Przegląd A ntropologiczny • tom 60, s. 3 5 -4 6 , Poznań 1997
A review of anthropological approaches to ageing
Maria Kaczmarek, Anita Szwed
Abstract
It is evident that the pattern o f ageing am ong hum ans has a unique character. Therefore, w hen undertaking any re search on hum an ageing one has to specify a proper m ethodology and m ethods w hich are available in th e anthropo logical perspective. The paper is aim ed at providing a review o f anthropological approaches to th e study o f ageing. On the basis o f the m eaning and scope o f the concept o f ageing, its sources and causal factors are discussed. Further, functional, physiological and m orphological indicators o f ageing are briefly described. M uch attention is focused on the concept o f biological age w hich is the key notion for assessm ent o f variation in the rates o f hum an ageing.
M aria K aczm arek, A nita Szwed, 1997; Anthropological Review, vol. 60, Poznan 1997, pp. 3 5 -4 6 , figs 2, tables 2. ISBN 83-86969-18-0, ISSN 0033-2003
“Eos, a mythological goddess, asked Zeus to allow Tithonus, the m ortal she loved, to live forever. Zeus granted Tithonus immortality, and the lovers lived happily — but not ever after. Tithonus began to grow old until he became so infirm that he could not move. Yet he was denied the gift o f death, and to this day he lives on, long after Eos had finally left him, 'a helpless, drivelling vegetable ’. Eos had made a grievous error - she had forgotten to ask Zeus to grant her lover eternal youth along with eternal life... ”
(from Greek Mythology)
The meaning and scope of ageing
An organism does not remain the same biologically over its life span. The structure and function o f one’s biological processes undergo various changes. Some o f these changes are termed growth, some development, others matu ration or ageing. Although these proc esses occur simultaneously, they have slightly different biological meaning. Growth is defined as an irreversible,
Institute o f Anthropology UAM Fredry 10, 61-701 Poznań
quantitative increase in size or mass, involving the production o f new proto plasm. Development is defined as a pro gression o f changes, either quantitative or qualitative, that lead from an undiffer entiated state to a well integrated, highly organized, matured form. M aturation in this approach means a functional capac ity o f the organs and the entire organism
[BOG IN 1993:2], W hen defining ageing, one should consider two quite distinct approaches to the definition. The first is through an individual level and describes a set o f deleterious changes which occur primarily in the postreproductive period.
36 M aria K aczm arek, A nita Szwed They are manifested as a gradual decline
in a variety o f body functions. The other way o f defining ageing refers to the population level and means that the overall process o f all deleterious changes affects the ability o f the organism to survive, which is m anifested in the in crease o f an age-specific death rate
[Co m f o r t 1979, St r e h l e r 1978, Ki r k w o o d, Ho l l i d a y 1986], This is strictly
meaningful only for a species where the individual organism is clearly defined and where the individuals are capable o f reproducing repeatedly during their life span. M oreover, it is noteworthy that ageing is not intrinsic to all living organ isms. For instance, due to some indefi nite regenerative mechanisms, Procary otic and many Eucaryotic microorgan isms are able to escape senescent changes [COM FORT 1979],
Going further into our considerations, the population concept o f ageing may be introduced in terms o f a survivorship pattern, as shown in Fig. 1. The pattern is well illustrated with an age-specific m ortality rate curve and a survival curve which are plotted against the lifespan o f a typical human population. Both curves, presented in Fig. 1 are closely approxi mated by a simple exponential (Gom- p ertzian curves) [G O M PERTZ 1825 quoted after K IR K W O O D , H O LLIDAY
1986, COMFORT 1979],
It is evident from Fig. 1 that the pat tern o f survivorship and mortality rate in humans is characterized by reasonably high survival and low mortality for the earlier part o f the lifespan, followed by, respectively, a steady decline and in crease thereafter, until the age when survivorship reaches zero and the mor tality rate reaches its maximum. This age has a constant value for the species.
