The origins of human ageing

Perception of uncontrollable mortality risk is associated with food insecurity and reduced economic effort among resource-insecure college students during COVID-19

In the framework of the uncontrollable mortality risk hypothesis, resource scarcity intersects with mortality risk, shaping resource allocation strategies with enduring impacts on human health and wellbeing. Despite rising economic and food insecurity among US college students, little is known about how these insecurities relate to mortality risk, or how scarcity and mortality risk interact to shape college students' resource allocation strategies. We examine perceptions of resource scarcity and mortality risk and their associations with food insecurity and resource allocation strategies among economically insecure college students during COVID-19 lockdowns. Participants were recruited through an economic crisis response center at a major public university in the United States. A total of 118 participants completed an online Qualtrics survey assessing sociodemographic characteristics, perceptions of mortality risk and resource availability, food security, economic effort, and time perspective; a subset (n = 51) also participated in a telephone interview assessing psychological distress. In general, participants reported more environmental adversity and economic effort during COVID-19 lockdowns compared to before. Students experiencing higher levels of uncontrollable (and not controllable) mortality risk report lower levels of economic effort, and the association was strongest among students perceiving the fewest resources. We also found significant associations between uncontrollable mortality risk and food insecurity. Our results highlight uncontrollable mortality risk's influence on human well-being. Public health efforts should target the experiences and root structural causes of uncontrollable mortality risk, which among economically insecure college students increasingly involves food insecurity.

Meta-analysis shows no consistent evidence for senescence in ejaculate traits across animals

Male reproductive traits such as ejaculate size and quality, are expected to decline with advancing age due to senescence. It is however unclear whether this expectation is upheld across taxa. We perform a meta-analysis on 379 studies, to quantify the effects of advancing male age on ejaculate traits across 157 species of non-human animals. Contrary to predictions, we find no consistent pattern of age-dependent changes in ejaculate traits. This result partly reflects methodological limitations, such as studies sampling a low proportion of adult lifespan, or the inability of meta-analytical approaches to document non-linear ageing trajectories of ejaculate traits; which could potentially lead to an underestimation of senescence. Yet, we find taxon-specific differences in patterns of ejaculate senescence. For instance, older males produce less motile and slower sperm in ray-finned fishes, but larger ejaculates in insects, compared to younger males. Notably, lab rodents show senescence in most ejaculate traits measured. Our study challenges the notion of universal reproductive senescence, highlighting the need for controlled methodologies and a more nuanced understanding of reproductive senescence, cognisant of taxon-specific biology, experimental design, selection pressures, and life-history.

Expanding evolutionary theories of ageing to better account for symbioses and interactions throughout the Web of Life

How, when, and why organisms age are fascinating issues that can only be fully addressed by adopting an evolutionary perspective. Consistently, the main evolutionary theories of ageing, namely the Mutation Accumulation theory, the Antagonistic Pleiotropy theory, and the Disposable Soma theory, have formulated stimulating hypotheses that structure current debates on both the proximal and ultimate causes of organismal ageing. However, all these theories leave a common area of biology relatively under-explored. The Mutation Accumulation theory and the Antagonistic Pleiotropy theory were developed under the traditional framework of population genetics, and therefore are logically centred on the ageing of individuals within a population. The Disposable Soma theory, based on principles of optimising physiology, mainly explains ageing within a species. Consequently, current leading evolutionary theories of ageing do not explicitly model the countless interspecific and ecological interactions, such as symbioses and host-microbiomes associations, increasingly recognized to shape organismal evolution across the Web of Life. Moreover, the development of network modelling supporting a deeper understanding on the molecular interactions associated with ageing within and between organisms is also bringing forward new questions regarding how and why molecular pathways associated with ageing evolved. Here, we take an evolutionary perspective to examine the effects of organismal interactions on ageing across different levels of biological organisation, and consider the impact of surrounding and nested systems on organismal ageing. We also apply this perspective to suggest open issues with potential to expand the standard evolutionary theories of ageing.

Network analyses unveil ageing-associated pathways evolutionarily conserved from fungi to animals

The genetic roots of the diverse paces and shapes of ageing and of the large variations in longevity observed across the tree of life are poorly understood. Indeed, pathways associated with ageing/longevity are incompletely known, both in terms of their constitutive genes/proteins and of their molecular interactions. Moreover, there is limited overlap between the genes constituting these pathways across mammals. Yet, dedicated comparative analyses might still unravel evolutionarily conserved, important pathways associated with longevity or ageing. Here, we used an original strategy with a double evolutionary and systemic focus to analyse protein interactions associated with ageing or longevity during the evolution of five species of Opisthokonta. We ranked these proteins and interactions based on their evolutionary conservation and centrality in past and present protein-protein interaction (PPI) networks, providing a big systemic picture of the evolution of ageing and longevity pathways that identified which pathways emerged in which Opisthokonta lineages, were conserved, and/or central. We confirmed that longevity/ageing-associated proteins (LAPs), be they pro- or anti-longevity, are highly central in extant PPI, consistently with the antagonistic pleiotropy theory of ageing, and identified key antagonistic regulators of ageing/longevity, 52 of which with homologues in humans. While some highly central LAPs were evolutionarily conserved for over a billion years, we report a clear transition in the functionally important components of ageing/longevity within bilaterians. We also predicted 487 novel evolutionarily conserved LAPs in humans, 54% of which are more central than mTOR, and 138 of which are druggable, defining new potential targets for anti-ageing treatments in humans.

