Sexual selection and Arctic environments

My 2006 paper is often criticized on one point. If Arctic environments did intensify sexual selection of women among ancestral modern humans, and if this selection did create inter alia the color traits of Europeans (diversification of eye and hair color, extreme depigmentation of skin color), then why are these traits absent among the native peoples of northern Asia and North America? Surely they too are products of Arctic environments.

Yes, they are. But it was not Arctic environments per se that intensified sexual selection of women. It was essentially two changes to the sexual division of labor that, among hunter-gatherers, generally correlate with distance from the equator. First, hunting distance increases with decreasing numbers of game animals per square kilometer, thereby increasing male mortality. Second, food gathering decreases with longer winters, thereby increasing women’s reliance on men for provisioning and increasing the costs of polygyny for men. It is this combination of higher male mortality and limited polygyny that intensifies sexual selection of women.

These points are made by Hoffecker (2002, pp. 7-8):

Hunter-gatherer diet is strongly influenced by latitude and temperature. To begin with, energy demands increase significantly in cold climates and caloric intake in arctic environments may be as much as 30 percent higher than it is in tropical regions. The percentage of meat and fish in the diet of recent hunter-gatherers increases as temperature, moisture, and primary productivity decline, and equals or exceeds 80 percent among most peoples who live in areas with an effective temperature of 10 degrees C or less. …

The high protein-fat diet and hunting and fishing subsistence of hunter-gatherers in northern environments has major implications for foraging strategy. Although cold maritime settings often provide rich concentrations of aquatic resources that require limited mobility, hunter-gatherers in northern continental environments who subsist on terrestrial mammals must forage across large areas in order to secure highly dispersed and mobile prey. Among peoples who rely primarily on nonaquatic foods, there is a correlation between temperature and the average distance of residential moves and a related correlation between the percentage of hunted food in the diet and territory size. Another consequence of low temperatures and a high meat diet is that males procure most or all food resources, generating a more pronounced sexual division of labor.

Hunting distance and male food provisioning of women seem to be at a maximum in a special kind of Arctic environment: ‘continental’ steppe-tundra, where almost all food is in the form of highly dispersed and mobile game animals. Today, this environment is confined to the northern fringes of continental Eurasia and North America. During the last ice age, it lay further south and covered more territory. This was especially so in Europe. The Scandinavian icecap had pushed the steppe-tundra zone far to the south and on to the broad plains stretching from southwestern France through northern Germany and into eastern Europe. This combination of treeless tundra and temperate latitudes created an environment quite unlike the northern barrens we know today. As Jochim (1983, p. 214) notes: “The low-latitude tundras and park-tundras of glacial Europe were richer than any modern northern counterparts.” Long intense sunlight favored a lush growth of mosses, lichens, and low shrubs that fed herds of large herbivores, mainly wild reindeer (a.k.a. caribou) but also mammoth, woolly rhinoceros, horse, bison, red deer, roe deer aurochs, ibex, chamois, saiga antelope, muskox, giant deer, wild ass, elk, and wild boar (Butzer, 1964, p. 138).

Though substantial, this kind of biomass is a volatile food source. Caribou herds in Alaska fluctuate considerably in any one area, in part because they cover long distances in the space of one year but also because they go through long-term demographic cycles of expansion and contraction (Burch, 1972). Among caribou-dependent Inuit, “at least 1 period of hunger or starvation is part of the normal annual cycle” (Burch, 1972, p. 350). In good times, caribou herds do provide a bountiful food source, but at the price of continual camp moves and extensive reconnoitering on foot. This is the real man-killer in Arctic groups that have not yet domesticated reindeer, as Krupnik (1985, p. 126) notes when explaining the Chukchi’s low ratio of men to women:

The herdsmen guarded the herd on foot. There were no herd dogs, and reindeer were not used for transport during the summer months, so that the men had to travel with the herds over the tundra with a minimum of portable possessions. All of this must have sharply intensified the physical burdens on adult, able-bodied men, and caused a higher mortality rate and consequently a proportional decrease of their numbers in the population.

Thus, on the steppe-tundra of the last ice age, the population of human hunters was probably as volatile as its resource base, all the more so if one also considers the climatic oscillations during this period. Moreover, because this population was dispersed over a wide area, its density was not necessarily high enough for long-term viability. Hoffecker (2002, pp. 8-10) writes:

The high mobility requirements of northern continental environments not only incur added time and energy costs, but also carry potential social and reproductive costs for dispersed populations in such environments. Populations must maintain a minimum threshold density in order to remain viable and avoid extinction, and it is estimated that the “minimum equilibrium size” for a mating network of modern hunter-gatherers is between 175 and 475 individuals. The degree of dispersal of these individuals across the landscape (typically grouped into bands containing roughly 25 individuals) cannot exceed their ability to sustain a social network through at least periodic contact and aggregation.

All of these factors hindered sustained human occupation. When temperatures fell during the last glacial maximum (19,000-18,000 BP), depopulation seems to have occurred throughout the steppe-tundra zone, but more so in some areas than in others. Least affected were the warmer and moister areas in Western Europe and the Carpathian basin, where continuous occupation is well attested throughout the glacial maximum (Hoffecker 2002, p. 194). Most affected were the colder and drier areas in northern Asia, where the steppe-tundra zone lay close to the Arctic Circle and far from the Atlantic. Goebel (1999) writes:

During the last glacial maximum (19 to 18 kya), Siberia was devoid of human populations, except perhaps in small refuges like the southern Yenisei or Transbaikal region. … The central Asian steppe also lacks archaeological sites spanning the last glacial maximum, suggesting that increased aridity, lower temperatures, and a lack of woody plants severely limited human settlement in this region as well.

Bioproductivity was clearly lower in northern Asia. This factor, on top of other factors affecting the entire steppe-tundra zone (volatile resource base, critically low density of human population), made any human presence vulnerable to extinction during periods of environmental stress. There likely were repeated cycles of colonization, extinction, and recolonization.

In conclusion, substantial and continuous human settlement seems to have been confined to the European end of the steppe-tundra zone. Only there did all conditions fall into place for sustained sexual selection of women.

References

Burch, Jr., E.S. (1972). The caribou/wild reindeer as a human resource. American Antiquity, 37, 339-368.

Butzer, K.W. (1964). Environment and Archaeology. Chicago: Aldine.

Frost, P. (2006). European hair and eye color - A case of frequency-dependent sexual selection? Evolution and Human Behavior, 27, 85-103

Goebel, T. (1999). Pleistocene human colonization of Siberia and peopling of the Americas: An ecological approach. Evolutionary Anthropology, 8, 208‑227.

Hoffecker, J.F. (2002). Desolate Landscapes. Ice-Age Settlement in Eastern Europe. New Brunswick: Rutgers University Press.

Jochim, M.A. (1983). Palaeolithic cave art in ecological perspective. In Hunter-Gatherer Economy in Prehistory. A European Perspective. G. Bailey (Ed.). Cambridge: Cambridge University Press.

Krupnik, I.I. (1985). The male‑female ratio in certain traditional populations of the Siberian Arctic. Inuit Studies, 9, 115‑140.