Why Disease Conquered the Americas

Posted on May 17, 2013


History of Aboriginal America 15.

Disease and the conquest of the Americas, pt.2

The scale of what happened in the Americas is unparalleled in human history.  An estimated 80 to 100 million people died in the century or two following Columbus. Some are dying still.  For comparison, Europe itself contained only about 80 million people when the hemispheres met.

smallpox-epidemic-native-americans-granger-sThe great dying, that widowing of the Americas, had and continues to have an immeasurable impact on the human story.  Arguably, because of its central role in Europe’s rise, the event (if the many plagues which swept through America can be characterized as a single event) is modern history’s first cause, the single most important factor creating modernity and the world we live in today—but that’s another argument.  For now it’s worthwhile to inquire why and how such a major demographic collapse could have happened in the first place.

There were the diseases, of course.  For those who died in the great post-Columbian pandemics, the names of their deaths are the names of the plagues which the Europeans brought with them.  These were infections which the Americas had never seen before:  malaria, yellow fever, scarlet fever, measles, whooping cough, chicken pox, influenza, cholera, typhus, typhoid, diphtheria, leprosy, smallpox, the bubonic plague, etc.  It’s a remarkable list.  Every item on the list, humanity still has some experience of except for smallpox—which some will say was the worst of them—but the list is only partially useful and the common experience of today is misleading, and not only because of the existence of modern medicines.  What happened when Europe and America met was particular and different.

The list is not the list.  In the Americas the list should be printed in bold.  The people of the Americas had a fatal weakness to the infections which the Europeans brought with them—even measles and chicken pox could slaughter them—a weakness which only begins to be accounted for by the well-known deadliness of “virgin soil” epidemics.  The version of the bubonic plague which killed 1/3 of Europe in the middle of the 14th century was considered a “virgin soil” epidemic.  The “virgin soil” effect is a powerful effect.  But on examination the American plagues were more devastating than that effect can explain by itself.  Science has recently added a contributing cause.  The founding peoples of the Americas, in fact, had an enhanced and documented susceptibility to infectious diseases, a susceptibility which appears to be a direct artifact of Americas’ deep history.

The history of everybody begins in Africa, of course.  The deepest richest strains of humanity are still found there.  Of all places on the planet, Africa displays the most human genetic diversity, more than the rest of the planet taken together.  It requires time to diverge, and deep time to diverge and diverge again.  That process had to be happening for a long time in Africa to account for its human diversity.  Africa, of course, offers direct evidence of its connection to the human story through a continuous archeological record over millions of years, tracking humanity’s emergence there.  Our nearest genetic relatives flourish there too, as you would expect, the chimpanzees and bonobos, who share 98% of human DNA.

The human story began in Africa, but then we wound up everywhere.  Some clarity about how exactly that happened has eluded us until recently.  Archeology is and has been critically hampered, for instance, by the scale of the time depth we are dealing with, by the fact that, since humanity emerged from Africa, the very shape of the world has changed.  Ice ages have come and gone; ancient shorelines have risen and then sunk again under the sea. With critical evidence of the human past ground up by time—buried, erased, inundated—it has taken the emerging scholarship on human genetic history—where populations worldwide have been administered ultra-detailed paternity and maternity tests—to begin to provide us with a testable and coherent picture.  By unraveling the thousands of generations of begats whose memory is embedded in our genes, scientists can now suggest an outline of the early history of human migration and the peopling of the planet.

The present evidence suggests that most of the human families which have flourished outside of Africa were seeded by a single and particular human migration or series of migrations, perhaps 70 thousand years ago.  The date of when it happened is still scientifically wobbly, but that such an event did happen is revealed compellingly in the genetic evidence.

According to the genetic trail, after leaving Africa, one stream of humanity spread from the Middle East north and west into Europe.  Another stream followed the coast eastward into Asia and the Orient.  A rivulet of the South Asian stream broke off, found and settled in Australia, then Tasmania some 50 thousand years ago.  At the farthest edge of the eastward and Arctic reach of humanity, the ones who had wandered farthest of all discovered and settled the Americas.