The 90 plus: longevity and COVID-19 survival

The world population is getting older and studies aiming to enhance our comprehension of the underlying mechanisms responsible for health span are of utmost interest for longevity and as a measure for health care. In this review, we summarized previous genetic association studies (GWAS) and next-generation sequencing (NGS) of elderly cohorts. We also present the updated hypothesis for the aging process, together with the factors associated with healthy aging. We discuss the relevance of studying older individuals and build databanks to characterize the presence and resistance against late-onset disorders. The identification of about 2 million novel variants in our cohort of more than 1000 elderly Brazilians illustrates the importance of studying highly admixed populations of non-European ancestry. Finally, the ascertainment of nonagenarians and particularly of centenarians who were recovered from COVID-19 or remained asymptomatic opens new avenues of research aiming to enhance our comprehension of biological mechanisms associated with resistance against pathogens.

Shifting epigenetic contexts influence regulatory variation and disease risk

Epigenetic shifts are a hallmark of aging that impact transcriptional networks at regulatory level. These shifts may modify the effects of genetic regulatory variants during aging and contribute to disease pathomechanism. However, these shifts occur on the backdrop of epigenetic changes experienced throughout an individual's development into adulthood; thus, the phenotypic, and ultimately fitness, effects of regulatory variants subject to developmental- versus aging-related epigenetic shifts may differ considerably. Natural selection therefore may act differently on variants depending on their changing epigenetic context, which we propose as a novel lens through which to consider regulatory sequence evolution and phenotypic effects. Here, we define genomic regions subjected to altered chromatin accessibility as tissues transition from their fetal to adult forms, and subsequently from early to late adulthood. Based on these epigenomic datasets, we examine patterns of evolutionary constraint and potential functional impacts of sequence variation (e.g., genetic disease risk associations). We find that while the signals observed with developmental epigenetic changes are consistent with stronger fitness consequences (i.e., negative selection pressures), they tend to have weaker effects on genetic risk associations for aging-related diseases. Conversely, we see stronger effects of variants with increased local accessibility in adult tissues, strongest in young adult when compared to old. We propose a model for how epigenetic status of a region may influence the effects of evolutionary relevant sequence variation, and suggest that such a perspective on gene regulatory networks may elucidate our understanding of aging biology.

Age-associated epigenetic change in chimpanzees and humans

Methylation levels have been shown to change with age at sites across the human genome. Change at some of these sites is so consistent across individuals that it can be used as an 'epigenetic clock' to predict an individual's chronological age to within a few years. Here, we examined how the pattern of epigenetic ageing in chimpanzees compares with humans. We profiled genome-wide blood methylation levels by microarray for 113 samples from 83 chimpanzees aged 1-58 years (26 chimpanzees were sampled at multiple ages during their lifespan). Many sites (greater than 65 000) showed significant change in methylation with age and around one-third (32%) of these overlap with sites showing significant age-related change in humans. At over 80% of sites showing age-related change in both species, chimpanzees displayed a significantly faster rate of age-related change in methylation than humans. We also built a chimpanzee-specific epigenetic clock that predicted age in our test dataset with a median absolute deviation from known age of only 2.4 years. However, our chimpanzee clock showed little overlap with previously constructed human clocks. Methylation at CpGs comprising our chimpanzee clock showed moderate heritability. Although the use of a human microarray for profiling chimpanzees biases our results towards regions with shared genomic sequence between the species, nevertheless, our results indicate that there is considerable conservation in epigenetic ageing between chimpanzees and humans, but also substantial divergence in both rate and genomic distribution of ageing-associated sites. This article is part of the theme issue 'Evolution of the primate ageing process'.

Understanding the Frontiers of Human Longevity in India: Imperative and Palliative Care

This article provides a theoretical and empirical insight on the study of population aging in India, with the special reference to the causes that have made it extremely significant. It evidently looks into the factors that are extensively associated with the process of population aging and have contributed to the Indian society. Demographically speaking, in the Indian context, the process of demographic transition has resulted from a falling birth rate, a slowing death rate, and spike in life expectancy. In the context of developing countries, the concept of population aging has been brought from developed countries. Initially, the outcomes of demographic transition had been experienced by developed regions followed by the rest of the world. Finally, it examines the consequences of complications that arise due to growth in life expectancy at birth, and further suggests the probable remedies to both strategy developers and policy-makers.

Host demise as a beneficial function of indigenous microbiota in human hosts

The age structure of human populations is exceptional among animal species. Unlike with most species, human juvenility is extremely extended, and death is not coincident with the end of the reproductive period. We examine the age structure of early humans with models that reveal an extraordinary balance of human fertility and mortality. We hypothesize that the age structure of early humans was maintained by mechanisms incorporating the programmed death of senescent individuals, including by means of interactions with their indigenous microorganisms. First, before and during reproductive life, there was selection for microbes that preserve host function through regulation of energy homeostasis, promotion of fecundity, and defense against competing high-grade pathogens. Second, we hypothesize that after reproductive life, there was selection for organisms that contribute to host demise. While deleterious to the individual, the presence of such interplay may be salutary for the overall host population in terms of resource utilization, resistance to periodic diminutions in the food supply, and epidemics due to high-grade pathogens. We provide deterministic mathematical models based on age-structured populations that illustrate the dynamics of such relationships and explore the relevant parameter values within which population viability is maintained. We argue that the age structure of early humans was robust in its balance of the juvenile, reproductive-age, and senescent classes. These concepts are relevant to issues in modern human longevity, including inflammation-induced neoplasia and degenerative diseases of the elderly, which are a legacy of human evolution.

