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Aging and senescence are two concepts of biological evolution that usually occur simultaneously. Aging is an inevitable process among all humans and animal species. The survival method and length may be different, but the body’s equipment will begin to deteriorate with time. Time is a process that cannot be reversed biologically or socially; therefore the body adapts overtime to such processes. In this, how the body is treated over time will result in rapid or reduced aging. The body will still age, but the rate of deterioration will be decreased. All of these mechanisms play into all bodily functions, the only separation is depending on the person, their diet, their physical activity and genetic makeup.

One in every 100,000 persons survive as long as 120 years old. Most men and women will not live to see this age. The average age for men before developing signs of senescence in 70 years old. At 70 years of age, Alzheimer’s and other degenerative conditions began to take its peak conditions. As mentioned earlier, the life conditions are based on the individual and his or her lifestyle. In an article entitled, “Late Life Human Development: Boosting of Buffering Universal Biological Aging” Kolling and Knopf introduces biological aging and senescence by describing telomere length (TL), theories of aging (evolutionary, stochastic, and deterministic), and free radical involvement.

In evolutionary aging theories, Weismann elaborates upon the idea of natural selection. This theory promotes Darwin’s ideas, but also the idea of group selection. Group selection states that aging benefits the group, even though it may be detrimental to the individual. He later proved this theory to be discrediting and presented the idea of disposable soma theory instead. This theory states that the organism separates germ and soma in order to maintain reproductive power due to evolutionary pressure. We all know this to be true, seeing as if the soma cells are all other cells other than the reproductive cells, and that reproduction only takes place during a certain lifespan, and it is not presented as an opportunity after a certain age in certain groups of men and women. Moving along, stochastic aging theory relies on the core idea that while individuals age, a significant amount of biological damage is accumulated over time in a random fashion as a by-product of normal living. My favorite of the three stochastic theories is the “wear and tear theory”. It proposes that the usage of the body and over time, cumulative damage occurs within the body leading to death of cells, tissues, organs, and finally the organism itself. Next, the “free radical theory” expounds upon how free radicals are produced during aerobic respiration later causing rapid death. These radicals place a threat to the aging organisms because of oxidative damage.

Finally deterministic aging theories presume that aging occurs because of genetically and endogenously programmed processes. One example is the absolute metabolic scope theory. This is a rather old theory which states, that the greater an organism’s oxygen basal, the shorter the lifespan. In all of these theories, they relate to telomere length of an individual’s chromosomes. In a study, longer telomeres at baseline were associated with reduce risk of death and dementia; however, research on the associations between telomere length as a predictive biomarker of mortality is still controversial.

Kolling, T., Monika Knopf (2014). "Later Life Human Development: Boosting or Buffering Universal Biological Aging."' GeroPsych Review. vol 27. (3), 103-108.

 

This weeks' reading revolved around stress on the human body. These blueprints to hormone level production, neuron response, and the neurological development was displayed in a case study by Aaron Kindsvatter and Anne Geroski. In this article entitled, “The Impact of Early Life Stress on the Neurodevelopment of the Stress Response System, they present a study with both an animal model and humans. This article is used to describe the functioning of the stress response system of the brain and to review literature pertaining to the impact of early life stress on the development of the stress response mechanisms.

The primary mechanisms of the stress response system include the sympathetic-adrenal-medullary axis (SAM), the limbic system and the hypothalamic-pituitary adrenal axis (HPA). The SAM axis involves direct connection between the norepinephrine center of the brain stem and the adrenal glands and is associated with the activation of the sympathetic branch of the autonomic nervous system. Most would also refer to the system in regards to the para-sympathetic nervous system in reference to a “flight or fight” response. Under this division of the autonomic nervous system, this response also helps to regulate homeostasis and allostasis. The HPA plays a key function in stress response by initiating a series of neurotransmitters and Nero hormonal cascades that, among other functions, serve to regulate the body’s response to stress.

These researchers used both rats and cats as control groups to help correlate stress among early development to adult hood. Among the cats, they inhibited some of the cats from using their right eye upon birth. In this they found that these now grown cats developed conditioned eye sight even after eye sight with both eyes were permitted. This carried on neuro-response was also relate able to maternal instincts among mothers of human under post and prenatal stress. Among this study, the same results among the cats and rats were valid. These lasting effects per-conditioned the brain to produce more stress relieving hormones. More ACTH and CRF resulted in the diagnosis of PSTD. This study was also compared to cortisol levels of adult survivors of child abuse. Within all of the new found diagnosis, the idea of threat experienced at an early age resulted in lifetime effects of such bodily productions to counteract future threats.

In all, the body’s response to previous witnessed ideas of threat will promote certain synaptic response with different levels of hormone production; hence, why some people experience strokes more rapidly. Stress can cause major body dysfunctions. It is even one of the leading cause of death in the U.S. Stress will condition the brain to adapt to certain experiences at certain times, without this modification through early stress level identification, the explanation of stressful adulthood might be still in question.

