Introduction
Even though the concept of evolution is a typical narrative, it remains one of the most highly contentious of the topics. Major disagreements persist in the field, particularly when the evolutionary origin and physical modification of primates are the subject matter of discussion. Because of these differences, lots of significant finds in the field have attracted considerable amounts of opposition from experts depending on the species, or the side angle of evolution one recognizes (Ravosa and Dagosto). Disputes often center around locomotion and posture – whether a particular species was fully upright or could walk bipedally, brain size (intelligence), sexuality and social organization, diet and dentition, olfaction and vision, and the so often locomotion and body configuration (Berkeley Education). Simply put, evolution is the process of change that occurs over time. When applied to biology, the term refers to changes in life forms over time (Berkeley Education). For example, the theory of biological evolution (closely associated with Darwin) asserts that all life forms on earth are related through common descent. In this proposition, parent organisms pass down inheritable traits to their offspring and that these changes over time have resulted in the remarkable similarities and the incredible diversity of life we see on earth today (Kingdon, Hapold, Butynski, Hoffman, Happold, and Kalina). Such is the diversity of life we observe in the family Hominidae – one of the two families (the other being Hylobatidae) – of the ape superfamily Hominoidea. The superfamily includes such class of organisms as the great apes (gorillas (genus Gorilla), chimpanzees, orangutans (Pongo), and bonobos (Pan) as well as human beings (Homo) (Kingdon et al.). Humans, who have long since abandoned arboreal environment, have over the years only partially lost the physical adaptation for active tree climbing; their forelimbs (hands), in particular, remains in the arboreal mold. Only the hind limbs (the feet) have lost their primordial prehensility to upright (bipedal) walking (Schmidt). However, morphological and molecular studies show that humans share a lot of attributes with other members of the Hominidae family such as the chimpanzees, indicating that humans are closely related to them. On the other hand, the gorillas are more distant while the orangutans are further distant away from man (Berkeley Education). Within the primate world, such are the changes that have occurred in locomotion and body configuration through evolution. While majorities of these changes are easily seen, other modifications require in-depth observation. This paper, therefore, discusses the evolutionary modifications of primate locomotion and body configuration as well some of the theories put forth in the explanation of the same.
Discussion
According to Napier, primates first evolved from an arboreal lifestyle in woodlands and forests before occupying the grounds in open grasslands. Through time, the primates represented and modified many of their characteristics not only to adapt to the needs of the changing environment but also to ensure the survival of their lineage. Because the conditions of the earth change over time – geologically and climatically speaking – primates existing at these times had to adjust themselves especially in locomotion and body configuration to better survive. All through the evolution process stemming from the changes in the environment directly interacting with the spin-off effects of natural selection, the primates forebears developed own and unique ways to move better (locomotion) depending on the body types (configuration). Variations in the body structure and movement not only helped the primates to adapt to the prevailing conditions but also enabled them to take advantage of the new resources and cope with the demands of the changing environment (Hunt). For instance, by improving the structure of their bodies that helped in the speed of their movements, primates were able to withstand environmental pressures as well as escape dangers (such as predators) (Napier). In the evolution of locomotion and body configuration of primates, an organism belongs to one of the four grades of evolution (Wasser).
In anthropology, a grade is a taxonomic group of organisms united by a shared degree of morphological and physiological similarity (Wasser). In the context of evolution, it refers to a group of organisms with shared morphological traits from an identical ancestral condition (Wasser).
As pointed out previously, primates have evolved over time from the trees to the ground. Today, the ultimate of that evolution is the human. Having to adapt to the new environments, each line of primates brought about unique and crucial evolutionary changes in their lineage with most of them becoming less arboreal and more terrestrial. While for the most parts primates are classified as quadrupedal, there is significant diversity in their travel methods from arboreal brachiation, tree vertical clinging and leaping, and terrestrial knuckle-walking, to pure bipedalism in man. Through the forces of natural selection and mechanisms of evolution, the primate family developed an increase in body hair density, acquired significant changes in their body configuration, and shifted towards having an upright stance (Schmidt).