IMPORTANCE

The extended longevity of modern humans is a very recent societal artifact, although it is inherent in human evolution. The age structure of early humans was balanced by fertility and mortality, with an exceptionally prolonged juvenility. We examined the role of indigenous microbes in early humans as fundamental contributors to this age structure. We hypothesize that the human microbiome evolved mechanisms specific to the mortality of senescent individuals among early humans because their mortality contributed to the stability of the general population. The hypothesis that we present provides new bases for modern medical problems, such as inflammation-induced neoplasia and degenerative diseases of the elderly. We postulate that these mechanisms evolved because they contributed to the stability of early human populations, but their legacy is now a burden on human longevity in the changed modern world.

Cerebral ageing-the role of insulin and insulin-like growth factor signalling: A review

Cerebral ageing is a complex biological process associated with progressing cerebrovascular disease and neuronal death. It does not always, however, associate with a functional decline, as the ageing mammalian brain retains considerable functional plasticity which supports successful cerebral ageing where age-related cognitive decline is modest. On the contrary, pathological cerebral ageing results in memory impairment and cognitive deterioration, with Alzheimer’s disease (AD) being a florid example. Trophic/growth factors promote brain plasticity; among them are peptides which belong to the insulin family. Preclinical research suggests that the evolutionarily conserved brain insulin/insulin-like growth factor-1 (IGF-1) signalling system controls lifespan and protects against some features of AD such as neurodegeneration-related accumulation of toxic proteins and cognitive deficiencies, as observed in animal models. Insulin and IGF-1 activate cell signalling mechanisms which play protective and regenerative roles; abnormalities in the insulin/IGF-1 system may trigger a cascade of neurodegeneration in AD. AD patients show cerebral resistance to insulin which associates with IGF-I resistance and dysregulation of insulin/IGF-1 receptors as well as cognitive deterioration. This review is focused on the roles of the insulin/IGF-1 signalling system in cerebral ageing and its potential involvement in neurodegeneration in the human brain as seen against the background of preclinical evidence.

Ageing of trees: application of general ageing theories

The main questions posed in ageing theories are how ageing evolved and whether or not it is programmed. While these questions have not yet been clearly resolved, several groups of possible theories have been published on this topic. However, most of these theories do not consider plants, and the specific traits involved in their ageing mechanisms. The first trait covers clonality and sectoriality and the second concerns the lack of a differentiated germ line. The lack of a germ line prevents telomere shortening which can lead to the transfer of somatic mutations into sexual offspring, while sectoriality in trees causes isolation of potentially catastrophic events in one tree part, thus creating a population of more or less independent modules within one axis. The processes of population dynamics, including ageing, can act within the framework of an individual tree as well as in that of the population as a whole, although the processes involved differ and consequently result in different effects.

Mate choice and the origin of menopause

Human menopause is an unsolved evolutionary puzzle, and relationships among the factors that produced it remain understood poorly. Classic theory, involving a one-sex (female) model of human demography, suggests that genes imparting deleterious effects on post-reproductive survival will accumulate. Thus, a 'death barrier' should emerge beyond the maximum age for female reproduction. Under this scenario, few women would experience menopause (decreased fertility with continued survival) because few would survive much longer than they reproduced. However, no death barrier is observed in human populations. Subsequent theoretical research has shown that two-sex models, including male fertility at older ages, avoid the death barrier. Here we use a stochastic, two-sex computational model implemented by computer simulation to show how male mating preference for younger females could lead to the accumulation of mutations deleterious to female fertility and thus produce a menopausal period. Our model requires neither the initial assumption of a decline in older female fertility nor the effects of inclusive fitness through which older, non-reproducing women assist in the reproductive efforts of younger women. Our model helps to explain why such effects, observed in many societies, may be insufficient factors in elucidating the origin of menopause.

Diet mediates the relationship between longevity and reproduction in mammals

The disposable soma hypothesis posits a negative correlation between longevity and reproduction, presumably because these aspects of fitness compete for a limited pool of nutrients. However, diet, which varies widely among animals, could affect the availability of key nutrients required for both reproduction and longevity, especially protein. We used a comparative database of mammal life history data to test the hypothesis that carnivores experience less of a negative relationship between reproduction and longevity than herbivores. Annual reproduction and adult mass were significant predictors of longevity among all mammals; although, the relative importance of reproduction and mass for explaining longevity varied among trophic levels. In herbivores, reproduction was a stronger predictor of longevity than mass. Carnivores showed the opposite pattern with reproduction explaining much less of the variation in longevity. Omnivores showed an intermediate pattern with mass and reproduction explaining similar amounts of variation in longevity. In addition, longevity and reproduction were significantly higher in omnivores than herbivores and carnivores, which were not different from each other. Higher dietary protein at higher trophic levels may allow mammals to avoid potential conflicts between reproduction and longevity. However, there may be potential costs of carnivorous diets that limit the overall performance of carnivores and explain the peak in reproduction and longevity for omnivores.

The evolutionary origin and significance of menopause

Contemporary women have long life expectancy (81 y, United States), especially relative to the age at menopause (51 y, United States). Menopause is a consequence of reproductive aging and follicular depletion (ovarian failure), yielding very low circulating estrogen serum concentrations and biologically disadvantageous metabolic alterations. Stated in terms of antagonistic pleiotropy, the ongoing hypoestrogenic endocrine environment, beneficial during lactation, results in acceleration of several age-related illnesses after menopause (ie, late postmenopausal osteoporosis, cardiovascular disease, and cognitive decline). Specifically, the similar hypoestrogenic hormonal milieu present during postpartum lactation provides biologic advantages (fitness) to both mother and newborn. These precepts of evolutionary medicine encourage a reassessment of hormone therapy, and on the basis of data presented the authors propose additional opportunities for disease prevention and morbidity reduction in postmenopausal women.

