Chapter 14 - The Human Growth Curve Different people grow to different sizes. Different growth rates occur during different developmental phases. Growth is regular over all cases in a population (Bell Curve). Different parts of the body grow at different rates. Brain and head grow fastest, followed by lymphatic system, general body, and reproductive system. Overall growth is a very regular process, and the idea that the genes target certain eventualities for the body, even in the event of growth pauses and stresses. The idea that canalization occurs to constrain certain growth processes until other processes can have a chance to "catch up" is interesting, and the theory of "sensitive periods" during which a growth parameter has an expected environment may be one of the few stresses which can derail the canalization of growth directionality. Chapter 15 - The Adolescent Growth Spurt and Developmental Age One of the most disruptive aspects of the human growth curve is the adolescent growth spurt, but as we have seen from the last chapter, even this conforms to a bell curve of expectations for growth between both boys and girls within the population. Sex difference results in differences in velocity curves between boys and girls, where boys peak adolescent growth velocity is between the years 14 and 15, and for girls it occurs between 12 and 13. The body is differentially represented, as the adolescent growth spurt involves the trunk over the legs skeletally, but the musculature and body organs also undergo a growth spurt here too. The development of the reproductive system is more variable for boys and girls than other growth factors, although the sequence of growth parameter changes is less variable. Time to maturity is also a variable measure during this growth spurt, but can be measured for morphological parameters where consistent form is realized for all normal individuals. This is not the case for the determination of shape, because any growth parameter being measured is dependent upon the final shape reached at maturity, and so cannot be estimated from transition forms. Chapter 22 - Nutritional Stress The chapter deals with the biological stresses on human populations and human adaptability as they relate to obtaining food energy and the structural components of the human body. Studies have shown that the many variables associated with geography, climate, population size, quality, and quantity of available energy resources, genetics, body size, physical activity, and disease, among others, have a significant impact on human health and nutrition. Genetically determined population differences in average adult body size have a significant effect on the energy needs of populations. There is a balance between energy intake and expenditure in most people that is governed by a complex homeostatic system - the average weight of people in most societies seems to be only slightly guided by conscious thoughts and actions. Energy expenditure and intake appear to be controlled more by an interaction between genetics and environmental controls. In addition to meeting energy needs, the diet of an animal species must provide the basic material its genetic information requires to accomplish physical growth. The natural diet provides the necessary nutrients and energy - deficiencies usually occur only as a result of undernutrition or altered environment. Many groups have compensated for inadequacies in some nutrients and minerals in their diets through cultural practices (knowingly and unknowingly). For example, South American groups soaked maize in lime water enhanced the calcium in the ingested food. Studies have shown that meat was never the major energy source at any stage in human evolution. This hypothesis is based on at least three factors: first meat does not contain all of the necessary nutrients to meet human physiological needs
second, hunting is a very inefficient source of food
third, contemporary H-G do not rely solely on meat sources. Adequately available natural resources provided some early agricultural societies with the basic requirements necessary to support towns and cities that lead to urbanization and civilization. These changes in population size, domestication crop usage and availibility, availability of nutrients and other resources resulted in different disease patterns and nutritional diseases. A strict maize diet, for example, can lead to niacin deficiency, pellagra, and death. The food source and diet has a significant impact on the adaptive capacity of a species - for example, the absence of lactase appears to be under genetic control (associated with milk use in human populations). It is not known if lactose intolerance is due to genetic adaptation to lactose in the diet, or some other growth adaptational response. A population is best adapted to its traditional nutrient environment and changes in that envrionment are likely to be stressful.