The demand for products that reduce infant stress is high, supported by the desire to protect vulnerable infants from environmental and culturally induced stressors. New research demonstrates scientifically sound reasons for seeking these protections. The findings suggest that protecting infants from environmental stresses yields long-term benefits because individuals who experience unhealthy infancies experience less healthy adulthoods.
This research has its roots in medical perceptions of health, particularly cardiovascular disease and diabetes. For instance, for a long time, people saw higher rates of heart disease, diabetes, and hypertension in African American communities as a racial statistic. However, as most introductory biological anthropology students quickly learn, race is not a valid category for classifying human populations.
Human biological variation is not racially patterned. Instead, it is the developmental environment that holds important answers to cardiovascular disease risk later in life. Specifically, research findings that hint at the “plasticity” of infants—their ability to adapt to changes in their environments—and consequences of early life stress, predict results in future environments.
For example, among humans, poor nutrition during the early stages of life is associated with greater deposits of fat and insulin-resistant tissue. The challenging nutritional environment faced by some African American children as a result of socioeconomic inequality makes for a more likely source of elevated risk of heart attack, stroke, and diabetes during adulthood. Moreover, new genetic studies demonstrate that these stress experiences may be inherited from earlier generations. That is, the consequences of stress exposure early in one person’s life may be transmitted to that person’s children through epigenetic inheritance, the phenomenon in which traits are passed down outside an organism’s DNA.
This result may occur as genes are degraded through a process called methylation. Many of these genes are important in controlling immune system development, fat deposits, and insulin-resistant tissue deposits. The questions facing biological anthropologists focus on the evolutionary relevance of this response, and whether the process represents plasticity and adaptation.
Plasticity references the range of phenotypes (observable characteristics of an organism) that may be expressed by a single individual genotype (the genetic make up of an organism) in response to environmental pressures. In infants, the ability to survive these pressures is an indication of plasticity, evidenced by the trade-offs experienced during this process. For example, stress in a growing organism results in reduced energy directed toward growth, so that the narrowed energy budget can be directed to growing essential tissue (think heart, lungs, endocrine system). The results produce individuals with shorter stature, but the individuals survive stressful events. Many scientists use the term “adaptive plasticity” to describe this process.
Here, the term “adaptive” refers to the fact that the plasticity in question permits immediate survival and the possibility of future reproduction. That is, organisms that have the capacity to survive stress events may experience
reproductive success later, while those experiencing limited plasticity may simply succumb to stressful events.
Dental Evidence
Teeth provide the key to applying the concept of plasticity to ancient populations; they are great sources for understanding the consequences of stress during infancy.
Human front teeth form between 1 and 6 years of age. These teeth are very sensitive to environmental disturbances, and enamel secretion is frequently disrupted in response to them. These disruptions, known as linear enamel hypoplasia, appear as lines where enamel is depressed. Teeth also have incremental markings called striae of Retzius (Image 1). Striae of Retzius form over approximately eight-day intervals, which map the developmental chronologies of teeth: scientists can count striae of Retzius to pinpoint the precise age when a section of enamel formed. The location of linear enamel hypoplasia on a tooth crown allows scientists to visualize growth disturbances on teeth and estimate the age when these disturbances occurred.
At Mason, one laboratory has been investigating these questions among the Jomon culture in prehistoric Japan (Image 2). This culture, which depended upon a hunting and gathering subsistence economy, existed between 10,000 and 2,300 years ago. Today, we can study the teeth of members of the Jomon that have been preserved in archaeological sites. High-resolution casts of them viewed under a microscope reveal that the former owners of the teeth studied suffered disturbances in their growth patterns. One important question about these growth disturbances involves their relationship to adaptive plasticity and the trade-offs in the investment of biological energy that this plasticity entails. For example, scientists know that infants with linear enamel hypoplasia survived the growth event, as enamel continued to grow after the defect was produced. Less is known regarding the relationship between these defects and life history.
To explore this relationship, scientists use silicone to collect high-resolution impressions of teeth. They then coat the impressions with resin in order to create a full replica of the tooth, which now can be studied under an engineer’s measuring microscope (Image 3). Scientists are seeking the precise spacing between the perikymata (the external evidence of the striae of Retzius) and the depth of the tooth enamel. The measurements provide a surface profile and a spacing profile of each tooth (Image 4). Grey bars indicate tooth depth, while black bars indicate perikymata spacing. Linear enamel hypoplasia is found where perikymata spacing is accentuated (Image 5).
Because perikymata provides an accurate chronology of each tooth, it remains possible to calculate the ages when each linear enamel hypoplasia formed. Once the chronology of linear enamel hypoplasia is established within each individual, researchers in our bioarcheology lab integrate information from the skeleton with the findings from the tooth samples.
Skeletons are vast storehouses of information about the life history of individuals. Information such as age at death, number of growth disturbances experienced during development, body size, chronic infectious disease, and chronic metabolic disease can be elucidated. We used Mason’s bioarchaeology lab to compare the age when linear enamel hypoplasia was first observed with the total number of linear enamel hypoplasia and age at death of each individual. Our results found that individuals who experience linear enamel hypoplasia at comparatively younger ages were at greater risk for developing more linear enamel hypoplasia and dying early. This research suggests that life history trade-offs are possible to measure in prehistoric humans. The individuals who experienced growth disruptions at earlier ages invested less energy in preventing these disruptions when they were older, and these individuals died when they were relatively young.
The totality of this research suggests the importance of avoiding infant stress exposure. Not because infants are frail—infants have a great deal of “built-in” plasticity to survive stressful events. But the expense associated withsurviving these events has dire consequences at older ages.
All these findings are environmentally and culturally contingent. Anthropologists studying these questions have found that individuals with evidence of linear enamel hypoplasia from lower socioeconomic status have higher risks of death than those from upper classes. Therefore, the ability to elude death following stress during infancy is also patterned along financial boundaries—those with limited access to resources may have experienced more marked trade-offs simply as a consequence of how frequently they encountered deprived environments.
These findings suggest that humans pay an important physiological consequence for stressors experienced early in development and consequences may be more severe when socioeconomic inequalities place the individual at greater risk for stress exposure.
ABOUT THE AUTHOR: Daniel Temple, a faculty member in the Department of Sociology and Anthropology, earned a PhD in biological anthropology with a minor in anatomy. He studies the effects of stress that humans encounter at early ages, with significant results for the subjects' later lives, and has worked with skeletal and dental remains from Japan, Siberia, Alaska, Florida, New Mexico and Arizona. Here he describes the phenomenon and how it is used to learn about ancient cultures.
August 19, 2015
