Energetics, adaptation, and adaptability

The now and future of human biology at the extremes: An introduction to the special issue

OBJECTIVES

A hallmark of the human species is our adaptability to a wide range of different ecologies and ecosystems, including some of the most extreme settings. Human biologists have long studied how humans have successfully (and sometimes unsuccessfully) adapted to such extremes, particularly ecological extremes like environments at lower limits of temperature and high altitude. In this special issue, we revisit traditional definitions and explore new conceptions of work in extreme environments. We argue that our definitions of extremes should change with our changing world, and account for extremes unique to the Anthropocene, including environments of inequality and precarity, pandemic landscapes, climate-impacted settings, obesogenic environments, and the environments of human spaceflight. We also explore the future of work at the extremes and provide some suggested guidelines on how human biologists can continue to build and expand on foundational work in this area.

CONCLUSION

While human biologists have done critical work on groups living in extreme environments, our definitions of humans at the limits continue to change as the world around us also changes. Scholars in this area have a responsibility to re-examine the parameters of extremes to stay at the forefront of scientific exploration and collaboration so human biology, as a discipline, can continue to shape our understanding of adaptability, and thus contribute to the continued thriving of all humans as we endure new climatic, environmental, and societal extremes.

Morphological correspondence between brain and endocranial surfaces in mice exposed to undernutrition during development

The morphological changes of the brain and the skull are highly integrated as a result of shared developmental pathways and different types of interactions between them. Shared developmental trajectories between these two structures might be influenced by genetic and environmental factors. Although the effect of environmental factors on neural and craniofacial traits has been extensively studied, less is known about the specific impact of stressful conditions on the coordinated variation between these structures. Here, we test the effect of early nutrient restriction on morphological correspondence between the brain and the endocast. For this purpose, mice exposed to protein or calorie-protein restriction during gestation and lactation were compared with a control group in which dams were fed standard food ad libitum. High-resolution images were obtained after weaning to describe brain and endocranial morphology. By magnetic resonance imaging (MRI), brain volumes were obtained and endocasts were segmented from skull reconstructions derived from micro-computed tomography (microCT). Brain and endocranial volumes were compared to assess the correspondence in size. Shape changes were analyzed using a set of landmarks and semilandmarks on 3D surfaces. Results indicated that brain volume is relatively less affected by undernutrition during development than endocast volume. Shape covariation between the brain and the endocast was found to be quite singular for protein-restricted animals. Procrustes distances were larger between the brain and the endocast of the same specimens than between brains or endocasts of different animals, which means that the greatest similarity is by type of structure and suggests that the use of the endocast as a direct proxy of the brain at this intraspecific scale could have some limitations. In the same line, patterns of brain shape asymmetry were not directly estimated from endocranial surfaces. In sum, our findings indicate that morphological variation and association between the brain and the endocast is modulated by environmental factors and support the idea that head morphogenesis results from complex processes that are sensitive to the pervasive influence of nutrient intake.

Human eating behaviour in an evolutionary ecological context

Present-day human eating behaviour in industrialised society is characterised by the consumption of high-energy-density diets and often unstructured feeding patterns, largely uncoupled from seasonal cycles of food availability. Broadly similar patterns of feeding are found among advantaged groups in economically-emerging and developing nations. Such patterns of feeding are consistent with the evolutionary ecological understanding of feeding behaviour of hominids ancestral to humans, in that human feeding adaptations are likely to have arisen in the context of resource seasonality in which diet choice for energy-dense and palatable foods would have been selected by way of foraging strategies for the maximisation of energy intake. One hallmark trait of human feeding behaviour, complex control of food availability, emerged with Homo erectus (1.9 x 10(6)-200000 years ago), who carried out this process by either increased meat eating or by cooking, or both. Another key trait of human eating behaviour is the symbolic use of food, which emerged with modern Homo sapiens (100000 years ago to the present) between 25000 and 12000 years ago. From this and subsequent social and economic transformations, including the origins of agriculture, humans have come to use food in increasingly elaborate symbolic ways, such that human eating has become increasingly structured socially and culturally in many different ways.

