Discovering the Chesapeake: The History of an Ecosystem

SEVEN: Human Biology of Populations in the Chesapeake Watershed

chapter seven Human Biology of Populations in the Chesapeake Watershed douglas h. ubelaker and philip d. curtin Some OldWorld pathogens stayed behind when humans migrated across Beringia, while others such as tuberculosis and other infectious diseases were transported to the New World. Human skeletal evidence shows that the health of the Indian population varied, depending on availability of food sources such as calcium-rich oysters . Increasing population density encouraged the spread of disease, especially new pathogens that were brought by the Europeans and Africans and depended on a nucleus of non-immune population. Relations between humans and their parasites in North America can be seen as a drama in three acts. The first act began with the arrival of humans and their pathogens during the Ice Age,up to about 10,000 years ago. The second act began in the late fifteenth century with the coming of Africans and Europeans and a new group of pathogens. The third act began in the second half of the nineteenth century, when humans began to conquer decisively the most serious infectious diseases that had troubled them. The initial mode of arrival over the land bridge from Asia helped to determine the kinds of pathogens humans could bring with them. Many Old World parasites, for example, had evolved in ways that required not only a human host but a secondary host as well—often an insect or a snail. Many that required a secondary host were left behind. Still another group of diseases failed to cross to the Americas because they had not yet evolved. Some pathogens had begun to parasitize human hosts before the agricultural revolution brought humans into close contact with domestic animals like dogs, cats, pigs, mice, and rats. But it was the new, postagricultural contact between humans and other animals that made it possible for diseases such as cowpox to mutate into smallpox . All known immigrants to the Americas before the 1500s were preagricultural , including the Inuit, who began to arrive about 2000 b.c.e. All these early immigrants came as small bands of hunters and foragers. Many diseases can maintain themselves only in a human community of a certain minimum size.This is especially the case for those that pass directly from one human host to another and for those that either kill quickly or leave the victim with a substantial immunity to further attack. In small communities, these pathogens can easily immunize such a large part of the population that the pathogens die out for lack of new hosts, though they may be reintroduced when a subsequent generation of nonimmunes appears. The community size required to maintain diseases of this type varies considerably from one pathogen to another. Measles is a classic example: recent estimates suggest that measles in isolated communities will die out if the population at risk is much less than about a half million.Other similar diseases, sometimes called crowd diseases, include mumps, smallpox, chickenpox, rubella, common cold, cholera, falciparum malaria, and perhaps influenza. The urban civilizations that developed in the Americas became dense enough to harbor these pathogens, but only when they were introduced from Europe and Africa after Columbus’s voyages. Ecologists have paid little attention to the role of human pathogens in the ecosystem, though medical specialists have taken an ecological approach to disease for some time.Pathogens behave in much the same way as other parasites and predators, and they follow a similar evolutionary process. The species that survive are those that have adapted and readapted to the changing ecological niches in which they find themselves . Microscopic parasites, like larger predators, have a symbiotic relationship to the host community. New and more virulent strains can appear and probably have appeared many times in the past, but increased virulence is viable only within limits. A superparasite that kills off the host community will die out for lack of prey. On the other hand, a pathogen that mutates toward less infectiveness may also disappear from its inability to infect new victims. Successful evolution for the parasite thus tends toward a balance—enough virulence to pass readily to new hosts, but not so much that it wipes out the host community. The hosts also adapt through evolution, but large primates such as humans evolve quite slowly. A human generation takes about twenty years. 128 Discovering the Chesapeake By contrast, some microorganisms pass through a complete generation in less than twenty...