Evolutionary Medicine

Historical Context

The Old World is defined as Europe, Africa and Asia, including the nearby islands before the 15 ^th century. The New World refers to the Americas and Australasia. When people first emigrated from Siberia and came to the Americas, they crossed a land bridge, made of frozen tundra where the Bering Strait is today. These immigrants are believed to be the ancestors of modern Native Americans.

The small size of the groups that crossed the land bridge likely limited the number of parasitic and infectious diseases that made the journey with these people. The frigid conditions along the trek across the frozen bridge prevented the survival of some pathogens, which are typically accustomed to warmer climates. This frozen trek also weeded out the weaker individuals and only the strongest people survived this difficult migration.

The Native Americans' nomadic life style prevented them from acquiring many diseases. Basically, prior to Columbus' discovery of the Americas the Native Americans were exposed to a few "hunter-gatherer" diseases such as chagas, leishmaniasis and amoebic dysentery. The Native Americans' lifestyle of small groups living in isolation resulted in a relatively "disease free paradise". The populations simply were not large enough to sustain and spread most infectious diseases. While this isolated lifestyle served the Native Americans well for a long time, it would ultimately leave them extremely vulnerable to disease later on.

The movement of human populations and contact across long distances leads to the spread of disease. When the Europeans colonized the Americas, the Native Americans had no memory, or stored B cells to fight Old World diseases. They essentially had no acquired immunity for Old World diseases. Acquired immunity occurs when an organism, a human in this case, is exposed to a virus or a bacterium. The person will either get sick and die or recover. If the person recovers, the body produces antibodies to that particular pathogen, thus "acquiring" long-term immunity. Native Americans lack of exposure to infectious disease resulted in a lack of knowledge in regards to disease and contagiousness. "One warrior could not understand how one could pass this damaging ailment (smallpox) when a battle wound couldn't be passed on." ¹⁸ Since the Native Americans were not exposed to the diseases of the Old World, the Native Americans faced high mortality rates upon contact. It is believed that there were about 20 million Native Americans living in the New World in 1492, and about 95% of them died over the next two centuries. ¹⁹

The transatlantic journey not only brought English colonists, but also diseases from the Old World that would decimate the population. One such colonist, Reverend Robert Hunt became ill on the ship and suffered for weeks. One of his symptoms was a high fever. He was the only colonist who showed signs of sickness on the journey. During the journey, hygiene was important. The bedding was aired out daily and the below decks were washed with vinegar. ²⁰ This most likely contributed to the overall health aboard the ship. The three English ships, the Godspeed, the Discovery, and the Susan Constant arrived in Jamestown, Virginia in the summer of 1607. The weather was hot and humid. The new colonists constructed an enclosed fort on a peninsula on the banks of the James River. It is believed that typhoid fever was the main disease that struck the Jamestown colony, and Reverend Hunt carried the disease across the ocean to the New World. The new colony would face starvation and high rates of disease and death throughout its early years.

The mortality rate during these early years was staggering. During the first year, the colony's population went from one hundred eight to thirty-eight. Then during the most difficult time, the Starving Time — the winter of 1609, the population went from five hundred to sixty in six months. The situation was so dire that colonists resorted to eating leather, rats and even the corpses of their fellow fallen comrades. Between 1619 and 1622, after 3,570 new settlers joined the colony, the population was 4,270. Within the three-year period beginning in 1619, about 3,000 colonists died. But, why was there such a high mortality rate?

There are numerous factors that contributed to the plight of the English colony. First, they arrived in the summer and many pathogens prefer warmer climate. Second, the site chosen for the settlement was surrounded by brackish water, which gave rise to mosquitoes and a poor supply of drinking water. The colonists initially drank the brackish water directly from the James River. Later, they dug shallow wells, but the salt water managed to seep into the drinking supply. Third, the colonists faced starvation. Nutrition has a negative impact on susceptibility to disease. Similarly, infection increases the level of starvation. A fourth reason is that the settlers had dirty living conditions. The last reason is the enclosed fort, which provided protection from the Native Americans, but the tight quarters contributed to the spread of infectious disease.

It has also recently been discovered that a severe drought coincided with the starving time. Scientists examined the tree rings of cypress trees and concluded that the drought lasted seven years and was the worst spell in 800 years. A drought would also explain the increased conflict that occurred between the Native Americans and the colonists. If the fresh water supply was drying up and there was more competition for food, it makes sense that there would be more fighting amongst the people. Additionally, since the James River is brackish, a drought most likely reduced the amount of fresh water entering the river thus increasing the salt content. The water quality certainly played a part in the poor health of the colonists.

It is difficult to know exactly what types of diseases struck the Jamestown colony because most fast killing diseases do not affect the skeletal system. Archaeologists study the bones of human remains in an effort to determine history. Some of the information about the types of diseases comes from archaeological research and some comes from primary source accounts. John Smith kept a detailed journal, but it is believed that some of his accounts were biased. However, journal entries have shed light on the signs and symptoms of the diseases that affected the English settlement. From the accounts of Reverend Robert Hunt's sickness, it is believed that typhoid fever was one disease that infected the colony. Malnutrition was also responsible for death in the colony, and probably made the colonists more susceptible to other diseases as well.

As the Jamestown colony survived, it became more dependent on subsistence farming, the first epidemiological transition: the shift to an agricultural subsistence economy began. Along with subsistence farming came the domestication of animals. Living amongst animals opened up new avenues for disease to infect humans. Subsistence farming created larger, more permanent settlements, limited food variety and poorer nutrition resulted, sanitation problems occurred and infectious disease increased. Clearly these problems came about with the emergence of subsistence farming. This transition extended through the 18 ^th century.

Smallpox is an example of a disease that evolved from a disease of animals. Recent evidence suggests that the smallpox virus is very closely related to viruses that infect rodents. ²¹ The smallpox virus of humans is also very closely related to the cowpox virus, which infects cows. This very close relationship led to the realization that an infection of the cowpox virus can elicit an immune response in humans and provide protection against the smallpox virus. The discovery of this relationship was key in the development of modern day vaccines. In the 1700's, Edward Jenner, an English physician, observed that milkmaids seemed to be more resistant to smallpox. He took the pus from a cowpox lesion and experimented with a vaccination for smallpox. In 1796, Jenner proved his cowpox vaccination prevented infection. ²² In 1806, Thomas Jefferson sent Jenner a letter to congratulate him. In 1807, Jenner received a wampum belt and a thank you letter from the chiefs of the five Nations of North American Indians. Smallpox killed hundreds of thousands of Native Americans. Jenner was most proud of his wampum belt. It would take one hundred eighty-two years from the time Jenner published his pamphlet to eradicate this deadly disease from the world. That year was 1980.

Yellow fever affected American history numerous times over the years. Since Africans had previously been exposed to yellow fever, they were more resistant to the disease. Native Americans worked the field and did chores for Europeans, but so many were dying from yellow fever. Europeans noticed that the Africans were not coming down with yellow fever as frequently as the Native Americans. This made Africans more desirable as a source of labor and increased slave trade in North America. ²³ Yellow fever also shut down the government in Philadelphia in 1793. Of the 40,000 residents, 4,044 died in a four-month span. ²⁴ The presence of yellow fever around the Mississippi Valley was one factor that Napoleon considered when he agreed to sell the Louisiana Purchase. The large American outbreak took place in 1878 in Memphis, Tennessee. During an eleven-week span following the first case, Memphis faced 17,000 cases. Five thousand of which were fatal. African Americans were not as resistant to this strain. ²⁵