Sexual Dimorphism in Human Evolution: A Paleontological Challenge - Research Paper Example

Running Head: Paleontology Sexual Dimorphism in Human Evolution: A Paleontological Challenge A Discussion Paper Date of Submission Introduction Evidently, there are several differences between the sexes that are strongly linked to the reproductive function and process; however, there are also several other anatomical variations between the sexes that are not that strongly related to the act and process of reproduction and are apparent throughout the body (Levinton, 2001). The entirety of these differences is referred to as sexual dimorphism. Though comparatively minor in human beings, sexual dimorphism is relatively huge in some of humans’ closest existing relatives, orangutans and gorillas (Sadava, Heller, Orians, Purves, & Hillis, 2006). The contemporary representation of this sexual dimorphism is that it is a “single, unidimensional phenomenon that is displayed to greater (e.g. gorillas, orangutans) or lesser (e.g. humans) degrees in the different primate species” (Oxnard, 1987, 2). Furthermore, it is commonly believed to be mainly related to variations in general size of the body between sexes (Levinton, 2001). The implication for evolutionary theory is, that human sexual dimorphism in the past must have been significantly greater than it is in the present day, possibly more like that in the living primates (Oxnard, 1987). Understanding the human ancestry is regarded as one of the challenges in exploring human evolution. Nonetheless, several fossil hunters appear to believe that this implies that their mission is to find the pieces of the exact human antecedent in the field (Elewa, 2004). Similarly, several laboratory examiners appear to believe that this implies that their mission is proving that a certain fossil relic is that ancestor (Serafini, 1993). Exploring human evolution, even in the mind of the public, appears to be this issue of moving from ‘missing’ to ‘found’ links (Oxnard, 1987, 2). The challenge appears to be the unearthing of ancestors. But what is the certainty of this undertaking? Even from a population as large and concentrated as that of any major metropolitan area, and over as many as hundreds of generations, the statistical changes of any particular individual ever becoming fossilized and found by a paleontologist millions of years later must be almost infinitesimal. How much less must be the chances of finding representatives of populations of perhaps only a few thousand, scattered over an area of the world as large as Africa or Asia, during periods of time measured in hundreds of thousands, even millions of years (Oxnard, 1987, 3). Once humans are thousand years ahead of a death, possibly tens of thousands of years beyond, the possibilities of ever unearthing anything fossilized that can be validly assumed to be a direct antecedent of anything living at present, are definitely small (Foote & Miller, 2006). Implications for the Human and Primate Evolution There is groundwork of accepted values, ‘for each sex for each genus’ (Oxnard, 1987, 28), that produces isolated constant blueprints of difference in two clusters of genera, and an additional number of general demonstrating, each, distinct differences (Oxnard, 1987). These discoveries have implication, primarily, for the core illustration of the occurrence of sexual dimorphism. It is now impossible to illustrate sexual dimorphism, at least as perceived through the general body proportions, as a particular occurrence with differential manifestation (Lockley, 2000). There are apparently several variations of sexual dimorphisms. A few patterns are each dispersed among various primary taxonomic primate groups: ‘prosimians, New World monkeys and Old World monkeys’ (Sadava, Heller, Orians, Purves, & Hillis, 2006, 738).Therefore, none of these patterns can be specifically believed to be primitive even though supplementary information might demonstrate that one among them was. Most likely, thus, at least one among these patterns should have occurred separately several times (Sadava et al., 2006). This results unavoidably, second, to the refutation of the thought that sex differences are merely an outcome of the differences on general size; because if that were the case, the most immense dimorphisms would be discovered in those animals which have the largest general size variations between the sexes (Oxnard, 1987). The vast range of the differences between the sexes for gibbons (Hylobates), tarsiers (Tarsius), spider monkeys (Ateles), bushbabies (Galago), and douroucoulis (Aotus), for instance, several among the genera that show the most insignificant size differences between the sexes is obviously opposing (Levinton, 2001). Certainly, this discovery indicates that there may be critical sexual dimorphisms in general proportions of the body that are insignificantly linked to size in any way. It would apparently be erroneous to assert that size is not concerned in at least a portion of the sexual dimorphisms that have been found out so far; but the information evidently show that there must also be numerous other portions of the sexual dimorphisms that are related to other factors (Levinton, 2001). And this results, third, in diverse conceptions of the ancestry of sexual dimorphisms. Due to the fact that several varieties of sexual dimorphisms are present it would appear untrustworthy to depend on only one underlying explanation. Hence the more basic concepts such as savannah-forest or terrestrial-arboreal variations, or the more intricate notions such as harem group are, independently, improbable causal factors (Elewa, 2004). Multifaceted accounts are far more plausible. And there have been several multifaceted explanations. Crook (1972 as cited in Foote & Miller, 2006) claims that the extent of sexual dimorphism is connected to a range in primate behavior spanning from slightest dimorphism in single, nocturnal, arboreal species to substantial dimorphism in sociable, diurnal, terrestrial species; Gautier-Hion (1975 as cited in Oxnard, 1987) indicates that sexual dimorphism is linked to both terrestriality and harem organization. Leutenegger and Kelly (1977 as cited in Serafini, 1993) suggest several possible explanations for what they refer to as body size dimorphism. But the presence of a number of various anatomical sexual dimorphisms indicates not merely that they are complexly established, but also that the interaction of the diverse factors is distinct in each case (Sadava et al, 2006). Hence, though there is no particular structure of reproductive efficiency, of troop defense, of feeding pattern, of locomotion, of territorial availability, of