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Human life expectancy is a statistical measure of the estimate of the average remaining years of life at a given age. The most commonly used measure is life expectancy at birth. This can be defined in two ways. Cohort LEB is the mean length of life of a birth cohort and can be computed only for cohorts born so long ago that all their members have died. Period LEB is the mean length of life of a hypothetical cohort assumed to be exposed, from birth through death, to the mortality rates observed at a given year. National LEB figures reported by national agencies and international organizations for human populations are estimates of period LEB.
Senescence or biological aging is the gradual deterioration of functional characteristics in living organisms. Whole organism senescence involves an increase in death rates or a decrease in fecundity with increasing age, at least in the later part of an organism's life cycle. However, the resulting effects of senescence can be delayed. The 1934 discovery that calorie restriction can extend lifespans by 50% in rats, the existence of species having negligible senescence, and the existence of potentially immortal organisms such as members of the genus Hydra have motivated research into delaying senescence and thus age-related diseases. Rare human mutations can cause accelerated aging diseases.
Biogerontology is the sub-field of gerontology concerned with the biological aging process, its evolutionary origins, and potential means to intervene in the process. The term "biogerontology" was coined by S. Rattan, and came in regular use with the start of the journal Biogerontology in 2000. It involves interdisciplinary research on the causes, effects, and mechanisms of biological aging. Biogerontologist Leonard Hayflick has said that the natural average lifespan for a human is around 92 years and, if humans do not invent new approaches to treat aging, they will be stuck with this lifespan. James Vaupel has predicted that life expectancy in industrialized countries will reach 100 for children born after the year 2000. Many surveyed biogerontologists have predicted life expectancies of more than three centuries for people born after the year 2100. Other scientists, more controversially, suggest the possibility of unlimited lifespans for those currently living. For example, Aubrey de Grey offers the "tentative timeframe" that with adequate funding of research to develop interventions in aging such as strategies for engineered negligible senescence, "we have a 50/50 chance of developing technology within about 25 to 30 years from now that will, under reasonable assumptions about the rate of subsequent improvements in that technology, allow us to stop people from dying of aging at any age". The idea of this approach is to use presently available technology to extend lifespans of currently living humans long enough for future technological progress to resolve any remaining aging-related issues. This concept has been referred to as longevity escape velocity.
Fecundity is defined in two ways; in human demography, it is the potential for reproduction of a recorded population as opposed to a sole organism, while in population biology, it is considered similar to fertility, the natural capability to produce offspring, measured by the number of gametes (eggs), seed set, or asexual propagules.
Pleiotropy occurs when one gene influences two or more seemingly unrelated phenotypic traits. Such a gene that exhibits multiple phenotypic expression is called a pleiotropic gene. Mutation in a pleiotropic gene may have an effect on several traits simultaneously, due to the gene coding for a product used by a myriad of cells or different targets that have the same signaling function.
The grandmother hypothesis is a hypothesis to explain the existence of menopause in human life history by identifying the adaptive value of extended kin networking. It builds on the previously postulated "mother hypothesis" which states that as mothers age, the costs of reproducing become greater, and energy devoted to those activities would be better spent helping her offspring in their reproductive efforts. It suggests that by redirecting their energy onto those of their offspring, grandmothers can better ensure the survival of their genes through younger generations. By providing sustenance and support to their kin, grandmothers not only ensure that their genetic interests are met, but they also enhance their social networks which could translate into better immediate resource acquisition. This effect could extend past kin into larger community networks and benefit wider group fitness.
Life history theory (LHT) is an analytical framework designed to study the diversity of life history strategies used by different organisms throughout the world, as well as the causes and results of the variation in their life cycles. It is a theory of biological evolution that seeks to explain aspects of organisms' anatomy and behavior by reference to the way that their life histories—including their reproductive development and behaviors, post-reproductive behaviors, and lifespan —have been shaped by natural selection. A life history strategy is the "age- and stage-specific patterns" and timing of events that make up an organism's life, such as birth, weaning, maturation, death, etc. These events, notably juvenile development, age of sexual maturity, first reproduction, number of offspring and level of parental investment, senescence and death, depend on the physical and ecological environment of the organism.