Climatic influences on basal metabolic rates among circumpolar populations

This article examines evidence for elevations in basal metabolic rate (BMR) among indigenous Northern (circumpolar) populations and considers potential mechanisms and the adaptive basis for such elevations. Data on BMR among indigenous (n = 109 males; 122 females) and nonindigenous (n = 15 males; 22 females) circumpolar groups of North America and Siberia are compiled and compared to predicted BMRs based on three different references: body surface area (Consolazio et al., 1963), body mass (Schofield, 1985), and fat-free mass (Poehlman and Toth, 1995). Regardless of which reference is used, indigenous circumpolar groups show systematic and statistically significant elevations in BMR ranging from +7% to +19% above predicted values for indigenous men and from +3 to +17% for indigenous women. Nonindigenous males also show elevations in BMR, although not to the same extent as in indigenous men (deviations = +3 to +14%), whereas nonindigenous females show no clear evidence of elevated BMRs (deviations = -7 to +5%). This pattern of variation between indigenous and nonindigenous groups suggests that both functional and genetic factors play a role in metabolic adaptation to northern climes. Recent studies on the ecology and genetics of thyroid function offer insights into the mechanisms through which indigenous circumpolar populations may regulate metabolic rates. Studies of seasonal variation in thyroid hormone levels suggest that indigenous circumpolar populations may have a greater capacity to elevate BMR during severe cold than nonindigenous groups. Recent twin studies indicate a significant genetic component of thyroid responses to environmental stressors. Further research exploring the genetics of seasonal variation in thyroid function and BMR among circumpolar groups would advance understanding of the role that selection may have played in shaping metabolic variation.

Lifetime dietary change and its relation to increase in weight in Spanish women

INTRODUCTION

Changes in dietary patterns and a decrease in physical activity have occurred in Western countries. These are factors in the variation in body composition observed in populations, characterized by a progressive accumulation of fat with age and a consequent increase in the risk of suffering from common chronic illnesses such as obesity, cardiovascular disease and cancer.

OBJECTIVE

To investigate weight gain throughout the life-cycle and its relation to modifications in dietary patterns, analyzing the causes of these modifications and their implications for patterns of adult overweight and obesity.

DESIGN

Cross-sectional sample of Spanish women from a socio-economically disadvantaged class.

SUBJECTS

1037 healthy perimenopausal women (age: 45-65 y).

MEASUREMENTS

Juvenile body mass index (BMI), current BMI, food frequency questionnaire, retrospective food habits.

RESULTS AND CONCLUSIONS

Of these women, 48.8% had changed their dietary habits during their lifetime. A change in diet due to migration or marriage occurred at approximately 20 years of age and was characterized by an increased frequency of consumption of foods rich in protein and complex carbohydrates, while a change due to illness occurred at around 50 years of age and was characterized by a decrease in the consumption of these types of food. The change in dietary behavior due to migration was associated with weight gain. Weight gain was also inversely associated with BMI during youth; women who in their youth had a BMI<18.5 kg/m2 gained an average of 21.4 kg, compared with those with a BMI>27 kg/m2 in their youth, who gained an average of 5.4 kg. International Journal of Obesity (2000)24, 14-19

Basal metabolic adaptation of the Evenki reindeer herders of Central Siberia

Previous research has suggested that basal metabolic rates (BMRs of indigenous circumpolar populations are elevated, perhaps as an adaptation to chronic, severe cold stress. This study examines variation in BMR among indigenous (Evenki) and nonindigenous (Russian immigrant) populations living in Central Siberia to determine: 1) whether the Evenki show evidence of increased metabolic rates, and 2) whether the metabolic responses of the Evenki are different from those of the recent Russian migrants ("controls"). BMRs were measured among 58 Evenki (19 men, 39 women) and 24 Russian (8 men, 16 women) adults (18-56 years of age) from three Siberian villages. Measured BMRs were compared to those predicted based on body weight and body SA (Consolazio et al., 1963; Schofield, 1985a,b). BMRs per unit weight and FFM were similar in Evenki and Russian men, whereas Evenki women had higher BMRs than their Russian peers. Relative to the Schofield (body weight) norms, Evenki men and women and Russian men all showed modest elevations in BMR, whereas Russian women had lower than expected BMRs. Compared to the Consolazio (surface area) estimates, both Evenki men and women showed significant elevations in BMR. Russian men also showed higher than expected BMRs, while those of Russian women were slightly below predicted levels. Age-related declines in BMR were evident among the women of both ethnic groups, but not among the men. Additionally, residence location was an important predictor of metabolic variation in the Evenki, with those of the more traditional village showing greater elevations in BMR. These results suggest that the Evenki display elevated metabolic needs, and this long-term adaptation reflects the interaction of genetics and level of acculturation. Am. J. Hum. Biol. 12:75-87, 2000. Copyright 2000 Wiley-Liss, Inc.