In this evolutionary process, the Lemuroids are the most primitive and the First Grade of the living primates. They include the Lorises, Galagos, and the True Lemurs (Debalfa). They are believed to have first evolved about 65 million years ago during the Paleocene epoch (Napier). These animals, particularly the lemurs and the lorises share significant traits with the original ancestral species such as their tendency to rely on olfaction (sense of smell) (Debalfa). This enhanced smelling ability is because of the presence of rhinarium (a moist, fleshy pad structure) situated at the end of their nose and having a protracted nose (Debafla). Some of their distinguishing identities include the presence of a grooming claw on the second digit of their hind legs and a dental comb for feeding and grooming (Debalfa). With the only a few exceptions such as the Sifakas lemur, this group of primates has hair covering their faces. Today, the true lemurs (or the brown lemur) are only naturally found in the Island of Madagascar and the adjacent islands of the Indian Ocean where they vary in species and ecological niches. Lemurs vary in size; from the smallest mouse lemur (5 inches) to the largest indri lemur (2-3 feet) (National Geographic as cited in Debalfa). In comparison to other primates, the corporeal size of the lemur is an adaptation to the limited ecological space on the island in which they evolved.
Locomotion in lemurs is diversified. Some are arboreal while others such as the ring-tailed lemurs, with their long and stripped tails, are terrestrial. It is common, however that some arboreal varieties are quadrupeds and others such as the sifakas, ringtails, and the indris show vertical clinging and leaping in movement (Debalfa; Schmidt). The latter trait (vertical clinging and leaping) is one of the natural flow characteristics they share with the tarsiers. Because lemurs are adept climbers, they have developed strong hands and muscular feet to aid jumping from tree to tree. They also have a flexible backbone structure that enables them to twist their bodies at different angles in midair and suspensory motion to grab branches. Due to their natural adaptation to climbing and jumping, most lemurs cannot walk uprightly. The Sifaka, for example, can only hop sideways on the hind legs to compensate for its inability to walk in an upright posture (Debalfa). Lorises resemble lemurs in most of their evolutionary traits. They are mostly found in the forests and woodlands of mainland Southeast Asia. Their forelimbs and hind legs are similar in length, and their short tails are of varied color. They are mostly arboreal with slow quadrupedalism, the latter which enables them to climb trees. Like the lemurs, the tarsiers have flexible anatomical structures that allow them the proper suspension, good grasping, and arboreal movement and adjustment.
Unlike other groups, tarsiers provide the least indication of primate change and evolution. They retained much of the primitive attributes that became obsolescent in higher primates (Schmidt). In size, the tarsiers are one of the smallest primate group of a single genus. Morphologically, these primates show a striking resemblance to fossil remains of prosimians from which they have retained the grooming toe on the second and third digits of their hind legs (Debafla). Some of the most obvious observable traits of the tarsiers are their largest eyes of the primate family, much longer legs than forelimbs, and the long less hairy tail. As a result of their specialized lifestyle (clingers and leapers), tarsiers show higher development on finger pads. On the ground, tarsiers are slow but can leap from branch to branch in arboreal movement. For efficient jumping, the anatomy of the tibia and fibula (the two lower bones of the hind limbs) of tarsiers are fused together midway their length. This bone fusion is an adaption to give the rear legs more strength to propel the organism in leap mode. They also have elongated ankle bones that help in jumping and which also is the basis of their name tarsier, in reference to the tarsal (ankle) (Sargis, Boyer, Bloch, and Silcox). Tarsiers have longer legs than arms. Were it not for their ability to grasp, leap, large eyes, and the ability to rotate their heads 1800 to compensate for their inability to move their eyes within their socket, tarsiers would most likely have not survived in their present habitat.
This category of primates is perhaps the most widely featured species in the narrative of evolution. In the course of evolution, monkeys have developed adaptive mechanisms to survive in several environments in several ways. Fossil records indicate that monkeys first evolved in Africa way back during the Eocene period (Kingdon et al.). Monkeys belong to the anthropoid suborder. Some of them are arboreal while others are terrestrial living in the open grasslands. Monkeys are of two categories; The New World and the Old World monkeys (Kingdom et al.). The New World monkeys – also known as the platyrrhine (meaning flat nose) – are found in the forests of the New World continents. This category is almost always in trees, spending a significant part of their lives climbing and swinging from tree to tree. These attributes set them apart from their Old World monkey brothers. Living in different climates partly explains why some monkeys of our modern times show different characteristics than others in various environments. For example, New World monkeys have long prehensile tails while those monkeys in Africa (Old World) those who happen to have tails have non-prehensile tails. Other Old World monkeys have no visible tail at all. (Kingdon et al.).