In-% SURVIVAL DEATH RATE
Fig. 1. Survivorship (continuous line) and m ortality rate (dashed line) as function o f age for a typical human
society
tuitively, underlying the population con cept o f ageing is the idea that the process o f ageing involves the entire life course. In this instance, growth may be seen as a part o f ageing. According to Riley: “Ageing is a life-long process o f growing up and growing old. It starts with birth (or conception) and ends with death... Ageing consists o f three sets o f processes - biological, psychological, and social; and these three processes are all sys tem atically interactive with one another over the life course” [RILEY 1979:4],
The process o f ageing is familiar in human societies and has attracted many investigators for a long time. The unique pattern o f ageing in humans, viz. the extent to which ageing is manifested and the existence o f a clearcut menopause, demands an appropriate methodology and techniques for the assessment o f this process. The anthropological perspective o f this survey reveals the nature and pe culiarities o f human ageing as far as the individual and the population are con cerned. It also displays the associations o f life-style, well-being and nutritional status with determinants o f variation in rates o f ageing. This article provides
A review o f anthropological approaches to ageing 37 readers with an overview on the meth
odology and methods of the ageing re search in modern human populations.
Why do we age? The origin of
ageing
Considering the idea that changes as sociated with biological ageing are those associated with a lessened chance of survival, one may ask about the causes o f this decreasing survival potential o f the population. There are a number of bio logical theories which focus on the proper answer to this question. However, it seems that each of these theories o f ageing is incomplete as it offers a suc cessful explanation o f only a portion of the ageing phenomenon, moreover, none of them has achieved general acceptance. Nevertheless, they are not mutually ex clusive, and there is a great deal of over lap among them. Furthermore, they share the common characteristics which have a genetic basis [SHOCK 1977]. Support for the genetic basis o f ageing comes from Hayflick’s experiments which show that certain cells of the body, grown in vitro, are only able to divide a limited number of times. He observed that the older the individual from whom the cell samples were obtained, the fewer doublings the cells would undergo. These two facts corroborate the concept o f the genetic basis of ageing [HAYFLICK 1965]. In most o f the theories on biological ageing two basic lines of thought may be distin guished. The first represents the deter ministic approach and postulates that ageing is an innate, genetically pro grammed process and as such is subject to only minor modifications. The other one follows the stochastic approach and postulates that ageing is the result o f the
accumulation o f harmful products of metabolism in tissues. These two meth odologically different approaches have different implications. If ageing is con sidered to be a programmed phenome non, nothing much can be done to mod ify this process. On the other hand, if ageing is taken to be a consequence of stresses, it may be possible to modify the life pattern by eliminating some o f those stresses. It seems that the truth is likely to lie in a combination o f these two ap proaches [Pr i n z i n g e r 1996].
While it is clear that ageing has a ge netic basis, there are a large number of various causes indicated as the most es sential for this process. Considering this, the biological theories may be classified into three major categories: genetic cellu lar theories, nongenetic cellular theories, and physiological theories [SHOCK
1977]. According to the genetic cellular theories, the major causes o f ageing may be perceived in the damage to the genetic information involved in the formation of cellular proteins. Several theories have argued that it is the breakdown o f these basic genetic mechanisms which cause ageing [ORGEL 1963, CURTIS 1966,
Ha h n 1970, Sin e x 1974, St r e h l e r 1978], The nongenetic cellular theories focus their interest on changes that take place in the cellular proteins after they have been formed. They suggest that ageing results from the accumulation of deleterious substances in the cells of the organism, or from damage to cell pro teins (Ha r m a n [1954], Failla 1958, Szilard 1959; cited after PRINZINGER
[1996], B J0R K STEN [1968]). A set of physiological theories suggest that age ing results from the failure o f some physiologically coordinating system, such as the immunological or endocrine
38 M aria K aczm arek, A nita Szwed systems, to integrate bodily functions
properly [FINCH 1976]. W hile each o f these theories focuses on a different as pect o f the ageing process, they may be found to share some common points. According to Strehler, the integrations are as follows: the loss o f the ability o f a cell to divide which is programmed in the genetic code o f the cell, the loss o f some o f the copies o f a group o f genes responsible for protein synthesis essen tial for cell repairing from environmental damage, an increased accumulation o f damaged enzymes and lipofuscin which impair the cell function, and decreased normal functioning at the cellular level. These processes lead to the decreased integration among the physiological sys tems o f the body; ultimately, the systems break down and the individual dies
[ St r e h l e r 1978].
