Post reproductive years are physiologically defined for women, but “old age” is a very ambiguous concept. In the United States, we tend to associate old age with physical ailments and decreased activity. Thus a person who’s vigorous and active at age 70 might not be regarded as “old,” whereas another who is frail and debilitated at age 55 may be considered old.
One reason we’re concerned with this definition is that old age is generally regarded negatively and is typically unwelcome in the United States, a culture noted for its emphasis on youth. This attitude is quite different from that of many other societies, where old age brings with it wealth, higher status, and new freedoms, particularly for women. This is because high status is often correlated with knowledge, experience, and wisdom, which are themselves associated with greater age in most societies. Such has been the case throughout most of history. But today, in technologically developed countries, information is changing so rapidly that older people may no longer control the most relevant knowledge.
By and large, people are living longer today than they did in the past because they aren’t dying from infectious disease. Currently, the top five killers in the United States, for example, are heart disease, cancer, stroke, accidents, and chronic obstructive lung disease. Together these account for more than 65 percent of deaths (Heron, 2010). All these conditions are considered “diseases of civilization” in that most can be accounted for by conditions in the modern environment that weren’t present in the past. Examples include cigarette smoke, air and water pollution, alcohol, automobiles, high fat diets, and environmental carcinogens. It should be noted, however, that the high incidence of these diseases is also a result of people living to older ages because of factors such as improved hygiene, regular medical care, and new medical technologies. Compared with most other animals, humans have a long life span (Table 16.2). The maximum life span potential, estimated to be about 120 years, probably hasn’t changed in the last several thousand years. But life expectancy at birth (the average length of life) has increased significantly in the last 100 years, owing to advances in standard of living, hygiene, and medical care. The most important advance is probably the treatment and prevention of infectious diseases, which typically take their toll on the young (Crews and Harper, 1998).
To some extent, aging is something we do throughout our lives (Finch, 2007). But we usually think of aging as senescence, the process of physiological decline in all systems of the body that occurs toward the end of the life course. Actually, throughout adulthood, there’s a gradual decline in our cells’ ability to synthesize proteins, in immune system function, in muscle mass (with a corresponding increase in fat mass) and strength, and in bone mineral density (Lamberts et al., 1997). This decline is associated with an increased risk for the chronic degenerative diseases, which are usually listed as the causes of death in industrialized nations.
As you know, most causes of death that have their effects after the reproductive years won’t be subjected to the forces of natural selection. What’s more, in evolutionary terms, reproductive success isn’t measured by how long we live. Instead, as we have emphasized throughout this textbook, it’s measured by how many offspring we produce. So organisms need to survive only long enough to produce offspring and rear them to maturity. Most wild animals die young of infection, starvation, predation, injury, and cold. Obviously there are exceptions to this statement, especially among larger-bodied animals. Elephants, for example, may live over 50 years, and we know of several chimpanzees at Gombe that have survived into their 40s or even 50s .
One explanation for why we age and are affected by chronic degenerative diseases like atherosclerosis, cancers, and hypertension is that the genes which enhance reproductive success in earlier years (and thus were favored by natural selection) may have detrimental effects in later years. These are referred to as pleiotropic genes, meaning that they have multiple effects at different times in the life span or under different conditions (Williams, 1957). For example, genes that enhance the function of the immune system in the early years may also damage tissue, so that cancer susceptibility increases in later life (Nesse and Williams, 1994). An example of this may be a gene responsible for lipid transport known as apolipoprotein E (apoE). One variant of this gene enhances immune function early in life but appears to be associated with an increased risk of Alzheimer’s and cardiovascular disease. In populations in which infectious agents are common, this variant is advantageous, but where infectious diseases are rare and people live longer, the variant that protects against Alzheimer’s and cardiovascular disease would be more beneficial (Finch and Sapolsky, 1999).