Even though monkeys are small in stature, the strength of their muscles is almost unparalleled which in later generations of today’s monkeys is being relied upon for fast movement. One monkey that stands out because of this evolutionary trait is the black-handed spider monkey. This animal uses several different types of locomotion to move from place to place. It is common to see them quadrupedally while walking or running, in suspension when in arboreal locomotion (hanging, climbing, or moving through trees), and in bipedal motion when leaping (Sargis, Boyer, Bloch, and Silcox). Quadrupedality is common when the monkey is in a stable position, walking, running, and climbing unlike in suspensory locomotion that happens when the monkey is brachiating while often maintaining a tail hold from branch to branch. The long hands and legs which the monkeys have developed in the course of evolution have made such movements possible. During locomotion, the long forelimbs allow the monkeys to swing when brachiating while the long legs enable them to move faster on the ground. In the forest biome, these animals developed speed as a way of protecting themselves from predators as well as secure food.
Compared to their ancestors, fossil records show that the body elongation of the monkey has increased during evolution (Ravosa and Dagosto). This elongation allows monkeys to leap further distances while on the ground than other primates do. This adaptive modification occurred when the forest biome attracted so many competition and inhabitants. It, therefore, gave the monkeys the lead of predator evasion and a chance for quality food (Schmidt). Another and perhaps the most significant body modification and configuration of the monkey, as a result of evolution, is the development of a unique tail that is both long and strong. The uniqueness of the tail is that it is longer and has stronger muscles than in any other primate. The monkey often uses this muscular, prehensile, and tactile tail which is sometimes longer than the body not just for locomotion but also for protection (Sargis, Boyer, Bloch, and Silcox). The monkeys have also developed a furry end at their tail to distract and knock off predators and other opponents.
Unlike the other primates, apes, and human beings lack the external tail, are the most intelligent, and equally more independent for survival. Literature contends that as the Grade 4 primates started moving into an upright posture, they lost the need for the tail and as such the structure protracted into the pelvic floor to support the internal organs (Sargis, Boyer, Bloch, and Silcox). Like humans, apes are found all over the world: Orangutans and Gibbons in Southeast Asia and Chimpanzees, Bonobos, and Gorillas in Africa. Compared to other primates in the ape family, Gibbons are the smallest (no more than 20 pounds in weight and just about 3 feet in height) thus are often referred to as the ‘lesser apes’ (Schmidt). Characteristically, Gibbons have long arms, light bodies, and permanently curved forelimbs for efficient brachiating. While Gibbons can occasionally walk on two atop trees, they are better at suspensory locomotion. Orangutans, on the other hand, are larger than Gibbons and can grow to about 4.5 feet and a mass 200 pounds in weight (Schmidt). Because of this body structure, orangutans are poor ‘brachiators’ and thus walk on the ground. The orangutans represent that timeline in primate locomotion evolution where the animals are coming off the trees and roaming the ground as they grow in body size. At this point, their body structure is becoming less adaptable to arboreal lifestyle to more of terrestrial dwellers. The Gorilla is the largest of the ape family known to grow as tall as 6 feet with arms longer than that (Schmidt). At this point, the primate is walking quadrupedally in a special type of locomotion known as ‘knuckle walking.' The short hind legs of the gorilla cannot support their massive body structure and are therefore forced to walk on all fours (Richmond, Begun, and Strait). During movement, gorillas walk on the soles of their hind limbs but do so on the palms of their forelimbs. They bend their fingers instead. Sargis, Boyer, Bloch, and Silcox observes that the long arms relative to the hind legs and the introduction of knuckle-walking in gorillas are the start point for the end of quadrupedalism and the beginning of bipedalism in the evolution of locomotion in primates (Schmidt).