W hatever hypothesis on ageing is taken into consideration, it is true that ageing is one o f the great paradoxes o f nature. As W illiams states, “It is indeed remarkable that after a seemingly miracu lous feat o f morphogenesis a complex metazoan should be unable to perform the much sim pler task o f merely main tainin g w hat is already form ed”
[ Wi l l i a m s 1957:398]. An attempt at resolving this problem refers to the ex planation in terms o f evolutionary proc ess, e.g. natural selection (W eismann
1891, cited after Ki r k w o o d, Cr e m e r
[1982]). There are two possible explana tions: either adaptive or non-adaptive in nature. I f ageing is seen as a beneficial trait in its own right, the explanation can be labelled as an adaptive one. If ageing is seen as detrimental, or at best neutral, and so its evolution explained indirectly, the explanation is non-adaptive in char acter. Explicit formulations o f the adap
tive view are rare, despite the fact that this type o f theory is probably the more popular. The difficulty is that, for the individual, ageing is clearly disadvanta geous, since senescence must reduce life expectancy and hence diminish the op portunity for reproduction. Among the beneficial characteristics o f ageing is the fact that this process accelerates the turnover o f generations and thereby in creases the chance o f a species adapting to a change in its environment, whereas without ageing the species might over crowd its environment and exhaust its resources [ Ki r k w o o d, Cr e m e r 1982, St e a r n s 1992, Pr i n z i n g e r 1996],
Considering non-adaptive theories o f the evolution o f ageing, again two lines o f thought may be distinguished: one suggesting that natural selection is sim ply unable to prevent the deterioration of older organisms because it becomes at tenuated with age, and another sugges tion that ageing is a by product o f selec tion for other beneficial traits. These two views share a common principle origi nally pointed out by Haldane and later clearly enunciated by Medawar which asserts that the force o f natural selection reduces progressively with age [H AL DANE 1941, Me d a w a r 1981], The rea
son for this is that selection acting early in life will affect a greater proportion o f individuals than genes acting late, when the proportion o f survivors is smaller and the remaining fraction o f their lifetime expectation o f reproduction is less. M edawar proposed that the attenuation o f the force o f natural selection with age was sufficient by itself to explain the evolution o f ageing. He suggested that selection acting on genes with age- specific times o f expression would tend to defer the age o f expression o f harmful
A review o f anthropological approaches to ageing 39 genes, so as to minimize their potential
for deleterious effects. However, there may be an accumulation o f deleterious genes which in the normal environment would combine to handicap any individ ual severely. This is a process, as Medawar claims, which could be ac counted for by the evolutionary origin o f senescence.
The second sub-type o f non- adaptive theory, that ageing is a by product o f selection for other benefi cial traits, was form ulated in general term s by W ILLIA M S [1957], W illiam s’ theory was derived from an argum ent sim ilar to M edaw ar’s, except for the crucial difference that the genes in question w ere assum ed to be pleio- tropic; the same genes having both, good effects in early life and bad ef fects in late. It w as argued that natural selection should favour retention o f the genes on the basis o f their benefits, but defer as far as possible the tim e o f the expression o f their deleterious effects to ages when survivorship w as low. In W illiam s’ theory, ageing was attrib uted to the positive action o f selection on the pleiotropic genes, but w as not in itse lf regarded as beneficial. In con clusion he stated th at even in the losses caused by environm ental m ortality the events occurring at later ages had lesser evolutionary significance.