social organization, or ecological niche, or even of developmental change that matches the findings so far, it is likely that some or all of these as a whole, in different values for each genus, could be really drawn in (Sadava et al., 2006). It is also probable that other, thus far ignored, factors might be considered. The discoveries also result in to yet more exploratory insights. Fourth, it is feasible to declare something about the progression of these sexual dimorphisms. A number of sexual variations in the primates, at least, must have evolved multiple times within the group (Lockley, 2000). In apes, monkeys, prosimians the primary patterns that were discovered should, in each case, have progressed more than a few times. Similarly, the different individual patterns should stand for a chain of free evolutionary energies (Elewa, 2004). Fifth, it is feasible to state something about related occurrences. As far as can be evaluated from the fairly limited illustration of primate genera in the data of Schultz, it could be indicated that one of these primary groups certainly embodies the ‘polygynous social structure/male troop defense grouping (Oxnard, 1987, 25) of several authors, containing, as it does, howler monkeys (Alouatta), macaques (Macaca), mangabeys (Cercocebus), and proboscis monkeys (Nasalis)’ (Oxnard, 1987, 25). In contrast, the segregation of several polygynous great apes, langurs, and cebids from this group, and the addition of the monogamous slow loris (Nycticebus), has a tendency to refute such an assertion. The second primary group, comprised of, as it does, gibbons (Hylobates), douroucoulis (Aotus), and tamarins (Leontocebus) may embody chiefly the ‘monogamous/equal male and female troop defense grouping’ (Oxnard, 1987, 25) of these particular authors. Once more, though, the addition of langurs (Presbytis) and spider monkeys (Ateles) within this group, and the segregation of tarsiers (Tarsius) from the group lessen the possibility of that thought (Oxnard, 1987). And in the various genera of prosimians and hominoids that each have their own unique sexually dimorphic patterns there must have been other evolutionary factors perhaps unrelated to the concepts involving the monogamous-polygynous social structure alternatives and the equal/unequal troop defense options (Oxnard, 1987, 33). Finally, these discoveries imply that when it is attempted to perceive sex differences in fossils of pre-human, pre-hominid, or pre-hominoid species, it cannot be longer assumed that an exclusive type of sexual dimorphism with contrasting manifestation encompasses all genera (Oxnard, 1987). Paleontologists at present should be well equipped to ask (Oxnard, 1987, 34): Are the sexual dimorphisms of, for instance, Homo sapiens neandertalensis, Homo erectus, Gigantopithecus, the various australopithecines and the various ramapithecines, like those of humans, or of gorillas, or of chimpanzees, or even of orangutans (species presumably very far removed from human ancestry)? Are any of them even like one of the two general patterns that exist more widely among the primates? Indeed, it would be entirely appropriate that we might even ask: are the sexual dimorphisms of any of these fossil creatures like those of no extant Primate at all, but presenting yet other patterns unique unto themselves? Most probably, at present, whenever it is attempted to evaluate sexual dimorphism in any fossilized remain, a more complex dimension emerges (Levinton, 2001). Before scholars can be sure about the ideas originating from examination of existing forms they require a far bigger exploration than is facilitated by Schultz’ findings (Sadava et al., 2006). Such a new exploration, or explorations, needs much bigger representative samples of each primate group; more significant representation of the various primate groups; groups of primate embodied at the species rank, or the geographic zone level, instead of the generic; more comprehensive morphometric descriptions of the anatomical constitution of each animal, and; more exhaustive relationships with systematic, developmental, ecological, functional, and social information (Sadava et al., 2006). Such wide-ranging studies will not be immediately or effortlessly conducted; but they at least nowadays seem far more remarkable that just verifying the presence of a particular pattern of sexual dimorphism with diverse manifestation in each genus (Sadava et al., 2006). But the presence of these new recommendations also has repercussions for the investigation of fossils, even though scholars can never openly identify these aspects for any specified fossil. A comparable progression of more comprehensive studies is needed for those fossils thought to be related to human and primate evolution (Sadava et al., 2006). What is needed then, in this case, are investigations of much larger numbers of fossils than have thus far been made public, with the thought of understanding differences in population, and with the notions of several sexual dimorphisms and several intermingling ancestries in mind. Conclusion It has been discovered that in terms of general proportions of body, sexual dimorphisms in primates is a much more multifaceted occurrence than it has been thus far been lead to accept as true. Each species’ sexes do not have lesser or greater degree of manifestation of an exclusive sexual dimorphism, along several unidimensional patterns greatly reliant upon variations in general size of the body in each sex. Instead, they possess an intricacy of structural form. The division of these is such, as to indicate that the different sexual dimorphisms have emerged separately several times in several million years, the duration of the Order’s evolution. And the division of the distinct sexual dimorphism patterns discovered within each of the big hominoids indicates that sexual dimorphism within this group have been capable of evolving within just the last million years of their departure. References Elewa, A. (2004). Morphometrics: Applications in Biology and Paleontology. New York: Springer. Foote, M. & Miller, A.I. (2006). Principles of Paleontology. W.H. Freeman. Levinton, J. S. (2001). Genetics, Paleontology and Macroevolution. New York: Cambridge University Press. Lockley, M. (2000). The Eternal Trail: A Tracker Looks at Evolution. Cambridge, MA: Perseus Books. Oxnard, C. E. (1987). Fossils, Teeth, and Sex: New Perspectives on Human Evolution. Hong Kong: Hong Kong University Press. Sadava, D., Heller, H.G., Orians, G.H., Purves, W.K., & Hillis, D.M. (2006). Evolution, Diversity and Ecology. W.H. Freeman. Serafini, A. (1993). The Epic History of Biology. New York: Plenum Press. Read More