Chimpanzees, on the other hand, is the closest cousin to man in the primate table. Their arms are only slightly longer than their feet and grow to almost the same height as the average man. Due to the evolution the spine, chimpanzees can walk upright like human beings even though they tend to prefer quadrupedal locomotion (Sargis, Boyer, Bloch, and Silcox). The desire and the need to hold equipment and use tools differentiated the evolution of human beings from their primate cousins. While the apes developed thicker bodies and muscle tissues covered with thick fur due to the cold terrain which they usually inhabit, man invented fire to keep him warm at night and thus developed less body hair (Hunt). From the times of Homoerectus and the Neanderthal man, human beings have been developing elongated spines as their locomotion gradually shifted to bipedalism. During the early stages of the bipedal movement, the human body structure started to change focusing more on their hind limbs to support the weight of their body mass which unlike in gorillas had reduced to significant proportions. Their forelimbs also gradually metamorphosed into hands with separate and elongated digits (phalanges) ideal for holding and using tools (Richmond, Begun, and Strait). During the early stages of the Pleistocene era, the human ancestor – Homohabilis – developed higher intelligence because of the enlarged brain and discovered fire. With this discovery, humans began shedding much of their body fur that had long kept him warm (Richmond, Begun, and Strait). In the late Pleistocene epoch, both man and apes started doing away with the short tail and the man stood upright, a trait he has maintained to date.
Literature Analysis
The discussion above present that by improving their body structure which in turn influenced their locomotion in such aspects as speed and stability, primates found it easier to survive in the changing environment (Ravosa and Dagosto). Over time, therefore, primates restructured their bodies and consequently locomotion to ensure much more chances of survival of their kind. Locomotion, as used in the discussion, refers to how a primate gets around while the mode of locomotion defines how the animal in question frequently gets around. As used in evolution, a mode of locomotion plays a significant role in generating the selective forces that function to change the body. For example, primates that exhibit arboreal quadrupedalism such as the Black Howler monkeys and Langurs, have both their hindlimbs and forelimbs equal in size and length. Those that show terrestrial quadrupedalism such as the Old World monkeys like the baboon have not only long and narrow bones of the pelvis but also forelimbs equal to hindlimbs (Richmond, Begun, and Strait). These modifications are different from those shown by primates such as tarsiers whose primary mode of locomotion is vertical leaping (Schmidt). These animals have Feet much larger and longer than hands and lengthened and fused tarsal bones in Hind limbs much longer than forelimbs. Several theories explain the evolution of primate locomotion and body posture.
One such theory is the Postural Feeding Theory (Hunt). This model presupposes that the hominid mode of locomotion developed first as a feeding technique before evolving into a regular form of movement. For primates with bipedal movement, the Postural Feeding Theory asserts that the need for meeting food requirements forced these animals to walk on two as bipedalism offered efficiency in long distance travel for hunting and foraging for food in open landscapes. For those in wooded habitats, this theory asserts that the animals developed low-frequency bipedalism like the one seen in modern day gibbons for arboreal feeding posture (Hunt; Schmidt). Another theory is the Hunter-Scavenger hypothesis that is used to explain the evolution of bipedalism in human beings. According to this theory, man developed an upright stance not only for greater stamina to hunt over extended distances, but also to see far and wide in open fields, as well as to free his forelimbs to carry and use tools (Richmond, Begun, & Strait). There is also the theory of Threat-Display (Evasion). This theory asserts that all organisms developed unique survival mechanisms independent of one another. For those who could not fight and defend themselves, they developed those features that enabled them to escape danger. Therefore, primates such as monkeys developed elongated body structure and strong tactile tails to flee danger while the orangutan and siamangs developed longer hands and firmer grips for efficient arboreal suspension (Williams).
The visit to the Los Angeles Zoo was both entertaining and educational. At the chimpanzee exhibit, the sight of the animals swinging from object to object was unmistakable. I also to saw the chimps transform from quadrupedal to bipedal. I also got to observe the animals especially the chimps take care of their young. There were also the Coquerel’s Sifaka that showed a curving back when it walked about, the gorillas and the Siamangs. Even though I was not able to observe all the animals in the zoo as some of them were asleep, the experience was educative.
Conclusion
The argument presented in the discussion above contend that the evolution of the body structure and locomotion of primates gives them a better chance of survival. With an appropriate posture and body structure, the animals can fend for themselves better, defend themselves against opponents, as well as flee from danger. Depending on the conditions of the environment in which they evolved, each primate species evolved to suit better the prevailing circumstances. The monkeys, for example, was increased in elongation with powerful tails and longer legs to escape predation while the tarsiers developed shorter but stronger hind limbs to cling to trees for more extended periods to hide from danger. In the end, therefore, those organisms that failed to evolve the required body structure and mode of locomotion were naturally unselected and were extinct as much of the locomotion evolutionary changes arose from the need to escape predation and fend for food.
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