Among the non-adaptive theories, t one which offers the most sp e d mechanism to account for the gene: evolution o f ageing, is the disposal soma theory [KIRKWOOD, 1 9 7 7 ] . T1
theory postulates that the optimum
i
vestment o f resources in somatic main nance and repair is always less than t minimum which would be required 1 indefinite somatic survival. In con:quence, ageing results from a progressive accumulation o f somatic defects and damage.
Table 1 gives a summary o f the dis cussed explanations o f the origin and causes o f ageing.
How do we age? A defence of a
multifactorial concept of ageing
Ageing is a m ultifactorial process which takes place sim ultaneously at each level o f the living structure in the course o f an individual life. The distinguished levels are: subcellular, cellular, tissue, organs, individual and population
[SU SA N N E 1986]. There are structure specific manifestations o f the process o f ageing which are presented in Table 2.
Considering some o f the molecular, cellular and physiological changes that may be responsible for the growth and ageing processes, it may be noted that these changes affect virtually all major systems o f the human body: skeletal, muscle, skin, pulmonary, cardiovascular, neural, endocrine, reproductive, gastroin testinal, and excretory. It is quite evident that, although biological changes associ ated with growth and aging are inevitable and universal they do not affect all the systems equally. There is a wide varia
rle-40 M aria K aczm arek, A nita Szwed
T a b le 1. Sum m arized representation o f the theories o f causal factors and origin o f ageing
1. C a u s e s o f A g e in g
S to ch a stic approach D e te rm inistic approach
Thesis: Ageing is an accidental consequence of
depietionai and accretionai processes
grammed process
Thesis: Ageing is an innate, genetically pro
The tear and wear theory [Pearl 1927] The absolute metabolic theory [Rubner 1908] The free radicals theory [Harman 1956] The cell doubling theory [Hayflick 1969] The collagen-/cross-Hnkages theory
[VerzAr 1957]
The somatic mutation theory [Failla 1958, Szilard 1959]
The error and fidelity theory [Orgel 1963] The immune theory [Walford 1969]
2. O r ig in o f A g e in g
A da ptive approach Non-adaptive approach
Thesis: Ageing is a benefi
cial trait in its own right
be explained indirectly
Thesis: Ageing is detrimental, or at best neutral; its evolution must
The force of natural selection reduces progressively with age and is unable to prevent the deterioration of older organisms The evolutionary origin of senescence theory:
[M e d a w a r 1982]
Ageing is a by-product of selection for other beneficial traits
The pleiotropic origin of senescence theory:
[W illia m s 1957]
The disposable soma theory:
[K ir k w o o d , H o ll id a y 1979]
T a b le 2. Indicators o f the process o f ageing at various levels o f a living organism
Level Manifestation
Subcellular Biochemical changes in various subcellular structures Cell Changes or loss of cells
Tissue Histochemical changes in tissue Organ Decrease in functional ability
Individual Decrease in the adaptive ability to stress Population Increased probability of death
breathing capacity, maximum work rate and maximum oxygen.
There have been numerous cross- sectional analyses o f age-specific size and physique changes designated as the processes o f ageing Data from longitudi nal studies indicate the fourth decade o f life as the critical point when decrement
in stature begins, but there is o f course considerable variability in the time o f the onset o f stature decline. It has been hy pothesized that the decline in stature with age results from vertebral body fractures, compression o f intervertebral discs, and postural changes [FRIEDLANDER et al. 1977, BORKAN et al. 1980], There is also
A review o f anthropological approaches to ageing 41 % OF PERFORMANCE % DECREMENT
Fig. 2. S chem atic linear representation o f the decline in various physiological functions w ith age [Sh o c k 1977]
a longitudinal decline with age in meas urements o f the extremities and other body length and breadth dimensions
[D AM O N 1972], A secular trend was found in this process [H lM E S, MUELLER
1982, BORKAN et al. 1983]. The effects o f the secular trend are also found in body size.