Why are there social gradients in preventative health behavior? A perspective from behavioral ecology

Background

Within affluent populations, there are marked socioeconomic gradients in health behavior, with people of lower socioeconomic position smoking more, exercising less, having poorer diets, complying less well with therapy, using medical services less, ignoring health and safety advice more, and being less health-conscious overall, than their more affluent peers. Whilst the proximate mechanisms underlying these behavioral differences have been investigated, the ultimate causes have not.

Methodology/Principal Findings

This paper presents a theoretical model of why socioeconomic gradients in health behavior might be found. I conjecture that lower socioeconomic position is associated with greater exposure to extrinsic mortality risks (that is, risks that cannot be mitigated through behavior), and that health behavior competes for people's time and energy against other activities which contribute to their fitness. Under these two assumptions, the model shows that the optimal amount of health behavior to perform is indeed less for people of lower socioeconomic position.

Conclusions/Significance

The model predicts an exacerbatory dynamic of poverty, whereby the greater exposure of poor people to unavoidable harms engenders a disinvestment in health behavior, resulting in a final inequality in health outcomes which is greater than the initial inequality in material conditions. I discuss the assumptions of the model, and its implications for strategies for the reduction of health inequalities.

Evolution of the menopause: life histories and mechanisms

A long postreproductive lifespan is a characteristic feature in the life history of human females, which is not shared with other primates. The ultimate cause of menopause has been the focus of much study and has generated a number of evolutionary explanations, most prominently the mother and grandmother hypotheses. Generally, these theories propose that menopause was an adaptive response to changes that led to the divergence of humans from their ancestors, and are based on observations such as the long-dependency time of human infants, early age of weaning of human neonates, high maternal mortality, the supportive role of grandmothers in childcare, and intergroup female transfers. While ongoing debate continues to refine evolutionary theory, the proximate cause of menopause currently receives less attention. Our knowledge about the mechanisms underlying menopause has been largely confined to ovarian exhaustion, the progressive loss of oocytes beginning before birth and continuing to the age of menopause. Most efforts have to date been focused on fitting curves to the few data available, rather than trying to explain why the dynamics of oocyte depletion follows a particular pattern. A few recent studies have attempted to address this problem by demonstrating that oocyte dynamics is a regulated process under tight physiological control, e.g. that ovaries sense both number and quality of their oocytes. In this review we assess our current knowledge from an evolutionary perspective and emphasize the benefit of combining a mechanistic and life history approach.

Testing evolutionary theories of menopause

Why do women cease fertility rather abruptly through menopause at an age well before generalized senescence renders child rearing biologically impossible? The two main evolutionary hypotheses are that menopause serves either (i) to protect mothers from rising age-specific maternal mortality risks, thereby protecting their highly dependent younger children from death if the mother dies or (ii) to provide post-reproductive grandmothers who enhance their inclusive fitness by helping to care and provide for their daughters' children. Recent theoretical work indicates that both factors together are necessary if menopause is to provide an evolutionary advantage. However, these ideas need to be tested using detailed data from actual human life histories lived under reasonably 'natural' conditions; for obvious reasons, such data are extremely scarce. We here describe a study based on a remarkably complete dataset from The Gambia. The data provided quantitative estimates for key parameters for the theoretical model, which were then used to assess the actual effects on fitness. Empirically based numerical analysis of this nature is essential if the enigma of menopause is to be explained satisfactorily in evolutionary terms. Our results point to the distinctive (and perhaps unique) role of menopause in human evolution and provide important support for the hypothesized evolutionary significance of grandmothers.

Understanding ageing from an evolutionary perspective

There is clear heritability of human longevity. However, the genetics of ageing is likely to be complex. Evolution theory tells us not to expect genes that have been selected to promote ageing. Ageing is not programmed but results from accumulation of somatic damage, owing to limited investments in maintenance and repair. Genes controlling the levels of activities, such as DNA repair and antioxidant defence, thus regulate longevity. In addition, there may be contributions either from late-acting deleterious genes that escape the force of natural selection or that trade benefit at an early age against harm at older ages. In some species, there is evidence that genes have evolved to detect and respond to changes in the environment, e.g. food supply. Evolutionary understanding can also help to understand important features of the human life history such as menopause.

Trade-off mediated effects on the genetics of human survival caused by increasingly benign living conditions

It is sometimes suggested that modern life, with its planned fertility and medically assisted compensation for genetic disadvantage, has greatly reduced the power of natural selection to produce significant further human evolution. However, this overlooks the very recent and dramatic changes that have occurred in the patterns of human mortality, driven by the development of increasingly benign living conditions. Where the genetics of survival are significantly influenced by life history trade-offs, as is now thought to be the case for ageing and longevity, a change in the environment can shift the optimal balance that was established through natural selection under previous conditions. We examine this possibility in the context of the dramatic recent change in the mortality pressure exerted by infectious disease using a model of a hypothetical immunogenetic trade-off that weighs survival against fecundity. Our results predict that such a change might have a significant effect on population genetics over relatively short-timescales, and they may also resolve the paradox of genes that impair fertility in present-day populations by showing how such genes might be a legacy from a powerful trade-off that has acted until very recently.

Spend less, live longer. The "thrifty aged" hypothesis

Evolutionary biology has rejected any argument in favour of planned ageing. In this article, the metabolic changes that follow in ageing have been analysed, and in accordance with that, an evolutionary explanation for the senescence program has been proposed. Ageing is characterised by a set of metabolic features all of them designed to reduce energy expenditure and to promote energy storage. Otherwise, the undoubted beneficial effect to fitness that means to live longer could generate a conflict between parents and offspring for energy resources. The solution to that conflict is an adult economical individual, a "thrifty aged". The way to obtain this cheaper individual is through the genetically induced physiological changes that finally lead the process. In conclusion, it is proposed that ageing is the condition, imposed by natural selection, that let organisms to outlast the obligatory reproductive time.