Enquiry into the evolution of ageing, or aging, aims to explain why a detrimental process such as ageing would evolve, and why there is so much variability in the lifespans of organisms. The classical theories of evolution suggest that environmental factors, such as predation, accidents, disease, and/or starvation, ensure that most organisms living in natural settings will not live until old age, and so there will be very little pressure to conserve genetic changes that increase longevity. Natural selection will instead strongly favor genes which ensure early maturation and rapid reproduction, and the selection for genetic traits which promote molecular and cellular self-maintenance will decline with age for most organisms.
The paternal age effect is the statistical relationship between the father's age at conception and biological effects on the child. Such effects can relate to birthweight, congenital disorders, life expectancy and psychological outcomes. A 2017 review found that while severe health effects are associated with higher paternal age, the total increase in problems caused by paternal age is low. Average paternal age at birth reached a low point between 1960 and 1980 in many countries and has been increasing since then, but has not reached historically unprecedented levels. The rise in paternal age is not seen as a major public health concern.
Ageing is the process of becoming older. The term refers mainly to humans, many other animals, and fungi, whereas for example, bacteria, perennial plants and some simple animals are potentially biologically immortal. In a broader sense, ageing can refer to single cells within an organism which have ceased dividing, or to the population of a species.
The antagonistic pleiotropy hypothesis (APT) is a theory in evolutionary biology that suggests certain genes may confer beneficial effects early in an organism's life, enhancing reproductive success, while also causing detrimental effects later in life, contributing to the aging process.
Semelparity and iteroparity are two contrasting reproductive strategies available to living organisms. A species is considered semelparous if it is characterized by a single reproductive episode before death, and iteroparous if it is characterized by multiple reproductive cycles over the course of its lifetime. Iteroparity can be further divided into continuous iteroparity and seasonal iteroparity Some botanists use the parallel terms monocarpy and polycarpy.
Negligible senescence is a term coined by biogerontologist Caleb Finch to denote organisms that do not exhibit evidence of biological aging (senescence), such as measurable reductions in their reproductive capability, measurable functional decline, or rising death rates with age. There are many species where scientists have seen no increase in mortality after maturity. This may mean that the lifespan of the organism is so long that researchers' subjects have not yet lived up to the time when a measure of the species' longevity can be made. Turtles, for example, were once thought to lack senescence, but more extensive observations have found evidence of decreasing fitness with age.
The patriarch hypothesis is a hypothesis that explains the occurrence of menopause in human females and how a long post-fertile period could confer an evolutionary advantage. It is an alternative theory to the grandmother hypothesis which tends to ignore male benefits of continued spermatogenesis and their roles in assistance.
The reproductive-cell cycle theory posits that the hormones that regulate reproduction act in an antagonistic pleiotrophic manner to control aging via cell cycle signaling; promoting growth and development early in life in order to achieve reproduction, but later in life, in a futile attempt to maintain reproduction, become dysregulated and drive senescence. Rather than seeing aging as a loss of functionality as we get older, this theory defines aging as any change in an organism over time, as evidenced by the fact that if all chemical reactions in the body were stopped, no change, and thus no aging, would occur. Since the most important change in an organism through time is the chemical reactions that result in a single cell developing into a multicellular organism, whatever controls these chemical reactions that regulate cell growth, development, and death, is believed to control aging. The theory argues that these cellular changes are directed by reproductive hormones of the hypothalamic-pituitary-gonadal axis. Receptors for reproductive hormones have been found to be present in all tissues of the body. Thus, HPG axis hormones normally promote growth and development of the organism early in life in order to achieve reproduction. Hormones levels then begin to change in men around age 30 and more abruptly in women when they reach menopause, around age 50. When the HPG axis becomes unbalanced, cellular growth and development is dysregulated, and cell death and dysfunction can occur, both of which can initiate senescence, the accumulated damage to cells, tissues, and organs that occurs with the passage of time and that is associated with functional loss during aging.
Genetics of aging is generally concerned with life extension associated with genetic alterations, rather than with accelerated aging diseases leading to reduction in lifespan.