Senescent changes in body weight display a clear pattern o f sexual dimor phism. While in males a decrease in body weight begins after the age o f 55, in fe males it starts at a later age and advances more slowly thereafter [D A M O N et al. 1977, ROSSM AN 1977]. The percentage o f body fat increases into late adulthood, although in absolute terms fat remains rather constant while the lean body mass decreases, thus accounting for the in crease in relative fatness [NORRIS et al. 1963, Ma l i n a 1969, Ro s s m a n 1977,
Pa r i z k o v a, Ei s e l t 1980]. Despite the
relative constancy o f absolute fat mass, there are changes in fat distribution throughout the body. There is an age
specific tendency to a larger amount o f intra-abdominal fat and a thinning o f subcutaneous trunk fat [G A R N & Y O U NG
1956, Bo r k a n & No r r i s 1977, Mu e l l e r 1982].
The results o f numerous works dem onstrate a substantial modification o f the skeleton in late adulthood characterized by considerable loss o f bone mass (osteoporosis). The causes o f osteoporo sis are presently unknown, although a number o f explanations are being pro posed. As the incidence o f osteoporosis is considerably higher in females, there are some suggestions o f a link between bone loss and estrogen, a link between postmenopausal calcium loss through the parathyroid hormone and between degree o f bone loss and growing sensitivity to the parathyroid hormone with age
[W lSK E et al. 1979].
The demography o f ageing is also a domain o f anthropological interest. It is a specific pattern o f the human demo- graphical structure that females outlive males [BlELICKI et al. 1994]. Among living creatures, humans are those who have the highest lifespan potential. An ageing population is defined as one in which the triangle-shaped demographic pyramid moves to a more rectangular one as a consequence o f the increase in the mean age o f its members over time. There is a considerable variation in life expectancy (as opposed to the life span) among contemporary human populations, even when the distorting effects o f infant mortality are removed. It is also claimed that the longer life span in women has no perceptible influence on the upper age limit o f fertility. According to Malina, the age at menopause has remained in the 45 to 50 range at least since the Graeco- Roman times [Ma l i n a 1979].
42 M aria K aczm arek, A nita Szwed
An assessment of variation in the
rates of human ageing
A major goal in biological gerontol ogy is the description of normal ageing. The traditional belief that disease is pathological while ageing is normal is beginning to be questioned by human biologists, since the precise point at which several conditions constitute a disease is problematic and the boundary between disease and normality is very subtle (for instance atherosclerosis which is present in may individuals at the age of thirty). It is emphasized that every physiological process involved in the maintenance o f homeostasis becomes less effective with increasing age. It is characteristic o f biological systems that each has a certain redundancy o f func tional capacity or a functional reserve. There is also a minimum level required to sustain a death threshold (cut-off point). When vitality or functional ca pacity drop below this critical point death occurs. The level o f the death threshold reflects the severity o f the conditions to which an individual is ex posed. As the environmental challenges and stresses that individuals cope with are never constant, the death threshold fluctuates around a mean value. It is logical, that if conditions are severe, the threshold will be higher, and life shorter. The process o f ageing can be modified by an appropriate intervention, such as lifestyle, diet, or activity. The extent of interventions can influence the rate of ageing. The latter may vary considerably among individuals. The understanding of such interpersonal variations may reveal the special characteristics of individuals who retain their functional capacities to age slowly or fast. Estimating the rate of
