Human Biogeography

5. How Are We Adapted to Our Environment?

89 5 How Are We Adapted to Our Environment? [Humans’ regional] differences cannot be accounted for by the direct action of different conditions of life, even after exposure to them for an enormous period of time. [166, Darwin, 1871, part I, p. 246] summary Several human traits give every impression of being evolved responses to the environment , so accounting for several regional differences between human populations . Dark and light skin, respectively, prevent damage from the sun and allows benefit from it. Long limbs allow heat loss in hot climates, whereas short limbs help retention of energy in cold climates. Peoples from different latitudes respond differently to extreme cold but not, seemingly, to extreme heat, perhaps because summer temperatures at mid-latitudes and exertion at high latitudes mean that temperate and arctic peoples can experience the same high temperatures as do tropical peoples. Although, the temperatures of high elevation can match those of high latitude , most studies of high-elevation peoples have concerned oxygen deprivation rather than cold. People resident at high elevation perform better in the thin atmosphere than do new arrivals. While acclimatization closes the difference, it does not do so completely. The data come mostly from Andeans and Tibetans, who seem to have evolved different adaptations to oxygen deprivation, though equally efficacious ones. Some differences between the sexes match those seen between latitudes, with females equivalent to high-latitude peoples. Females have shorter limbs for a given stature and more subcutaneous fat than males have. Consequently, females are more efficient at retaining energy and probably perform better in extreme cold than do males. The greater efficiency of females is likely related to the benefits of reserving energy for care of their offspring. Human cultures occur at greater density in the tropics than at higher latitudes : humans, like other forms of life, are more diverse in the tropics. Several 90 | Environmental Influences on Humans explanations exist for the latitudinal gradient in biological diversity (the Forster effect). Most can explain the Forster effect in human cultures, but some do not. Those that do not work for human cultures are also invalid for several nonhuman taxa too. Thus, anthropology substantiates both the valid arguments for the Forster effect and the invalidity of some of the other explanations. Even so, the best explanations for the Forster effect might differ between nonhuman species (whose ranges overlap) and human cultures, which tend to be quite territorial. One of the simplest explanations for the Forster effect among nonhuman species is that whereas the tropical climate places no limits on form, high-latitude winters severely restrict what can survive. For humans, the lesser productivity of high latitudes forces cultures to use large ranges (more in chapter 6) and hence constrains the density of cultures. Within latitudes (for instance, within the tropics), hotspots of both biodiversity and cultural diversity exist. Explanations for the Forster effect should also explain hotspots compared to coldspots. However, the match is not good. Scale effects could be in operation; i.e., mechanisms at one scale do not operate at another. In addition, analyses of environmental explanations for hotspots, based as they are on current environmental conditions, often lack information on the vital components of history of both climate and biogeographic distributions. Humans are distributed very unevenly across the globe. Half of the world’s human population occupies less than 3% of its land area. Correspondingly, only a few cultures live at high population density; most live at low density. In general , the distribution of densities of present-day humans does not show any cline with latitude or climate. Nevertheless, extreme high and low densities correlate with good and poor climate. We are at our highest regional densities in regions of clement climate and lowest at most adverse climatic extremes. The fact that our largest cities are usually coastal indicates the importance of trade to our geographic distribution. Hunter-gatherer cultures, more closely dependent on the immediate environment , are at highest population densities in the tropics, and at ever lower densities at higher latitudes. This gradation of density fits an explanation for the decline of cultural diversity with increasing latitude, namely, that the higher productivity of the tropics allows populations of large enough size to be viable to live in smaller geographic ranges and hence to allow closer packing of cultures. CLIMATE AFFECTS FORM AND FUNCTION Darwin did not consider that the conditions of life could produce the variation that he knew existed between peoples from different parts of the world (opening...