Primates are assume to have began evolving in Eocene during the Tertiary Period between 75 & 60 million years back for living in Miocene covered by lush forests. In later part of Oligocene roughly 25-30 million years back when drier Savannah grassland replaced the evergreen forests, certain primates living in trees returned back to the ground and formed the ancestors of apes and man. Therefore, evolution of humans and apes originated in unison from some common ancestor living in the trees about 25-30 million years back. It was during the Pliocene era about five million years back humanization i.e. the achievement of human organization or emergence of the genus Homo began.
Modern man i.e. Homo sapiens, belongs to class Mammalia, order Primates and suborder Anthropoidea as per Goodman, M., Tagle, D. A., Fitch, D. H., Bailey, W., Czelusniak, J., Koop, B. F., Benson, P. & Slightom, J. L. (2000) PRIMATE HERITAGE. Primates is considered as a diverse group and huge in size. Under this group tree shrews, lemurs, lorises, tarsiers, monkeys, apes and of course man is included. Each of them is essentially adapted for arboreal life with certain common features.
1. Grasping limbs: In case of primates, the thumb or the big finger lies opposite to the rest of the fingers. This helps the forelimbs to become magnificent clasping organs. For primates, the forelimbs can be used for griping branches of trees while climbing on them and also used for picking up objects and tools.
2. Rotating forelimbs: Primates can rotate their hands at 180o as the radius and ulna bone of the arm can twist about each other on the wrist. Besides, the ability of the upper arm to rotate in the shoulder socket (glenoid cavity) gives then additional flexibility to swing on branches of trees.
3. Nails replace claws: In higher primates like humans, the tips of the digit bone have flat nails in place of claws. These are rich in nerve endings that help in manual agility.
4. Binocular vision: Placement of eyes in the frontal portion of the head that looks forward so that these are able to focus on the same object separated differentiated by a specific angle. This gives a 3D effect of depth allowing stereoscopic vision.
5. Visual Acuity: In the retina, apart from rods, there are cones aiding in color vision and greater visual definition. The core concentrates at a specific point i.e. the central area.
6. Large Brain: In the brain, the visual centres are proportionate to the visual advancement. In the cerebrum, the sensory and motor areas are very well developed and the surface of the cerebrum is deeply fissured. This lends accuracy in control of sensitive hands manipulation and ability to think, reason and analytical power.
7. Enlarged skull: To have room for cerebrum, primates have an enlarged cranial cavity. The cranial portion of the skull is enlarged and the facial portion is shortened. The foramen magnum is shifted ventrally.
8. Lowered olfactory sense: The olfactory lobes being reduced in primate brain, the sense of smell is not well developed and thus reduced in the primate brain.
9. Fewer offspring: Unlike other mammals, the litter size is small and their babies are dependent on the parents longer with longer gestation periods. Extended periods of interaction between adult and child has resulted in cultural evolution.
10. Social dependency: Group inhabitation has resulted in the evolution of social interaction and formation of companies.
1. Thumb placed at opposite end of fingers helps in gripping with power and precision.
2. Hands are capable of rotation at 180 degrees with rotating forelimbs
3. Eyes looking forward, embedded on the face with parallel optical areas for stereoscopic vision.
4. Higher volume of rods and cones with their own nerve cells for visual acuity.
5. Higher sensory and motor areas with deep fissures, larger brain area for increased intelligence.
6. Expanded cerebrum; foramen, magnum ventral with enlarged skull
7. Snout reduced, face flatted with less protrusion, lowered olfactory sense
8. Increased parental care, less number of offspring and longer gestation period
9. Community living, corporate behavior and socially dependent amongst themselves.
The evolution of primates is supposed to have started during the great adaptive radiation of mammals somewhere between 75 & 60 million years back during the Eocene period. Their ancestors are supposed to be small primitive four-legged ancestors consuming insects. They resemble tree shrew that belonged to the order Insectivora and they look like a transition between Insectivora and Primates. Broadly two lines of evolutions diverged resulting in the two suborders of the present day i.e. Prosmians and Anthropoidea. The Prosmians chose living in the trees and includes loris, lemurs, tarsiers and tree shrews. On the other hand anthropoids include the following three groups:
1. Monkeys of the New World i.e. Marmosets and Spider monkeys
2. Monkeys of the Old World i.e. Baboons and Monkeys
3. Hominoids I,e, Apes-Gorilla, Chimpanzee and Oranguttan and Humans
Systematic of Primates depicting relationship of Man to other Primates
Suborder 1. Prosimll
Superfamily (i) Tupaiodea: Tree shrews Super family (ii) Lemuroidea : Lemurs, lorises
Superfamily (Hi) Tarsioidea: Tarsiers Suborder 2. Anthropoidea
Superfamily (i) Ceboidea: New world monkeys
Family (a) Cebidae: Spider monkeys
Family (b) Callithricidae: Marmosets Superfamily (ii) Cercopithecoidea: Old world monkeys
Family cercopithecoiae: Macaques, baboons Superfamily (iii) Hominoidea
Family (a) Hylobatidae: gibbons
Family (b) Pongidae: Apes: Gorilla, Chimpanzee and Oranguttans
Family (c) Hominidae: Humans and subhumans
In the opinion of renowned anthropologist, Sherwood Washburn, departure of human and apes from their primate ancestors must have ushered following the development of brachiation as they continued to retain some of the brachiating features like broad trunk, well-built collar bone and a flexible anus. However climatic changes must be the cause for them to return to the ground from the trees.