ageing of an individual is of considerable interest because it is important for identi fying basic ageing processes, and thus for preventive medicine. The most pre valent approach to the assessment of ageing in individuals is the concept of biological age, widely used by auxolo- gists for assessing normal child growth
[TANNER 1992], The basic point o f this concept is that the normal sequence of developmental processes is provided by chronological age. The index o f biologi cal age in adults has a slightly different interpretation, as it must reflect the prob ability o f death at any chronological age; this probability increases with chrono logical age after about the first year of life [Br o w n, Fo r b e s 1976]. Some mathematical models, the multiple re gression procedure, have been proposed which relate the progressive decline in physiological functions to the increased risk o f death with age [BROW N, FORBES
1974, FURUKAWA et al. 1975], These models are built under the assumption that there is a risk factor, corresponding to an index o f biological age, the mean value of which may change with age for the population. There is also a cut-off level in the distribution o f the parameter. The individuals for whom those values lie on the unfavourable side of this cut off level are at an increased risk of dis ease or death. However, the multiple re gression analysis approach has met with severe criticism [COSTA, McCr a e 1980]
Another approach to the estimation of biological age o f adult is to transform data from any battery test in the pattern profile system [BORKAN, NORRIS 1980]. Twenty four physiological, morphologi cal and functional parameters were se lected from the data bank as a represen tative subset o f the characteristic changes
A review o f anthropological approaches to ageing 43 o f ageing. The criteria for selection were
that the variables had a positive or nega tive linear trend with age, good reliability and reflected a wide range o f physical functions. Analyses were performed us ing data on lifestyle to delineate sub groups for comparison as mean biologi cal age profiles. The results obtained indicate that men who engaged in organ ized physical activity were biologically more youthful than those who did not. Other comparison showed that being married and better educated were also associated with greater biological youth fulness [BlELICKI et al. 1988, ROGUCKA
1995]. This approach, although success ful in comparison between different groups is limited in use, since the data come from cross-sectional studies. The data from cross-sectional study are effi cient for description o f the status o f bio logical phenomena at a particular mo ment o f time but not for the rate o f the ageing processes. The latter one requires some additional assumptions and is based on the data from longitudinal ex amination o f actual change with age. Although, such studies are known to be difficult and time consuming but may provide evidence on how to delay dele terious changes.
Summarizing, it may be said, that the ageing processes in human species are a subject o f great popular interest and an thropological approaches to the study o f ageing seem to offer considerable poten tial for the future.
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Streszczenie
Pragnienie nieśm iertelności i tęsknota za w ieczną m łodością w yrażona, m iędzy innym i w zacytow anym n a w stę pie fragm encie m itologii greckiej, a rów nocześnie św iadom ość nieuchronności procesów starzenia się i śm ierci pasjo now ały ludzi od zarania dziejów. Proces starzenia się je s t doskonale znany we w szystkich populacjach ludzkich, a ujęcie antropologiczne w ram ach gerontologii, nauki o starzeniu się i starości żyw ych organizm ów , pozw ala poznać naturę i osobliw ości procesu starzenia się człowieka, je j źródła i przyczyny oraz im plikacje tego procesu d la jed n o stek i społeczeństw a. Celem niniejszej pracy je s t przedstaw ienie koncepcji w yjaśniających źródła i przyczyny procesu starzenia się organizm ów w ujęciu antropologicznym .