In biogerontology, the disposable soma theory of aging states that organisms age due to an evolutionary trade-off between growth, reproduction, and DNA repair maintenance. Formulated by British biologist Thomas Kirkwood, the disposable soma theory explains that an organism only has a limited amount of resources that it can allocate to its various cellular processes. Therefore, a greater investment in growth and reproduction would result in reduced investment in DNA repair maintenance, leading to increased cellular damage, shortened telomeres, accumulation of mutations, compromised stem cells, and ultimately, senescence. Although many models, both animal and human, have appeared to support this theory, parts of it are still controversial. Specifically, while the evolutionary trade-off between growth and aging has been well established, the relationship between reproduction and aging is still without scientific consensus, and the cellular mechanisms largely undiscovered.
Human reproductive ecology is a subfield in evolutionary biology that is concerned with human reproductive processes and responses to ecological variables. It is based in the natural and social sciences, and is based on theory and models deriving from human and animal biology, evolutionary theory, and ecology. It is associated with fields such as evolutionary anthropology and seeks to explain human reproductive variation and adaptations. The theoretical orientation of reproductive ecology applies the theory of natural selection to reproductive behaviors, and has also been referred to as the evolutionary ecology of human reproduction.
The mutation accumulation theory of aging was first proposed by Peter Medawar in 1952 as an evolutionary explanation for biological aging and the associated decline in fitness that accompanies it. Medawar used the term 'senescence' to refer to this process. The theory explains that, in the case where harmful mutations are only expressed later in life, when reproduction has ceased and future survival is increasingly unlikely, then these mutations are likely to be unknowingly passed on to future generations. In this situation the force of natural selection will be weak, and so insufficient to consistently eliminate these mutations. Medawar posited that over time these mutations would accumulate due to genetic drift and lead to the evolution of what is now referred to as aging.
Few animals have a menopause: humans are joined by just four other species in which females live substantially longer than their ability to reproduce. The others are all cetaceans: beluga whales, narwhals, orcas and short-finned pilot whales. There are various theories on the origin and process of the evolution of menopause. These attempt to suggest evolutionary benefits to the human species stemming from the cessation of women's reproductive capability before the end of their natural lifespan. Explanations can be categorized as adaptive and non-adaptive:
References
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- 1 2 Shokhirev MN, Johnson AA (2014-01-21). "Effects of extrinsic mortality on the evolution of aging: a stochastic modeling approach". PLOS ONE. 9 (1): e86602. Bibcode:2014PLoSO...986602S. doi: 10.1371/journal.pone.0086602 . PMC 3897743 . PMID 24466165.
- ↑ Reznick DN, Bryant MJ, Roff D, Ghalambor CK, Ghalambor DE (October 2004). "Effect of extrinsic mortality on the evolution of senescence in guppies". Nature. 431 (7012): 1095–9. Bibcode:2004Natur.431.1095R. doi:10.1038/nature02936. PMID 15510147. S2CID 205210169.
- ↑ Chen HY, Maklakov AA (November 2012). "Longer life span evolves under high rates of condition-dependent mortality". Current Biology. 22 (22): 2140–3. doi: 10.1016/j.cub.2012.09.021 . PMID 23084993.
- 1 2 Pepper GV, Nettle D (September 2014). "Perceived extrinsic mortality risk and reported effort in looking after health: testing a behavioral ecological prediction". Human Nature. 25 (3): 378–92. doi:10.1007/s12110-014-9204-5. PMID 24990431. S2CID 24682307.
- ↑ Dańko MJ, Burger O, Kozłowski J (2017). "Density-dependence interacts with extrinsic mortality in shaping life histories". PLOS ONE. 12 (10): e0186661. Bibcode:2017PLoSO..1286661D. doi: 10.1371/journal.pone.0186661 . PMC 5648222 . PMID 29049399.
- ↑ Carnes BA (April 2004). "Darwinian bodies in a Lamarkian world". The Gerontologist. 44 (2): 274–279. doi: 10.1093/geront/44.2.274 .
- ↑ Blell M (2017-09-29). "Grandmother Hypothesis, Grandmother Effect, and Residence Patterns". John Wiley & Sons, Ltd. pp. 1–5. doi:10.1002/9781118924396.wbiea2162. ISBN 9780470657225.
{{cite book}}:|work=ignored (help); Missing or empty|title=(help) - 1 2 Quinlan RJ (2010-06-01). "Extrinsic Mortality Effects on Reproductive Strategies in a Caribbean Community". Human Nature. 21 (2): 124–139. doi:10.1007/s12110-010-9085-1. S2CID 39977514.