Life history, disease, and trauma
Critical to understanding disease and trauma on bone is what is generally referred to as “host resistance” (Goodman et al. 1984). Because of both biological and cultural differences in the availability of and access to resources and reserves, not all individuals are equally at risk. A host who is in good health can often meet the challenge of even a severe disease stressor. On the other hand, an individual who is not in good health may find it difficult to resist even a relatively minor one. For example, an infectious disease resulting in gastroenteritis will have a much greater impact on a poorly nourished individual than on one who is well nourished. Selye (1976) has demonstrated that an individual who is continually stressed may eventually exhaust the physiological capacity to resist any stressor.
Certain segments of the population may be at greater risk at certain ages because their biological requirements are not matched by biological resources. Newborns, for example, are born with very immature immune systems. They must rely on immunity conferred during their time in utero and transferred via breast milk from the mother. Because of their state of biological immaturity, infants are frequently unable to rally from stressors that have only mild effects on a more mature individual. Mortality is particularly high during the first year in many marginal communities.
Once weaning begins, a second peak in both morbidity and mortality is frequently seen. Infants and young children become dependent on their own natural defenses at a time when these defenses are just beginning to develop. If nutrition is inadequate, as it frequently is at this age in marginal communities, those defenses will be further hindered. Thus, it is not unusual to see weaning age infants and children undergoing repeated bouts of chronic diarrhea, upper respiratory disease, and malnutrition.
Sometimes infants and children rebound from illness and make it through these high-risk periods. Despite recovery, the repeated insults may have a lasting adaptive cost in terms of such functional abilities as growth, reproduction, activity patterns, cognition, behavior, and social performance. In documented cases from living populations, infants and children do continue to succumb to repeated exposures to stressors, and these negative effects can last for generations (Clarkin and Levy 2004).
In the discussion of host resistance, some clarification of the unit of analysis is necessary. In any study of stress, the first level of analysis is the individual. The study of disease in archaeological populations begins with the evaluation of an individual skeleton. However, it is critically important to move to a population level to understand the full impact of diseases on host resistance and fitness at the community level.
The inability of an individual to resist disease will result in physiological disruptions. The severity of the disruption depends on many factors. Age, sex, health status, genetic composition, and nutritional constitution are especially critical factors. For example, a nutritional deficiency that occurs during a critical phase of growth may affect several biological systems. Decreased activity, increased use of fat stores, and decreased skeletal growth are a few of the possible responses (see chapters in Lewis 2009; Thompson et al. 2014). A similar deficiency that occurs after growth ceases may have little lasting effect on the biological system.
Target organs must be considered in studying the impact of stressors. For example, the adult human skeletal system is relatively immune to mild and short-term nutritional stress. However, the skeletal system is in constant communication and cooperation with other systems. The primary functions of the skeleton are support and locomotion, storage and regulation of minerals (especially calcium and phosphorus), protection of the brain, spinal cord, and other organs, and production of red blood cells. The diverse set of functions in one system indicates the degree to which the entire body is actually dependent on the skeleton. Thus, a careful reading of a variety of subtle morphological changes can be very revealing of physiological disruptions.
Although the record is far from complete, many stressors leave markers on bones and teeth. These markers can be used to reconstruct the history of morbidity (disease) and mortality (death) experienced during infancy and childhood. From the record of type, severity, frequency, and distribution of ill health, we can begin to draw inferences about its functional and adaptive effects on the individual and on society.
The adult skeleton may not show effects of mild stressors, but the growing bones and teeth of children are often altered in measurable ways. Specifically, chronic or episodic physiological stress can disrupt growth, and the disruptions often leave permanent markers on bone and teeth that persist into adulthood. Retrospective indicators of previous physiological insults are among the most useful indicators of diet and disease for prehistoric skeletal remains.