Humans have very long lifespans, a significant portion of which is spent in a non-reproductive state, in comparison to other primates. Although lifespan has increased over the past century, there is good evidence that the pattern of a long, post-reproductive lifespan is not novel, and that recent increases in longevity are just extreme manifestations of the general human pattern. This research project attempts to test several hypotheses for this extension of the human lifespan, focusing on support by mothers of their own offspring; support by grandmothers of grandchildren; the support by fathers of their offspring; and a model of support focusing on embodied capital. Several of these hypotheses emphasize the quantity and quality of help that grandparents provide for their descendants, which can be interpreted as a form of indirect reproduction. The researchers will examine behavioral pathways by which older individuals impact their descendants' well-being, the impacts grandparents have on the growth, health, and morbidity of grandchildren, and the effects such assistance may have on fitness outcomes such as child fertility and grandchild survivorship. The project focuses on the Tsimane, a group of forager-horticulturalists in Amazonian Bolivia. The researcher will work with anthropologists and physicians from Bolivia and the U.S., and study over 2,500 Tsimane living in 22 villages. Methods include demographic and life-historical interviews; analysis of extant data from missionary and public sources; biomedical assessments of health and morbidity; anthropometrical measures; time allocation and resource production observations and interviews; and food consumption and resource transfer observations and interviews. The project represents the first extensive tests of theories concerning longevity that examine the overall fitness effects among kin and the proximate means by which such effects are realized in a traditional small-scale population with a sufficiently large sample size. Such small-scale foraging-horticultural populations are the closest contemporary analogues to our human ancestors, so the new knowledge to be gained will help to advance understanding of our human history. The opportunity to do research with these sorts of populations is fast disappearing due to development and urbanization. The broader significance includes the value of the empirical data and ethnography of aging for a variety of health and welfare planners. The project will actively involve community members in data collection and train local people in primary health care, as well as provide some primary care assistance. In addition, the researchers will organize data on the health status of children and elderly, and the role of inter-generational support networks for the purpose of planning future local health initiatives.
Rationale A significant portion of the adult human lifespan is spent in a post-reproductive state. Although lifespan has increased over the past century, there is robust evidence among subsistence-level populations without access to modern healthcare that the pattern of a long post-reproductive lifespan is not novel, and that recent increases in longevity are just extensions of a general human pattern. Why human longevity? This project sought to address this question among Tsimane forager-horticulturalists of the Bolivian Amazon. The major objectives are to: (1) examine patterns of economic productivity, functional ability, morbidity, mortality and fertility by age and sex; (2) test competing hypotheses about the role of learning subsistence skills in shaping unique human life history traits; and (3) investigate the relationship between life history traits, food transfers within and across families, and biological aging. Methods Data were collected by an inter-disciplinary team of over 50 individuals between 2002-2010. We used a combination of systematic behavioral observations, interviews, field measurements and experiments. Most anthropological data were collected from focused work in 11 villages with a combined population of roughly 1500 individuals. Medical, anthropometric, and functional status data were collected by medical teams among 9,000+ individuals of all ages from 80+ villages. Demographic data were also collected on the full sample of adults. Findings Age-profile of food production and food transfers Tsimane do not produce more than they consume until the late-teens to early 20s. This caloric deficit accumulated in childhood is offset by a caloric surplus produced throughout much of adulthood. By age of peak efficiency men and women share over half of all production with children. Men and women maintain high levels of production into the early 60s, with older men producing more calories per day than adults in their 20s. Parents and grandparents account for a large percentage of calories consumed by children and teenagers. At advanced ages (65+) men and women no longer have sub-adult children to invest in and direct much more of their production to grandchildren than to children. Around this time production drops dramatically. Functional ability Performance of common tasks is significantly compromised at advanced ages. Over 60% of Tsimane age 60+ complain about hearing loss, over 80% have trouble seeing close distances, and over 70% can no longer chop large trees. Over 90% of men over age 60 can no longer carry a heavy weight a distance of 50 meters. Over 70% of men no longer hunt by age 70, often complaining about becoming too tired, having poor eyesight, hearing loss, and being too weak. Although Tsimane are more active than Western populations throughout life, older Tsimane take 91% longer than age-matched Japanese and 29% longer than Americans to stand up five consecutive times after sitting down; older Tsimane are significantly slower in walking a distance of three meters. Morbidity About one-third suffer from respiratory illness, one-fourth from gastrointestinal illness, and over three-fourths from intestinal parasites or pathogenic protozoa. These illnesses often occur simultaneously. About half of men and women have anemia, with children and adolescents showing highest risk. The cumulative toll of high levels of childhood infection is reflected in stunted growth. Tsimane serum shows high levels of inflammation. Levels of one indicator of inflammation, C-reactive protein (CRP), were found to be much higher among Tsimane at all ages compared to all other populations where it has been measured. Obesity, high cholesterol, and high blood pressure are all related to much of the cardiovascular disease among adults in Western countries. Our investigation of peripheral arterial disease (PAD), a pre-cursor to fully developed artherosclerosis, indicates that PAD is absent among our sample of Tsimane adults (n=258). In every population it has been studied PAD increases with age except among Tsimane. Low BMI and cholesterol among Tsimane may protect against CVD by reducing inflammation and improving metabolism. Demography: Mortality and fertility Before 1990, life expectancy at birth among Tsimane was 43 years. Despite recent improvement in life expectancy, Tsimane death rates at all ages are similar to those observed in Europe in the 1800s. Since 1990 we found a great reduction in death rates for adults but not for infants or children. We think that this is due to differences in access to medical interventions for adults and children, since adults have a greater ability to seek treatment. In general the Tsimane population is very young, where 51% are younger than age 15 (4% are age 60+). Tsimane total fertility rate (TFR) is 9, and has changed very little since the 1950s in all areas of Tsimane territory. Conclusions This research is beginning to result in an integrated understanding of the human life course, including economic productivity, resource transfers, functional ability, morbidity, and demography. There has never been a similar project of this scope with an indigenous population living a subsistence-based lifestyle.