Geriski and Aaron Kindsvatter (2011). "The Impact of Early Life Stress on the Nuerodevelopment of the Stress Response System". American Counseling Association.

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William R. Leonard is a leading anthropologist in the field of human nutrition. He was born in Jamestown, NY and received his PhD in biological anthropology from the University of Michigan at Ann Arbor in 1987. He is now an Abraham Harris Professor in the Department of Anthropology and the Chair of Anthropology at Northwestern University. He is also the Director of the Global Health Studies Program.

Siberia-2
Dr. William R. Leonard (left) with former student Josh Snodgrass, Univeristy of Oregon, conducting fieldwork in Siberia. (Photo provided by William Leonard)

Much of his research focuses on nutrition, energetics, and child growth in both modern and prehistoric human populations. He has traveled and studied in regions of South America, including Bolivia, Ecuador, and Peru, and also Siberia. In these regions, Leonard conducts research on population adaptation to their specific nutritional environment and how these adaptations affect their health, as well as contribute to chronic disease risks. Additionally, Leonard compiles information about human and primate ecology in order to examine the evolution of nutritional requirements in our hominid ancestors. This research leads to insight regarding the origins of obesity and metabolic diseases in contemporary human populations.

One recently published paper by Leonard, titled “The global diversity of eating patterns: Human nutritional health in comparative perspective” highlights Leonard’s work surrounding human nutrition, dietary trends, and the raising rates of obesity in the US. In the paper, he focuses on the different types of subsistence in the US versus less modern, more traditional societies. He notes that the energy intake between industrialized and non-industrialized societies is not different, but that the composition of nutrition includes higher levels of fats and carbohydrates in industrialized cultures. He also compares humans’ nutritional needs to primates, noting that the increase in brain size in higher-level primates such as humans has led to humans requiring higher quality foods than some of our close evolutionary relatives. As rates of obesity and chronic metabolic diseases continue to rise in the US and other industrialized societies, research such as Leonard’s studying the causes and origins of such nutritional deficiencies is of growing importance.

References:

Leonard, William R.

2014 The global diversity of eating patterns: Human nutritional health in comparative perspective. Physiology & Behavior 134:5-14.

Background information based on biosketch provided by Dr. William R. Leonard.

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Sara Stinson is a professor at CUNY Queens College.  She received her Ph.D. from University of Michigan in 1978.  Her focus is on physical and developmental variations among living human populations.  She studies environmental influences on growth and the evolution of body size.  Her research has been mostly on South American populations, including variation in indigenous South American populations, effects of high altitude hypoxia, and growth of lowland tropical forest populations.

Stinson

 

Growth Variation: Biological and Cultural Factors

Variation in size is one of the most obvious ways that humans differ. These differences are also easy to measure and compare across populations, making this a well-studied topic in biology/anthropology. Many factors can contribute to growth variation across and among populations such as genetic factors, nutrition, environmental conditions, social conditions, and cultural conditions.

 

Genetic Factors:

Genotype can have a strong effect on growth as shown in the next two examples. Our ancestry and previous natural selection pressures on our ancestors affect growth patterns.

The range of the average height for the tallest and shortest populations is less than one foot. The shortest population, the Efe pygmies of central Africa, is less than 5 ft. tall on average.  While poor nutrition plays a role, it has been shown that these populations have a reduced number of sites for growth hormone to attach on their cells which would lower the effect the hormone has on them. Smaller body size could have been selected for since it would be advantageous to be small in this hot, humid environment, but no genes that could be attributed to natural selection have been identified.

Another genetic example of growth variation is the difference in males and females. In the US male and female average height is approximately 6 inches different.  The height ranges can be represented by two overlapping bell curves. While males tend to be taller, females tend to carry more subcutaneous fat than males, and females carry their weight in different areas than men.  These sexually dimorphic tendencies exist in every human population, and are contributed to by hormonal differences during adolescence and puberty.

 

Environmental Factors:

Most size variation in the world’s populations is attributed to environmental factors.

Socioeconomic Status is one of the major players that can contribute to size variation. Socioeconomic status can limit access to quality health care and nutritious food, while having an increased exposure to disease. Scientists can look at income, education, occupation, and household possessions to determine this status. In almost EVERY study done so far children from high-income families are taller than children from low-income families. This difference is more noticeable in developing countries than in industrial countries. Economic inequality exasperates this phenomenon. Some argue that short stature due to economic status could be attributed to higher levels of stress in low-income families. Low-income is also associated with higher rates of obesity in developing and industrial societies where high-caloric, low-nutrient foods are cheapest.

Ideology and Beliefs have also been shown to influence growth. Families in Niger would rather keep their possessions important for determining rank, instead of selling them for money, because in this culture status is very important. In the US parents may opt out of forcing their children to eat healthy foods in order to maintain a peaceful dinner environment because here an emphasis is placed on having positive family dinners. In Tanzania children from monogamous marriages were taller and thinner than children from polygamous marriages.

