Human Biogeography

9. Other Species Affect Our Biogeography

215 9 Other Species Affect Our Biogeography Without trypanosomiasis the whole of the sub-Saharan continent would, like Latin America, have been readily conquered by European forces long before the 19th Century. [505, Maudlin, 2006, p. 679] summary The organisms that cause disease vary in their geographic distribution, and therefore , so also do our physiological traits and genes. Thus, people native to areas where malaria is present have a different genetic complement than do those who live in areas free of malaria. Some diseases have prevented humans from occupying certain regions of the world. For instance, sleeping sickness might have slowed European hegemony over Africa and so preserved African cultural diversity. People who live in cities face a differently ranked set of pathogens than do those who live rurally, and therefore their physiologies are different. Thus, it was not only regional differences in susceptibility that led to the devastation of indigenous populations as Europeans expanded across the globe but the fact that the exploring Europeans were mainly city dwellers. Even now, the prevalence of measles corresponds to the size of the infected populations. And before we had developed advanced weapons, it is not impossible that predators and scavengers limited our numbers and our spread across the world. The organisms that cause disease vary in their geographic distribution. Malaria is more prevalent in tropical countries than in temperate ones and is absent from the Arctic. Measles is a disease of cities. And so on. Many aspects of our genetic makeup and physiology are related to evolved responses to disease organisms. If the diseases vary geographically, so does 216 | Interaction among Cultures and Species our physiology and our genetic constitution, our genotype. I present here some examples of this biogeographical coincidence of pathogens, disease, and human physiology. The aim is to provide examples of the biogeography of interspecific interaction and also to illustrate to biogeographers of nonhuman species the extent of information on human biogeography. DISEASE AND HUMAN BIOGEOGRAPHY Malaria and Sleeping Sickness Sickle cell anemia is the classic example of how regional variation in the incidence of a disease causes regional variation in the nature of humans, in other words of how disease affects human biogeography [332, ch. 10; 530, ch. 6; 640, ch. 7]. Malaria is induced by the protozoan Plasmodium, the most lethal form of which is P. falciparum. P. falciparum is particularly widespread and prevalent in tropical Africa, although it now occurs throughout the tropics [714]. Sickle cell anemia, a lethal condition, is also prevalent in tropical Africa. It is characterized by individuals with it having two copies of the HbS gene. The gene persists in falciparum malarial regions, and people homozygous for it are born, because a person heterozygous for HbS is resistant to falciparum malaria and hence survives better than the homozygous HbA person. Outside of malarial areas, the homozygous HbA person survives better than the heterozygote individual, which is why sickle cell anemia is effectively nonexistent where there is no falciparum malaria. Sickle cell anemia, or the gene that causes it when homozygous, does not occur everywhere where P. falciparum occurs. Rather, as with the ability to digest lactose (chapter 7), different genes, and hence chemicals, seem to have evolved to do the same job. For instance, in some areas of western Africa, another form of hemoglobin gene, HbC , confers resistance to malaria, indeed better resistance against severe malaria [332, ch. 10; 530, ch. 6]. It occurs together with HbS , but it is rare where HbS is common, which itself is rare where HbC is common, perhaps because HbC is more recently evolved than HbS and was in the process of replacing HbS in regions of severe falciparum malaria before malarial control campaigns [332, ch. 10; 530, ch. 6]. P. falciparum infects humans and gorillas. It used to be though that its closest relative was P.reichenowi, which infects chimpanzees, but it is now known that the closest relative is another P. falciparum of gorillas [460]. Other Species Affect Our Biogeography | 217 Elsewhere than Africa, malaria appears to be associated with variants of precursors of hemoglobin. While they might offer some protection against malaria, they have adverse effects in its absence, such as thalassemias , which were first recognized in and around the Mediterranean [332, ch. 10]. The argument for protection against malaria of the blood variants that produce the thalassemias is based, as with sickle cell anemia , on the fact that the otherwise...