In another view of aging and pleiotropy, anticancer mechanisms operating in early life may have opposite effects in later life (Hornsby, 2010). What’s more, epigenetic mechanisms affect not only aging itself but also the diseases associated with aging. Current research on these mechanisms points to possible epigenetic-based therapies and prevention strategies for dealing with the negative consequences of the aging process (Gravina and Vijg, 2010). Pleiotropy may help us to understand evolutionary reasons for aging, but what are the causes of senescence in the individual? Much attention has been focused recently on free radicals, highly reactive molecules that can damage cells. These by-products of normal metabolism can be protected against by antioxi dants such as vitamins A, C, and E and by a number of enzymes (Kirkwood, 1997). Ultimately, damage to DNA can occur, which in turn contributes to the aging of cells, the immune system, and other functional systems of the body. Additionally, there is evidence that programmed cell death is also a part of the normal processes of development that can obviously contribute to senescence.
The mitochondrial theory of aging proposes that the free radicals produced by the normal action of the cell’s mitochondria as by-products of daily living (for example, eating, breathing, walking) contribute to declining efficiency of energy production and accumulating mutations in mitochondrial DNA (mtDNA). When the mitochondria of an organ fail, there’s a greater chance that the organ itself will fail. In this view, as mitochondria lose their ability to function, the body ages as well (Loeb et al., 2005; Kujoth et al., 2007). Two of the most promising strategies for enhancing health in later life are calorie reduction (Anderson and Weindruch, 2012) and aerobic exercise, both of which appear to improve mitochondrial function (Fig. 16-17) (Lanza and Nair, 2010).
Another hypothesis for senescence is known as the “telomere hypothesis.” In this view, the DNA sequence at the end of a chromosome, known as the telomere, is shortened each time a cell divides (Fig. 16-18). Cells that have divided many times throughout the life course have short telomeres, eventually reaching the point at which they can no longer divide and are unable to maintain healthy tissues and organs. Changes in telomere length have also been implicated in cancers and other diseases associated with aging (Oeseburg et al., 2010). In the laboratory, the enzyme telomerase can lengthen telomeres, allowing the cell to continue to divide. For this reason, the gene for telomerase has been called the “immortalizing gene.” But this may not be a good thing, since the only cells that can divide indefinitely are cancer cells. Although this research isn’t likely to lead to a lengthening of the life span, it may contribute to a better understanding of cellular functions and cancer.
Far more important than genes in the aging process, however, are lifestyle factors, such as smoking, physical activity, diet, and medical care. Life expectancy at birth varies considerably from country to country and among socioeconomic classes within a country. Throughout the world, women have higher life expectancies than men. A Japanese girl born in 2009, for example, can expect to live to age 86, a boy to age 80. Girls and boys born in that same year in the United States have life expectancies of 81 and 76, respectively. Unfortunately, gains in life expectancy in the United States appear to be slowing relative to other industrialized nations, primarily because of smoking, obesity, and sedentary habits (Seppa, 2011). In 2006, the latest year for which data were available, the United States ranked 36th in life expectancy among nations of the world (United Nations Population Division, 2007).
In contrast to these children in industrialized nations, girls and boys in Mali have life expectancies of only 51 and 48, respectively (data from World Health Organization, WHO Global Health Indicators, 2011). Many African nations have seen life expectancy drop below 40 owing to deaths from AIDS. For example, before the AIDS epidemic, Botswanans had a life expectancy of almost 65 years; at the height of the AIDS epidemic, life expectancy in Botswana was slightly more than 40 (Fig. 16-19). In 1990, Zimbabweans could expect to live to 61; but by 2000, that figure had dropped to 45.
One consequence of improved health and longer life expectancy in conjunction with declining birth rates is an aging population, leading in some parts of the world to a shift toward older median ages and greater numbers of people older than 65 than younger than 20. In demographic terms, these two groups represent dependent categories, and there’s increasing concern about the decline in the number of working-age adults available to support the younger and older segments of a population. In other words, the dependency ratio is increasing, with significant consequences for local and global economies. This phenomenon is of growing concern in the United States and western Europe, where it is estimated that more than a third of the population will be older than age 65 and fewer than half will be in the workforce.