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Human Physical Growth – Anthropology Notes – For W.B.C.S. Examination.
মানব দেহের দৈহিক বৃদ্ধি – নৃতত্ত্ব নোট – WBCS পরীক্ষা।
As the physical anthropologist seeks to understand the meaning of the striking range of variability which confronts him he is constantly faced with the importance of the processes of physical growth and development. In addition to the obvious fact that the size and shape of the adult are the direct result of his own unique pattern of development, there are a number of other ways in which ontogeny (i.e., development) is responsive to the evolutionary forces which affect human populations.Continue Reading Human Physical Growth – Anthropology Notes – For W.B.C.S. Examination.
For example, the longer growth period of Homo sapiens relative to other Primates (successively longer periods being a feature observed as we proceed up the Primate scale) is of selective value in affording more time in which to learn the many facets of human culture while still socially and biologically dependent upon one’s parents. The study of growth and of the forces which which create and maintain its variability are basic parts of any discipline which seeks to understand man in his entirety. Small wonder it is, then, that anthropologists such as Boas, Krogman and Garn have contributed many of the basic advances in growth research, while other biological scientists, such as Todd, Skerlj and Tanner, who have likewise contributed much, have always been closely associated with anthropology. In other words, though human growth research has always been multi-disciplinary in nature, anthropologists have been prominent throughout its rather short history.
One of the most striking features of growth is its regularity. If we plot repeated measurements of, for example, the height of a single child as taken by a single trained measurer against the age of that child, as shown in Figure 1, we will immediately note the smoothness of the resulting curve. In fact, it is so smooth that it may be accurately described by a very simple mathematical equation. Even the course of adolescence, characterized by a rapid and intense spurt in growth, proceeds along a smooth pathway for the same individual. The regularity in the shape of the growth curve is such that, as early as five years of age, we may estimate the adult height of a particular child and be confident of accuracy within a range of about two inches on either side of our estimate.
Growth may be characterized as “target-seeking,” a description which is the more apt due to the trajectory-like shapes of the curves of most dimensions. The target is, of course, the adult and it is sought unerringly and constantly. The pathway to this target is apparently built into the genetic code and, when it is disrupted, corrections are made to allow the resumption of the original trajectory. Thus, when a healthy child suffers from a particularly acute illness, or is beset by some prolonged disease or chronic upset, his growth progress is usually affected and he falls off course, usually, but not always, dropping behind. When the disease is cured, or the condition corrected, growth resumes at a faster-than-usual pace and the individual is observed to “catch up.” The period of acceleration continues until the child reaches the approximate position on the curve that he would have been at had the disruption not occurred; at that point, the growth slows down to its normal rate.
Just as we may characterize growth as regular and predictable, we may also characterize it as highly variable. Such an apparent paradox may easily be resolved. The regularity and predictability refer to any single dimension, such as height, in a single child, while the variability refers to the relationship between two dimensions in the same individual, such as height and weight, or between the same dimension in two or more individuals. For example, during the adolescent phase of growth in males, there is a loss of subcutaneous fat in both the arm and the leg. Before adolescence, a boy who has a thick layer of fat on his arm will quite likely have a thick layer in his leg. However, there is very little relationship between the timing of this loss in the two extremities of an individual boy. In one person, the loss may occur first in the leg, then later in the arm; in another, this may be reversed; in yet a third the loss may occur simultaneously in both members. In other words, all possible combinations of variability can be seen in normal, healthy boys and the precise sequence cannot be predicted in a specific one with any confidence at all. The range of variability among individuals with respect to a single dimension is likewise striking and, in a sense, distressing. Twenty-five years ago, researchers were very much concerned with the construction of “standards” of growth, which would provide the reference norms for sizes of children, which could be applied to a wide variety of children and which could pinpoint the abnormal and, hence, unhealthy growers. We now know that such standards are of little value except in restricted groups and, often even then, serve more as a series of flexible guidelines. We can easily demonstrate that the growth of normal white children from Des Moines or Cleveland is not representative of the growth of normal white children from Philadelphia. If we desire to learn how a particular child is growing relative to a meaningful norm, then we must use, as our standard, the growth pattern of his or her peers, from the same group and representing the same unique combination of ethnic elements and environmental factors characteristic of any single population segment. Even when we do have adequate and representative standards of normal growth, the range of variation is so wide that we must be very conservative in our evaluations of the individual or group. Such a range of normal variation is a remarkable feature of our species and is indicative of both the many forces which can affect growth and the extreme plasticity of the growth process in the face of these forces.
The growth status of a child (or group of children) at any given moment in time is controlled intrinsically by two sets of factors: those which relate to his final adult size and those which relate to the speed at which that size is being attained. Thus, it is almost without exception that a tall adult was a tall child, and in about the same relative position. It is this feature which allows us to predict the adult height of a child by the age of five with a reasonable degree of accuracy for some purposes. If a five year old boy is in the tallest ten percent for his age, the chances are he will be in the tallest ten percent as a man (though his relative position may change somewhat during adolescence). The regularity of the growth pattern of individuals results in the “canalization” of development and adult size emerges as a very powerful determinant of size throughout growth.On the other hand, children vary considerably in the rate at which they move toward the adult. The average American boy will grow for eighteen years; however, some will take twenty years and others but sixteen to reach adult size. Called the rate of maturation, this feature is under direct genetic control, though subject to environmental forces, and the variability will affect the size at a particular age. Other things being equal, the faster maturers of a given age (say ten years) will be larger than the slower maturers. This is because a faster maturing ten year old is closer to the adult than a slower maturer. Variability in the rate of maturation is most notable during adolescence. A rapid maturing ten year old girl will be well into her adolescent spurt, perhaps even at her peak velocity, and will therefore be significantly larger than a slow maturing one, who will still be growing at the much-reduced preadolescent speeds. The degree of this difference is illustrated in Figure 2, which shows the curves of growth in height for samples of fast, average, and slow maturers. In this case, assignment to a maturation rate group is made on the basis of the age at menarche, a very sensitive indicator of such differences. It is clear that the same relationships exist among the three groups throughout the period of growth, though the differences become intensified during adolescence. It is interesting and important to note that these differenced disappear when adulthood is reached; the maturation rate does not really affect the final height of an individual in a significant way but it does add to the variation observed during growth.
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