolution illuminates diseases spread in the age of discovery

2:09pm, January 16, 2012
Sponsor Message

After European diseases arrived on American shores, chroniclers wrote graphic descriptions of depopulated villages, great leaders felled by illness and whole tribes wiped out by smallpox. Yet apart from such accounts almost nothing is known about the wave of disease that hit the Americas 500 years ago.

Some researchers have argued that the New World’s immunological naiveté caused as much as 90 percent of the Native American population to be wiped out almost as soon as Europeans and their African slaves began arriving. Others counter that those diseases had a relatively modest, short-lived effect that was concentrated in areas where European observers were most likely to take notice, like missions and trading posts. The debate has raged for decades, if not centuries.

But now, we have computers. Two new studies suggest possible ways to answer some long-standing questions about the effects of imported infectious diseases on the New World. One essentially rebuilds the genealogy of two whole continents using ancient DNA from archaeological remains and modern-day samples. The other is a spatial analysis of prehistoric settlement patterns in northeastern North America during the first half of the 17th century. It fills in a lot of gaps, both literally and figuratively, about how the epidemic was felt by different tribal groups.

First, the genetics: Brendan O’Fallon of ARUP Laboratories in Salt Lake City and Lars Fehren-Schmitz of Yale and the University of Göttingen in Germany used a computer program called BEAST that collects genetic profiles from a bunch of individuals in a population, both living and ancestral, and assembles them into family trees. Then the program estimates, based on those trees, how the population’s size and genetic structure has changed over time.

BEAST has answered all sorts of questions. It tracked the extinction of giant Ice Age bison in North America and concluded that the animals were already on the decline long before human hunters arrived. Another study documented recent population growth among cougars in Montana by sampling DNA from a virus that infects them. The software has even exonerated people wrongly accused of crimes: It showed that a 1998 HIV outbreak among children in a Libyan hospital predated the arrival of five nurses and a doctor who were blamed for causing it.

O’Fallon and Fehren-Schmitz booted up BEAST with DNA profiles from 137 present-day Native Americans and 63 people who lived in Canada, Illinois or Peru between 800 and 5,000 years ago. BEAST concluded that Native Americans must have experienced a bottleneck about 500 years ago that reduced their numbers from a few million to about half that, the researchers report in the Dec. 20 Proceedings of the National Academy of Sciences.

But the study is frustratingly vague on details. BEAST did reveal that certain branches of the Native American family tree were more affected than others by the population decline. Those differences may reflect higher vulnerability to disease among certain tribes or in particular geographic areas, though it’s too early to figure out from the genetics exactly what those might be.

Some of those gaps might be filled in by approaches like the one used by Eric E. Jones of Wake Forest University in Winston-Salem, N.C., and Sharon N. DeWitte of the University of South Carolina. They collected historical and archaeological data on Native American populations in northeastern North America during the first half of the 17th century and dumped the numbers into a geographic information system, a computer program that analyzes spatial data. Jones and DeWitte then used a statistical technique to fill in gaps in their data. The result was a shaded map of the Northeast showing the intensity of depopulation over the period. The darkest parts of the map were in the Connecticut and Hudson River valleys, among Algonquin-speaking tribes like the Abenaki, Mohegan and Massachusett.

Those groups may have been much harder-hit than Iroquois-speakers like the Seneca and Onondaga simply because the Algonquin lived in coastal and major river valley regions that were colonized earlier and more intensively by European settlers. Or it may be that genetic differences gave the Iroquois better immunity to new diseases, the researchers conclude in a paper published online November 15 in the Journal of Anthropological Archaeology.

That kind of hypothesis would have been impossible to test just a few years ago. With an ever increasing supply of genetic data, innovative analytical software and curious researchers, however, we may eventually be able to answer questions about how and when different European diseases were introduced to the Americas, which ones did the most damage and to whom. How odd it is that the prospect of reconstructing such a pivotal event exists only thanks to a dramatic revolution that has transformed our own age: Because now, we have computers.

More from Science News