Does mRNA level of microsomal carnitine palmitoyltransferase predict yield of peripheral blood stem cell apheresis?

Transplantation of autologous hematopoietic stem cells is a well established therapeutic procedure. Despite advances in efficacy of the stem cell mobilization and apheresis process until now a predictive factor for the expected stem cell yield before initiation of mobilization therapy could not be identified. The main objective of our study was to evaluate alterations in enzymes involved in fatty acid metabolism on the level of gene expression in mononuclear cells, as changes in relative mRNA levels of these enzymes could represent the hematopoietic regenerative potential. Data of 23 consecutive patients with different lymphoid malignancies undergoing stem cell mobilization were analyzed. Our results show that mRNA levels of microsomal carnitine palmitoyltransferase in peripheral blood mononuclear cells quantified before application of mobilization therapy correlate positively with the amount of CD34 positive cells in peripheral blood before first apheresis, in the first apheresis product and in the total harvest outcome. The association of enzymes involved in fatty acid metabolism with hematoopoiesis was further confirmed in healthy subjects on altitude-adaptation training and in proliferating or differentiating HL60 cells. This gives evidence for a possible predictive value of such analyzes though further data of a larger sample are to be collected to confirm our observations.

Environments and evolution: interactions between stress, resource inadequacy and energetic efficiency

Evolutionary change is interpreted in terms of the near-universal ecological scenario of stressful environments. Consequently, there is a premium on the energetically efficient exploitation of resources in a resource-inadequate world. Under this environmental model, fitness can be approximated to energetic efficiency especially towards the limits of survival. Furthermore, fitness at one stage of the life-cycle should correlate with fitness at other stages, especially for development time, survival and longevity; 'good genotypes' under stress should therefore be at a premium. Conservation in the wild depends primarily on adaptation to abiotically changing habitats since towards the limits of survival, genomic variation is rarely restrictive. The balance between energetic costs under variable environments and energy from resources provides a model for interpreting evolutionary stasis, punctuational and gradual change, and specialist diversification. Ultimately, a species should be in an equilibrium between the physiology of an organism and its adaptation to the environment. The primary key to understanding evolutionary change should therefore be ecological, highlighting energy availability in a stressed world; this approach is predictive for various patterns of evolutionary change in the living and fossil biota.

Life extension research: an analysis of contemporary biological theories and ethical issues

Many opinions and ideas about aging exist. Biological theories have taken hold of the popular and scientific imagination as potential answers to a "cure" for aging. However, it is not clear what exactly is being cured or whether aging could be classified as a disease. Some scientists are convinced that aging will be biologically alterable and that the human lifespan will be vastly extendable. Other investigators believe that aging is an elusive target that may only be "statistically" manipulatable through a better understanding of the operational principles of systems situated within complex environments. Not only is there confusion over definitions but also as to the safety of any potential intervention. Curing cell death, for example, may lead to cell cancer. The search for a cure for aging is not a clearly beneficial endeavour. This paper will first, describe contemporary ideas about aging processes and second, describe several current life extension technologies. Third, it analyses these theories and technologies, focusing on two representative and differing scientific points of view. The paper also considers the public health dilemma that arises from life extension research and examines two issues, risk/benefit ratio and informed consent, that are key to developing ethical guidelines for life extension technologies.

Evolution of Human Longevity, Population Pressure and the Origins of Warfare

In a protected environment, humans have the longest lifespan of all primates. However, during the emergence of Homo sapiens from pre-hominids, the expectation of life at birth would have been quite low. On the basis of reasonable assumptions, an average expectation of life of less than 20 years is sufficient to maintain a population of hunter-gatherers. As individuals became better adapted to their environment, the mortality rate would gradually decrease, and this would result in the survival of more offspring to adulthood. Thus, the population will increase, and one of the consequences in human evolution is the migration of human communities to many new habitats. The development of agriculture provided a more reliable source of food, and stimulated further the increase in population size. Villages became towns, and then cities, states and empires arose which had very large populations, and competed for land and other resources. Armies were raised and were often at war. All this was due to population pressure, as Malthus had realised more than 200 years ago. However, neither he, nor any of the others who discussed warfare, understood that the demographic changes that produced large human populations was a steady increase in the expectation of life at birth. This inevitably occurred at the same time as man gradually gained more control over his environment, and achieved far more reproductive success than is seen in hunter-gatherers living in a harsh, stressful environment.

The effects of background mortality on optimal reproduction in a seasonal environment

We consider optimal annual routines of reproductive behaviour in a seasonal environment. In our model the condition of the organism is adversely affected by hard work, but can recover during easy periods. Our analysis concentrates on the effects of background mortality (i.e., mortality that cannot be avoided) on the optimal strategy and how often an organism following this strategy breeds. In particular, we are concerned with whether reproduction occurs at specific times of year (entrained to the annual cycle), and if so then how many reproductive bouts occur per year. We find that an increase in background mortality can have various effects. If the animal is entrained to the annual cycle and has one breeding attempt per year, then breeding tends to occur earlier and there may be two breeding attempts per season. Another possible outcome is that breeding is no longer entrained. If the animal is entrained but sometimes skips reproduction so that it does not breed every year, then an increase in mortality may make it more likely that the animal breeds every year. We show that as background mortality increases the resultant increase in the frequency of breeding contributes to the increase in annual mortality. We also explore the effects of mortality on the timing of reproduction within a year, highlighting the tension between the interests of the parent and that of the young.