O rganizm w toku ontogenezy podlega ciągłym zm ianom , których przejaw em je s t w zrost, rozwój, dojrzałość, sta rzenie się. W praw dzie procesy te przebiegają rów nocześnie, jed n ak że ze w zględu n a istotę leżących u ich podstaw zjaw isk biologicznych przyjęto d la nich odrębne definicje. W zrost określa się ja k o nieodw racalne zm iany natury ilościowej, w łączając w to produkcję plazm y, prow adzące do zw iększania się m asy lub w ym iarów danego organizm u. Rozwój je s t określany ja k o progresyw ne zm iany ilościow e lub jakościow e, które prow adzą od prostej form y do w yso ko zorganizow anej form y dorosłej. Stan, w którym organizm osiąga form ę d orosłą nazyw a się dojrzałością. T ak więc, dojrzałość oznacza funkcjonalną zdolność organów i całego organizm u do pełnienia określonych funkcji. Starzenie się żywego organizm u określa się ja k o pow olne, nieodw racalne zm iany koloidalnej struktury m aterii. W yrazem tych zm ian są procesy degeneracyjne pow odujące osłabienie czynności układów enzym atycznych i horm onalnych. O bja wem tych zaburzeń są zm iany czynności w szystkich układów ustroju, takich ja k układ nerwowy, horm onalny, krąże nia, oddychania. U jm ując naturę procesów starzenia się w perspektyw ie antropologicznej, zjaw isko to opisuje się i w yjaśnia na poziom ie osobniczym oraz n a poziom ie populacyjnym . Starzenie się osobnika określa się n a podstaw ie zm ian degeneracyjnych, które p ojaw iają się przede w szystkim w okresie postreprodukcyjnym . P rzejaw iają się one stopniow ym spadkiem spraw ności różnych funkcji życiowych. D rugie ujęcie odw ołuje się do poziom u populacyjnego i zgodnie z tym podejściem starzenie się oznacza całość procesów destrukcyjnych, które w efekcie prow adzą do zw iększania się praw dopodobieństw a śm ierci. Starzenie się nie je s t w łaściw ością im m anentną w szystkim organizm om żywym. Procaryota i proste Eucariota nie podlegają procesom starzenia się, gdyż w ytw orzyły m echanizm y regeneracji, które um ożliw iają im nieograniczony rozwój. Populacyjną koncepcję procesu starzenia się organizm u m ożna przed stawić graficznie ja k o krzywe przeżyw ania i um ieralności. N a rys. 1 zaprezentow ano krzyw e przeżyw ania i w ym iera nia, w zależności od w ieku, dla przeciętnej populacji ludzkiej. Krzywe te o dzw ierciedlają specyficzne dla określonych kategorii w ieku tem po w ym ierania oraz przeżyw ania; w pierw szym okresie życia człow ieka charakteryzuje się ono wysokim stopniem przeżyw alności i n isk ą wym ieralnością, następnie stopniow ym i stałym spadkiem przeżyw alności oraz w zrostem um ieralności aż do w ieku, w którym przeżyw alność osiąga zero a w ym ieralność osiąga sw oje m aksi mum. Ten w iek m a sta łą w artość d la określonych gatunków . M ożna w ięc pow iedzieć, że starzenie się je s t procesem przebiegającym w ciągu całego życia osobniczego, w raz z w iekiem rośnie praw dopodobieństw o śm ierci osobnika. Zaproponow ano w iele teorii objaśniających przyczyny procesu starzenia się organizm u, je d n a k żadna z proponow a nych teorii nie je s t kom pletna i nie w yjaśnia tego zjaw iska w pełni. W szystkie proponow ane w yjaśnienia łączy to, że odw ołują się do genetycznej determ inacji procesu starzenia się, przy czym je d n e z nich u jm u ją ten proces ja k o deter m inistyczny, a w ięc nieunikniony, inne ja k o stochastyczny. Z godnie z pierw szym podejściem badaw czym proces starzenia się nie je s t m odyfikow alny i jakiekolw iek działania w kierunku opóźnienia procesu starzenia się człow ieka skazane są n a niepow odzenie. Przyjm ując, iż proces starzenia się m a charakter stochastyczny, m ożliw a je s t wówczas jego m odyfikacja; zależy o n a od stopnia zagrożenia organizm u przez w arunki środow iska, w ja k im ten organizm się rozwija. W śród postulow anych teorii w yjaśniających źródła i przyczyny starzenia się organizm u m ożna w yróżnić trzy kategorie: teorie genetyczne, niegenetyczne i fizjologiczne. Teorie genetyczne starzenia się organizm u u p atru ją źródło przyczyn starzenia się w uszkodzeniach m ateriału genetycznego, teorie niegenetyczne natom iast postulują, iż przyczy n ą starzenia się je s t nagrom adzenie substancji szkodliw ych dla organizm u, które w daw kach w ja k ic h w ystępują przyczyniają się do rozpadu organizm u, teorie fizjologiczne sugerują że starzenie się je s t w ynikiem degeneracji syste m ów fizjologicznych (im m unologicznego, endokrynnego).