The events summarized in this brief reporting occurred in era beyond the memory of history, and although it talks about migrations, most of the story is about settling in.  After Africa, the European, Middle-Eastern and Asian populations benefited from the broadest genetic streams.  Of course, a substantial population provides its own opportunities for diversity, and there was plenty of room and time to grow a population in Europe and Asia.  Time greatly favoured the parts of the human family living in Asia and Europe, since those places were settled and had began to expand their populations long before anybody started settling America.

In fact, it was only a part of a part of part of the human genetic heritage that first entered the Americas.  When the people got there, that genetic bottleneck began to diversify again.  The few became many.  The success of the American enterprise eventually created the numbers necessary for achieving genetic diversity over time, but the meeting of the two long-separated streams of humanity in 1492 interrupted the time, fatally interrupted it long before the people of the Americas could come even close to achieving the genetic diversity of the Old World—if that was possible.  The people of the Americas would die of their relative genetic purity.

It is a hazard, when facing epidemics, to be too much alike, as can be illustrated by the example of the 19th century Irish potato famine.  There are hundreds of varieties of potatoes, but they imported and planted only a very few of them in Ireland.  Thus when the potato blight arrived, large parts of the crop were infected and destroyed.  With more variety, a smaller proportion of the crop would be vulnerable to any given disease.  And the chances of a vulnerable crop being planted in a field next to another vulnerable crop, for instance, would have been lessened too, impeding the ability of the blight to spread.  Thus, even vulnerable varieties are less endangered by epidemics when the population under attack is genetically diverse.

That the people who originally settled the Americas started out as very few, perhaps a handful of tribes in the beginning, is clear in the DNA.  Nine out of 10, and almost 100% of those in South America, have type O blood, for instance. In Europe there are approximately equal numbers of type O and type A.  This uniformity among Aboriginal people applies also to their immune systems.

Human leukocyte antigens (HLAs) protect us from infection, and since infections come in many varieties, having a variety of HLAs will protect us better.  Not everybody has every variety of HLA, but in a given group you can usually expect a much larger spectrum of variation than any one individual possesses.  Thus, even if an infection comes along that is deadly to a particular individual, or even many individuals, there is still a good chance that others in the group will possess the HLAs to give them immunity.  Even the newest and most devastating plagues will thus permit survivors in a group possessing a sufficient variety of HLAs.

However the original people of the Americas, because of their relative genetic purity, simply didn’t possess the kind of variety existing elsewhere.

In his book, 1491, Charles C. Mann discusses the work of Yale virologist Francis L. Black among the Aboriginal people in South America:

In the 1990s, Black reviewed thirty-six studies of South American Indians.  Not to his surprise, he discovered that overall Indians have fewer HLA types than populations from Europe, Asia, and Africa.  European populations have at least thirty-five main HLA classes, whereas Indian groups have no more than seventeen.  In addition, Native American HLA profiles are dominated by an unusually small number of types.  About one third of South American Indians, Black discovered, have identical or near-identical HLA profiles; for Africans the figure is one in two hundred.  In South America, he estimated, the minimum probability that a pathogen will encounter a host with a similar immune spectrum is about 28 percent.  In Europe, the chance is less than 2 percent.

Thus the plagues that emptied the Americas were devastating for three reasons.  One, they were new, and the populations of the Americas had not had the opportunity to acquire immunity to them.  Two, the biological protections which the Aboriginal people had were less on both the individual and collective level than that possessed by Old World populations.  Three, the genetic uniformity of the American populations ensured that a disease which affected one in the population could often spread handily and without check to the rest.

Those who had traveled the farthest out of Africa and populated the Americas were in the end too few and too exceptional, a reality which even tens of thousands of years of change in the Americas and a hugely expanded population were unable to erase.  When Old World and New World finally met again, the consequence of that exceptionality shaped the fate of the Americas and ultimately created the world around us today.


See Charles C. Mann, 1491: New Revelations of the Americas Before Columbus, Vintage Books, NY, 2005.