Ageing purpose: another thrifty genotype

For evolutionary biology, ageing is a non-adaptive process. The 'disposable soma' theory proposes that senescence is the consequence of a reduction in the energy invested in the processes of cell maintenance and repair due to the fact that it is more beneficial to invest it in reproduction. Recently, various genes have been identified whose mutations modify the life span of certain animals. Most of these genes are related to energy metabolism, especially insulin, IGF-1 and their receptors. Furthermore, it has also been demonstrated that there is a modification in metabolic pathways during ageing. As a result, the energy-storing pathways are strengthened and there is a reduction in the pathways that use energy. All these findings suggest that ageing is a strategy designed by natural selection to save energy, in accordance with other saving strategies. This way the energy that is not used can be dedicated to offspring to improve their pre-reproductive survival.

The clinical pharmacology of ageing

The ageing of populations and individuals continues to be as vital, yet to some extent as neglected, a topic in pharmacology and therapeutics as was first realised about 30 years ago. In parallel with the realisation of the predicted demographic shifts in both the developed and developing world, there have since been major developments in the basic biological concepts of ageing, in the physiology of ageing, in the study of pathogenetic mechanisms underlying a variety of age-associated disorders and syndromes, and in the evidence base for therapeutic intervention in elderly patient populations. These all present new challenges both in the practical delivery of effective medical care and in clinical and biological research. The scale of prescribing for an ageing population has continued to rise as anticipated. Whether there has now been any improvement in the quality or rationality of prescribing, or in the previously demonstrated unacceptable level of susceptibility to adverse drug reactions in the (now expanded) older patient population is largely unknown. We urgently need to find out using up-to-date research methods. National and international guidelines for drug development and regulation have more recently been followed by broader policy initiatives on prescribing for older people, but the impact of these on standards of medication use and on clinical outcome remains to be seen. A new series in this journal on the clinical pharmacology of ageing is timely. The required focus and framework for research have often tended in the past to emerge as afterthoughts behind the merely disease specific, and it is to be hoped that a sequential review of some of the key topics may help to re-ignite a more sound and less short-sighted agenda than previously.

The role of stem cells in aging

The objectives of this review were first to critically review what is known about the effects of aging on stem cells in general, and hematopoietic stem cells in particular. Secondly, evidence is marshalled in support of the hypothesis that aging stem cells play a critical role in determining the effects of aging on organ function, and ultimately on the lifespan of a mammal. Aging has both quantitative and qualitative effects on stem cells. On balance, the qualitative changes are the more important since they affect the self-renewal potential, developmental potential, and interactions with extrinsic signals, including those from stroma. Although hematopoiesis is generally maintained at normal and life-supporting levels during normal aging, diminished function is acutely apparent when old stem cells are subjected to stress. There is ample evidence of diminished self-renewal capacity, restriction of the breadth of developmental potency, and decreased numbers of progeny of old stem cells subjected to hematopoietic demands. The prediction is made that whatever plasticity in developmental potential possessed by a young stem cell is lost during aging. Those parts of the world enjoying an ever-increasing standard of living are also inhabited by an increasingly elderly population. The effects of age on many physiological functions are not well studied or appreciated. A public health challenge to provide increased quality of life for this growing segment of the population requires more attention to the variable of age in experimental studies. Stem cell populations are likely to be a fruitful subject for studies of this type.

Human stem cells as targets for the aging and diseases of aging processes

While many theories have been proposed for the aging process, and many debates on the matter of aging and the diseases of aging being either the result of the same or independent processes, most have not considered humans as a hierarchical system made up of cybernetically interacting levels of organization. To understand the aging process and the diseases of aging, one must view the human as the result of the total genomic DNA in the single fertilized egg that proliferates, differentiates and develops into an individual of about 100 trillion cells, organized by different cell types (pluri-potent stem cells, progenitor stem cells, terminally differentiated cells) into multiple tissue, organ and organ systems which interact with each other via endogenous factors and with exogenous factors. Our hypothesis is that both aging and diseases of aging are dependent of the normal functioning of the pluri-potent stem cell pool. Specifically, the concept involves the cybernetic feedback between the 'quantity' of the stem cell pool in each tissue niche with the 'quality' of the stem cells in the pool. The process of gap junctional inter-cellular communication (GJIC), which has been implicated in the evolution from the single cell organism to the multi-cellular organisms, requiring growth control, differentiation, apoptosis, adaptive response capability of differentiated cells and senescence, is speculated to be a shared mechanism in stem cell biology and in many chronic disease processes (teratogenesis; carcinogenesis, atherogenesis, diabetigenesis, etc.). Specifically, stem cells are assumed to be 'immortal' until induced to express their connexin genes and have functional GJIC, at which time they can differentiate and become 'mortal'. As long as the stem cells are communicating with their differentiated daughters via some extra-cellular soluble negative growth factor, the homeostatic control of their growth and differentiation is maintained for the organism. However, if the stem cell pool is depleted by any process, replacement of tissue due to wear and tear is diminished. The dependence of this tissue/organ to maintain homeostatic control of other organ systems then diminishes, leading to 'systems failure'. In addition, if the stem cells in the pool have been exposed to agents that prevent the normal terminal differentiation of that cell, but whereby these 'initiated' stem cells can be expanded in any tissue, clones of partially differentiated and non-functional appear in the tissue. This diminishes the efficacy of that tissue to function properly and, thereby, also contributes to 'system failure' by contributing to the breakdown of homeostatic organ system control. One clear example, that of carcinogenesis, illustrates this point.