W ujęciu ew olucyjnym starzenie się je s t zjaw iskiem paradoksalnym , którego biologia ew olucyjna nie potrafi przekonująco wyjaśnić. Istnieją dw a m ożliw e wyjaśnienia: adaptacyjne lub nieadaptacyjne. Jeśli przyjąć, że proces starzenia się je s t korzystny dla populacji, w yjaśnienie będzie m iało charakter adaptacyjny. Jeśli u podstaw w yjaśniania leży założenie, że starzenie się je s t procesem szkodliw ym lub co najwyżej neutralnym , w ów czas je s t to w yjaśnianie nieadaptacyjne. W yjaśnianie adaptacyjne opiera się n a stw ierdzeniu, ze starzenie się przyspiesza w ym ianę pokoleń, tym samym zapobiega przeludnieniu i w yczerpaniu zasobów środow iska. Teorie nieadaptacyjne postulują, że starzenie się je s t ubocznym produktem selekcji dla innych korzystnych cech (teorie W illiam sa, M edaw ara). Interesującą kon
46 M aria K aczm arek, A nita Szwed
cepcja je s t propozycja K irkw ooda („disposable som a theory”), w której postuluje się, że optym alna inwestycja w zachow anie som atycznej rów now agi i reperacji szkód je s t zaw sze m niejsza niż m inim um , które bytoby wymagane do nieskończonego trw ania organizm u. O m aw iane wyżej teorie starzenia się prezentuje tab ela 1.
Proces starzenia się organizm u przebiega w różnym tem pie i n a różnych poziom ach, od struktur kom órkowych do poziom u populacyjnego. T ę w ielopoziom ow ość procesu starzenia się ilustruje tabela 2. T em po procesu starzenia się organizm u o cen ia się n a podstaw ie kryterium m orfologicznego, fizjologicznego i funkcjonalnego, na przykład, spadek w ydolności organizm u z w iekiem określany na podstaw ie funkcji fizjologicznych poszczególnych układów przedsta w iono schem atycznie n a rys. 2. Stopień w yrażenia cech stanow iących podstaw ę w ym ienionych kryteriów oceny procesu starzenia się b ad a się w ujęciu przekrojow ym lub podczas obserw acji długofalow ych. B adania przekrojowe pozw alają w nioskow ać o stanie organizm u w określonym m om encie czasow ym , n atom iast dynam ikę procesu starze n ia się m ożna określić tylko i w yłącznie n a podstaw ie obserw acji z badań długofalow ych. S ą one trudne do realizacji, czasochłonne i kosztow ne ale coraz częściej postuluje się tego typu badania aby m óc odpow iedzieć n a pytania, na przykład w ja k im stopniu określony styl życia przyczynia się do przyspieszania lub opóźniania procesów starzenia się.
Podstaw ę teoretyczną oceny procesu starzenia się stanow i koncepcja w ieku biologicznego, który w odniesieniu do fazy stabilnej i inw olucyjnej m a inne znaczenie niż w odniesieniu do fazy progresyw nej. W iek rozwojowy w odnie sieniu do fazy stabilnej i inw olucyjnej ontogenezy człow ieka pojm uje się ja k o zw iększające się praw dopodobieństw o śm ierci osobnika, em pirycznie w yznaczane za po m o cą m odelu regresji w ielokrotnej. Ten sposób postępow ania pod dano krytyce [C osta i M cC rae 1980] a Borkan i N orris [1980] zaproponow ali m etodę tak zw anych profili. W m eto dzie tej w ykorzystuje się k ilk a z określonego zestaw u cech fizjologicznych, d la których w ykreśla się profile grupując osobników w zależności od interesującego nas aspektu (np. aktyw ni fizycznie, nieaktyw ni).
W podsum ow aniu pow yższych rozw ażań należy stw ierdzić, że m im o niedoskonałości koncepcji oraz metod słu żących do oceny stanu i dynam iki procesu starzenia się człow ieka, antropologia oferuje now e perspektyw y badawcze tego fascynującego zjaw iska.
