Adaptations to water stress and pastoralism in the Turkana of northwest Kenya.

The Turkana people inhabit arid regions of east Africa-where temperatures are high and water is scarce-and they practice subsistence pastoralism, such that their diet is primarily composed of animal products. Working with Turkana communities, we sequenced 367 genomes and identified 8 regions putatively involved in adaptation to water stress and pastoralism. One of these regions includes a putative enhancer for STC1-a kidney-expressed gene involved in the response to dehydration and the metabolism of purine-rich foods such as red meat. We show that STC1 is induced by antidiuretic hormone in humans, is associated with urea levels in the Turkana themselves, and is under strong selection in this population (s∼0.041). This work highlights that partnerships with subsistence-level groups can lead to new models of human physiology with biomedical relevance.

Reservation food sharing among the Ache of Paraguay.

We describe food transfer patterns among Ache Indians living on a permanent reservation. The social atmosphere at the reservation is characterized by a larger group size, a more predictable diet, and more privacy than the Ache typically experience in the forest while on temporary foraging treks. Although sharing patterns vary by resource type and package size, much of the food available at the reservation is given to members of just a few other families. We find significant positive correlations between amounts transferred among pairs of families, a measure of the "contingency" component required of reciprocal altruism models. These preferred sharing partners are usually close kin. We explore implications of these results in light of predictions from current sharing models.

How can we distinguish between mutational "hot spots" and "old sites" in human mtDNA samples?

New research into variation in mutation rates across nucleotide positions in human mitochondrial DNA (mtDNA) calls into question population genetics models that assume a constant mutation rate for all sites in a sequence, particularly for hypervariable control region segments I and II. Related to this research is discovering the extent to which highly polymorphic sites are really mutational "hot spots" rather than "old" sites rooted early in the phylogenetic tree. This issue is addressed through the analysis of linkage disequilibrium patterns in the mtDNAs of 10 human populations. Hot spots can be expected to show little or no disequilibrium since they can be interpreted as having randomly expressed patterns. In fact, the results suggest that many highly polymorphic sites are not old sites, but instead are hot spots. Suspected hot spots are listed and compared with hypervariable sites given by Wakeley (1993) and Hasegawa et al. (1993).

"It's a Wonderful Life". signaling generosity among the Ache of Paraguay.

Intensive food sharing among foragers and horticulturists is commonly explained as a means of reducing the risk of daily shortfalls, ensuring adequate daily consumption for all group members who actively pool resources. Consistently high food producers who give more than they receive, however, gain the least risk-reduction benefit from this daily pooling because they are the least likely to go without food on any given day. Why then do some high producers consistently share food, and why do some average producers share proportionally more food than others? We propose that although these individuals may not receive the same amounts they give (i.e., strict Tit-for-Tat), one explanation for their generosity is that they receive additional food during hard times. These include brief episodes of sickness, disease, injury, or accidents-fairly common events in traditional societies that can render individuals incapable of producing food, thereby having long-term effects on morbidity and fecundity and ultimately on lifetime reproductive success. Data collected among the Ache, a group of South American forager-horticulturists, indicate that those who shared and produced more than average (signaling cooperative intent and/or ability to produce) were rewarded with more food from more people when injured or sick than those who shared and produced below average. These results, framed within the context of tradeoffs between short-term and long-term fitness, may provide insight into motivations behind costly expenditures for establishing and reinforcing status and reputation.

Genetic traces of ancient demography.

Patterns of gene differences among humans contain information about the demographic history of our species. Haploid loci like mitochondrial DNA and the nonrecombining part of the Y chromosome show a pattern indicating expansion from a population of only several thousand during the late middle or early upper Pleistocene. Nuclear short tandem repeat loci also show evidence of this expansion. Both mitochondrial DNA and the Y chromosome coalesce within the last several hundred thousand years, and they cannot provide information about the population before their coalescence. Several nuclear loci are informative about our ancestral population size during nearly the whole Pleistocene. They indicate a small effective size, on the order of 10,000 breeding individuals, throughout this time period. This genetic evidence denies any version of the multiregional model of modern human origins. It implies instead that our ancestors were effectively a separate species for most of the Pleistocene.