Energetic consequences of being a Homo erectus female

Body size is one of the most important characteristics of any animal because it affects a range of behavioral, ecological, and physiological traits including energy requirements, choice of food, reproductive strategies, predation risk, range size, and locomotor style. This article focuses on the implications of being large bodied for Homo erectus females, estimated to have been over 50% heavier than average australopithecine females. The energy requirements of these hominins are modeled using data on activity patterns, body mass, and life history from living primates. Particular attention is given to the inferred energetic costs of reproduction for Homo erectus females based on chimpanzee and human reproductive scheduling. Daily energy requirements during gestation and lactation would have been significantly higher for Homo erectus females, as would total energetic cost per offspring if the australopithecines and Homo erectus had similar reproductive schedules (gestation and lactation lengths and interbirth intervals). Shortening the interbirth interval could considerably reduce the costs per offspring to Homo erectus and have the added advantage of increasing reproductive output. The mother would, however, incur additional daily costs of caring for the dependent offspring. If Homo erectus females adopted this reproductive strategy, it would necessarily imply a revolution in the way in which females obtained and utilized energy to support their increased energetic requirements. This transformation is likely to have occurred on several levels involving cooperative economic division of labor, locomotor energetics, menopause, organ size, and other physiological mechanisms for reducing the energetic load on females.

Heritability of death from coronary heart disease: a 36-year follow-up of 20 966 Swedish twins

OBJECTIVE

The aim of the present study was to evaluate and distinguish between environmental and genetic effects for death from coronary heart disease (CHD) as well as to determine whether the importance of genetic influences is changing with age.

DESIGN

A cohort study with a follow-up time of 36 years.

SUBJECTS

The cohort drawn for the present study includes 20 966 twins born in Sweden between 1886 and 1925 where both twins within a pair still lived within the country in 1961.

METHODS

Concordances and correlated gamma-frailty model were used to assess and distinguish between genetic and environmental influences as well as to evaluate age-related changes in genetic influences.

RESULTS

A total number of 4007 CHD-deaths (2208 males, and 1799 females) was observed. The probability of dying from CHD given that one's twin partner already has died from CHD decreased with increasing age, particularly amongst males. The genetic variation in susceptibility to death from CHD was moderately large, and decreased slightly across time, particularly amongst males. The heritability was 0.57 (95% CI, 0.45-0.69) amongst male twins, and 0.38 (0.26-0.50) amongst female twins.

CONCLUSIONS

The genetic contribution to the variation in CHD-mortality was moderate both in females and males. Furthermore, although genetic effects appeared to be greater at younger ages of death, our findings clearly suggest that genetic factors are in operation throughout the entire life span.

Aging is a deprivation syndrome driven by a germ-soma conflict

Evolution through natural selection can be described as driven by a perpetual conflict of individuals competing for limited resources. Recently, I postulated that the shortage of resources godfathered the evolutionary achievements of the differentiation-apoptosis programming [Rev. Neurosci. 12 (2001) 217]. Unicellular deprivation-induced differentiation into germ cell-like spores can be regarded as the archaic reproduction events which were fueled by the remains of the fratricided cells of the apoptotic fruiting body. Evidence has been accumulated suggesting that conserved through the ages as the evolutionary legacy of the germ-soma conflict, the somatic loss of immortality during the ontogenetic segregation of primordial germ cells recapitulates the archaic fate of the fruiting body. In this heritage, somatic death is a germ cell-triggered event and has been established as evolutionary-fixed default state following asymmetric reproduction in a world of finite resources. Aging, on the other hand, is the stress resistance-dependent phenotype of the somatic resilience that counteracts the germ cell-inflicted death pathway. Thus, aging is a survival response and, in contrast to current beliefs, is antagonistically linked to death that is not imposed by group selection but enforced upon the soma by the selfish genes of the "enemy within". Environmental conditions shape the trade-off solutions as compromise between the conflicting germ-soma interests. Mechanistically, the neuroendocrine system, particularly those components that control energy balance, reproduction and stress responses, orchestrate these events. The reproductive phase is a self-limited process that moulds onset and progress of senescence with germ cell-dependent factors, e.g. gonadal hormones. These degenerate the regulatory pacemakers of the pineal-hypothalamic-pituitary network and its peripheral, e.g. thymic, gonadal and adrenal targets thereby eroding the trophic milieu. The ensuing cellular metabolic stress engenders adaptive adjustments of the glucose-fatty acid cycle, responses that are adequate and thus fitness-boosting under fuel shortage (e.g. during caloric restriction) but become detrimental under fuel abundance. In a Janus-faced capacity, the cellular stress response apparatus expresses both tolerogenic and mutagenic features of the social and asocial deprivation responses [Rev. Neurosci. 12 (2001) 217]. Mediated by the derangement of the energy-Ca(2+)-redox homeostatic triangle, a mosaic of dedifferentiation/apoptosis and mutagenic responses actuates the gradual exhaustion of functional reserves and eventually results in a multitude of aging-related diseases. This scenario reconciles programmed and stochastic features of aging and resolves the major inconsistencies of current theories by linking ultimate and proximate causes of aging. Reproduction, differentiation, apoptosis, stress response and metabolism are merged into a coherent regulatory network that stages aging as a naturally selected, germ cell-triggered and reproductive phase-modulated deprivation response.

Approach of evolutionary theories of ageing, stress, senescence-like phenotypes, calorie restriction and hormesis from the view point of far-from-equilibrium thermodynamics

B. L. Strehler wrote that "Any system that is not in thermodynamic equilibrium will approach that state at a rate that is a function of absolute temperature and the energy barriers to the rearrangements of components". Far-from-equilibrium thermodynamics allows a global systemic description of the cellular behaviour. This approach transcends the genetic and stochastic considerations on ageing as well as some evolutionary questions about ageing. The fundamental difference between the processes of development and ageing could reflect the intrinsic differences existing between biological systems where an increase in specific entropy production (SEP) is, respectively, still possible or not. The increase of the potential of SEP which probably occurred with evolution might explain in part why life span could increase. However, this SEP-driven increase in life span was possible only in those species which did not take advantage of their increased potential of SEP to ameliorate their reproductive capacity at the expense of possible increases in repair capacity. The criteria of stability of far-from-equilibrium open systems and the theory of attractors also help to sort the possible types of cellular stress responses: normal ageing, hormesis, stress-induced premature senescence, apoptosis or necrosis.