Environmental Pollutant (toxicant) exposure has been associated with reduced height. Prenatal exposure to cigarette smoke and/or alcohol shows babies that are born smaller. Children with high blood lead levels have slower prenatal and postnatal growth and delayed puberty. Polychlorinated biphenyls (PCBs) can increase maturation rates in girls. Girls with higher levels of PCBs have earlier menarche, but there is little affect on overall growth patterns.

Recent Trends in growth patterns show that height and weight have increased in populations around the world. Over the last 100 years in Sweden, female height has increased by almost 5 inches and male height by almost 8 inches. This next part is a little unintuitive, but trends show that we are maturing earlier as well as faster. The age of menarche has decreased from 15-16 in the 19th century to 12-13 years old in most industrialized societies. Increases in leg length are higher than the increase in trunk length. European populations are now on average taller than Americans. While the height of populations in Oaxaca, Mexico has remained the same for 500-3000 years, during the last 30 they got a health center and secular trends have showed up in children’s growth patterns. Recent Mayan immigrants to the US have children that are approximately 4 inches taller. 70% of this height increase is attributed to longer legs.

Why are these trends happening though? The change in the last 150 years has happened too fast for it to be attributed to genetics. The reduction of infectious diseases could be a factor-improvement of world populations overall. Greater gene flow between populations could also be a factor. The increase in body size appears to be slowing and plateauing in affluent countries. A downside to these trends is that an earlier age of menarche has been deemed risk factor for breast cancer, heart disease, and type 2 diabetes. Obesity is becoming an ever-increasing problem around the world. About 67% of US adults are overweight or obese.

 

Nutrition and Disease throughout the Life Cycle

Parental Growth usually assessed by birth weight can give indications on how an individual will grow. Babies less that 5.5 lbs are considered to have a “low birth weight” which can result from prematurity, slow growth during gestation, genetics, or multiple births. During WWII women in Russia and the Netherlands were under siege from the Germans reducing the amount of food entering the cities. Birth weight decreased in these populations, with 50% of babies in St. Petersburg being born with low birth weight. An increase in maternal nutrition during pregnancy shows an increase in birth weight. However, if a woman has been malnourished her entire life, increasing nutrition during pregnancy is not enough to increase birth weight. Babies can adjust their metabolism to what they (and their mother) are experiencing during pregnancy. So if a woman is normally well nourished, but during pregnancy is malnourished, this cannot only make the baby be smaller, but also change the child’s overall metabolism. Maternal cues to the fetus including placental transfer of nutrients or hormones would be an epigenetic effect. Studies have shown that a women’s birth weight is a strong predictor of her children’s birth weight.

Infancy and Childhood Growth is more important, however, in terms of its affect on body size and health. Disruptions in the growth of children are the main cause of small adult body size.  Human milk is relatively low in fat and protein content indicating frequent nursing and slow rates of postnatal growth. Breast-feeding infants is very beneficial for developing infants. Other feeding methods do not provide anti-infective agents that help in the development of the baby’s immune system. These agents include antibodies against specific bacteria and viruses; white blood cells; and proteins, fats, and carbohydrates that convey antimicrobial action. Bottle-fed infants have higher rates of illness and mortality than breast-fed infants. It can be “argued that breast milk is the nutritionally ideal food for infants because it is the food that has evolved to meet their needs. “ Breast-feeding is associated with lower risk of ear, respiratory and gastrointestinal infections; sudden infant death syndrome; asthma; and reduced risk of obesity and diabetes in the children.

During the first few months of development, infant growth rates are consistent in poor and developed countries. However after the age of 4-6 months the growth rate in infants in disadvantaged circumstances decreases. By two years, there is a substantial different in height.  The slowing of growth is referred to as growth faltering. By 5 years old, children in poor countries fall below the 25th percentile for height. In rural Mali cultures young children are not encouraged to eat, and must fend for themselves. But in rural highland Peru children are given preferential treatment, and in times of food shortage are given food over adults. Growth faltering is not only nutritional but also caused by diseases.  Children and infants exposed to pathogens may experience more growth faltering.

What did you think of the Mali cultures?

Adolescent Growth is less affected by environmental factors due to the fact that this period is characterized by slower growth rates. In some environments rapid maturation is necessary due to high mortality rates.  Rapid maturation results in smaller adult body sized, but the individuals can reproduce at an earlier age. In these cultures there is a trade off between rapid maturation and small adult stature.

Catch-up Growth is a period of faster growth that follows a disruption. Turkana populations are initially below the 50th percentile and sometimes lower than the 5th percentile in height, but when they reach maturation they are on the normal standard. This can be explained by a longer growth period in these populations.

Is small body size an adaptation? It has been argued that when a human is exposed to low-nutrition environments they may cut their losses and remain smaller so that the resources that they have can go father. This would allow for more fit individuals that are smaller than their maximum potential.

Human height variation is a complex outcome of gene and environmental interactions. Narrowing down the cause of height to one factor is impossible, and when looking at height distributions you must take into account genetic history, environment, nutrition during development, social status, and cultural influences.