Beneficial influences of systemic cooperation and sociological behavior on longevity

During his long research career in the field of aging, Dr Bernard Strehler developed a series of theories concerning the identity of genes that can promote longevity and their role in natural selection. As a tribute to Dr Strehler, we have taken this opportunity to summarize a selection of these theories and to illustrate how these insights have influenced our search for longevity genes within the immune system. The identification of longevity genes has proven difficult. We believe that, at least in part, this reflects the emphasis on the concept of survival of the 'physically' fittest. We have used the immune system as a model to demonstrate that, over and above the self-evident advantage of those genes that contribute the attributes commonly associated with survival of the 'physically' fittest, those genes that lead to a predisposition to cooperate also confer a competitive survival advantage. As the acquisition of cooperativity in a society is linked to support mechanisms provided by older individuals, the search for longevity genes should not be limited to those genes that are associated with extended expression of a youthful phenotype. Rather these studies should be expanded to include identification of those genes that regulate physiologic parameters that affect individual longevity, even if they do not correspond with the traditional view of reproductive competitiveness. At the societal level, longevity genes may encode attributes that regulate sociologic or psychological parameters that may contribute to a tendency to non-aggressive or cooperative behavior that leads to achievement of common goals necessary for the survival of the species. This view of the selection for longevity impacts the analysis of longevity genes and aging at the organismal level. Dr Strehler viewed organismal aging as an integrated functional state, in which he conceived the outcome as reflecting the net balance of functional decrementers and evolved compensatory features. We propose that, in more evolved species, the longevity genes will be those genes, or sets of genes, that counterbalance of age-related functional decrementers with the age-related manifestation of evolved compensatory features. Thus, as illustrated here through analysis of the immune system, the longevity genes may well be those genes that promote overall systemic cooperation and compensation within the immune system and associated systems, rather than the genes that prevent age-related alterations in only one or a limited number of pathways.

Evolution of ageing

Explaining why ageing occurs is a solution to the longstanding enigma of the role of senescence in nature. Even after half a century of progress, this solution continues to unfold. Evolution theory argues strongly against programmed ageing, suggesting instead that organisms are programmed for survival, not death. In the current view, ageing results from the twin principles that (i) the force of natural selection declines with age, and (ii) longevity requires investments in somatic maintenance and repair that must compete against investments in growth, reproduction and activities that might enhance fitness. In addition to explaining why ageing occurs, the evolutionary theory also provides insight into the mechanisms underlying the complex cellular and molecular changes that contribute to senescence, as well as an array of testable predictions. Some of the most interesting current problems are to understand how the genetic factors influencing ageing and longevity are predicted to respond to fluctuating environments, such as temporary periods of famine, as well as to other kinds of spatial and/or temporal heterogeneity. Rapid progress in human genomics raises the prospect of greatly increasing our knowledge of the determinants of human longevity. To make progress in understanding the role and evolution of genetic and non-genetic factors in human longevity, we need more detailed theoretical studies of how intra-population variables, such as socio-economic status, influence the selection forces that shape the life history.

Calorie restriction and aging: a life-history analysis

The disposable soma theory suggests that aging occurs because natural selection favors a strategy in which fewer resources are invested in somatic maintenance than are necessary for indefinite survival. However, laboratory rodents on calorie-restricted diets have extended life spans and retarded aging. One hypothesis is that this is an adaptive response involving a shift of resources during short periods of famine away from reproduction and toward increased somatic maintenance. The potential benefit is that the animal gains an increased chance of survival with a reduced intrinsic rate of senescence, thereby permitting reproductive value to be preserved for when the famine is over. We describe a mathematical life-history model of dynamic resource allocation that tests this idea. Senescence is modeled as a change in state that depends on the resources allocated to maintenance. Individuals are assumed to allocate the available resources to maximize the total number of descendants. The model shows that the evolutionary hypothesis is plausible and identifies two factors, both likely to exist, that favor this conclusion. These factors are that survival of juveniles is reduced during periods of famine and that the organism needs to pay an energetic "overhead" before any litter of offspring can be produced. If neither of these conditions holds, there is no evolutionary advantage to be gained from switching extra resources to maintenance. The model provides a basis to evaluate whether the life-extending effects of calorie-restriction might apply in other species, including humans.

Understanding ageing

A broad biological approach makes it possible to understand why ageing exists and also why different mammalian species have very different maximum longevities. The adult organism is maintained in a functional state by at least ten major mechanisms, which together comprise a substantial proportion of all biological processes. These maintenance mechanisms eventually fail, because the evolved physiological and anatomical design of higher animals is incompatible with continual survival. The lifespan of each mammalian species depends on the efficiency of maintenance of their cells, tissues and organisms, and there is much evidence that such maintenance is more effective in long-lived species, such as man, than in short-lived small mammals. It is also evident that there is an inverse relationship between reproductive potential and longevity, which would be expected if total metabolic resources are shared between investment in reproduction, and investment in the preservation of the adult body. It is proposed that the eventual failure of maintenance leads to the pathological changes seen in age-associated disease. Although we now have a biological understanding of the ageing process, much future research will be needed to uncover the cellular and molecular changes which give rise to age-associated diseases. The major aim of such research is to devise procedures to delay or prevent the onset of these diseases.