During the Pliocene period continental elevation happened resulting in more aridity of the climate. More aridity and tempering of tropical heat resulted in degeneration of forests which led to a shrinking effect towards the equator and the Savannah grasslands spread widely. The continual depletion of forest cover and lessening in the number of trees led to the tree-living creatures to descend to the ground. Then began the process of gradual adaptation needed to ensure survival in the new habitat. Prolonged childhood was one of the adaptations and lowered maturity of the skull and therefore greater increase in the size of the brain eventually the adaptation to dwell in the open countryside. These modification differentiated individuals that eventually led to humans.
Humans decent from trees ushered in the following changes in the organization:
(i) Considering erect progression.
(ii) Hands that were used to aid movement evolved to become organs of the mind
(iii) Hunting and food gathering function (forest depletion lessened the ready availability of food and this compelled to be on the lookout for other substances.
(iv) Need for clothes to protect during winters
(v) Consuming an omnivorous diet led to be free from climatic upheavals, clothing and use of fire leading to their dispersal.
(vi) Formation of societies.
Close resemblance among apes and humans show that they have evolved concurrently and from some common ancestry for a considerable stretch of time. Thereafter humans switched to bipedal locomotion and upright posture, while apes and their ancestors continue to be four-footed. The special features of humans are stated below that have been adopted during the process of becoming human.
1. Bipedal locomotion - Humans are terrestrial and walks on hind legs only. Forelimbs or hands have ceased to be used for walking and have evolved for use of other functions. Either bipedal gait liberated forelimbs to be used as hands for diverse manipulations or the pre-use of hand necessitated the ancestors of humans to assume bipedal upright position. Apes have remained tree dwellers and their forearms are longer than legs. They still walk on four arms.
2. Upright posture - The bipedal locomotion led to an upright posture mandating significant changes in the musculoskeletal system. These changes are :
(i) The hind limbs or legs became longer for balancing the body.
(ii) The abdominal area became short while the thorax became broad and flat.
(iii) The lumbar vertebrae are numbers 4 to 5 in man due to shortening of abdomen while in case of apes these are 6-7. The sacral vertebrae are fused in humans.
(iv) Over the years a lumbar curve has developed in humans which is not seen in apes.
(v) The iliac bones of the pelvic girdle are broad and expanded in case humans and are also expanded to assume a basin like structure that helps in housing the internal organs of the body cavity.
(vi) Tail is not present
(vii) In humans the skull lies balanced on the vertebral column straight but in apes the same is at right angles to the vertebral column.
(viii) The humans the occipital condyle and foramen magnum point downward while in apes these point backward.
3. Forelimbs: The enhanced power of the Brian should have the capability to use it. On similar lines, the evolution of the hands constitutes another key result in the evolution of humans. The forelimbs evolved into a capable clasping and manipulating mechanism for tool manufacturing, throwing arms and ammunition and also for ferrying objects. These constitute the way though which humans are able to transform his ideas into novel creations. (McKenna and Bell 2000)
4. Opposable thumb: With the evolution of hands, devices such as spears, bows and arrows and elementary tools for cutting carcasses were made. Due to this the thumb got lengthened which became opposable to the finger position. This development of opposable thumbs helped the ancestors to hold, grip and do simpler manipulations with accuracy. This resulted in the evolution of the hunting characteristics of life.
5. Brain and cranial cavity: The distinguishing feature of humans from all its primate relatives is the amazingly large brain and high intelligence. Because of this, humans have developed the power of rational thinking, anticipate the result of his actions and help in solving problems. In order to accommodate a huge brain, the cranial cavity has more volume with an average ranging from 1350-2000 cc or more than that. In case of apes who are the closest kin of man, the cranial cavity is 450-600 cc.
The enormous size of the frontal lobes has brought about in the development of a high forehead that is found sloping in case of apes.
(i) The face of apes is projected outwards like a muzzle due the presence of broad and long row of teeth which is known as Prognathous type. However, in case of humans, the face is orthognathous with protruding canines of the lower jaw.
(ii) According to Kavanagh M, in apes there is presence of Simian shelf, however the same is absent in humans. (The Simian shelf is something like a ledge of bones extending backwards from the symphysis of the lower jaw).
(iii) Humans have a chin which is formed by the straightaway extension of the lower margin of the lower jaw. Similar chin is absent in prognathous face.
(iv) In case of apes there are heavy ridges of bones projecting over the eye socket. (the ridges of the eyebrows or supra orbital ridges). These are absent in humans.
7. Structure of Teeth - As compared to humans, the structure, size, shape and arrangement of teeth is dissimilar.
(i) The dental arch of humans i.e. the arrangement of teeth is rounded parabola, however in case of apes these are found arranged in a straight sides U.
(ii) The canine teeth are huge and protruding in apes forming tusks which is absent in humans
(iii) The incisor teeth are bigger in apes which are small in humans.
(iv) In case of apes, a simian gap i.e. diastema is there in the upper jaw between the incisor teeth canine teeth. It provides the space for the lengthy canine of the lower jaw.
8. Food habits- Humans were herbivorous in the past, but now omnivorous, where apes continue to be herbivorous.
9. Intelligence - Due to the massively developed cerebral hemisphere, the highest level of intelligence is found in humans. Due to this, humans developed the capability of thinking, planning, logical analysis, reasoning and manifestation of emotions.
10. Binocular vision - In humans through evolution large forwardly directed eyes are placed with higher power of accommodation and it is through binocular vision, perception of depth and estimation have become possible. Rod cells that react to light and cone cells that are sensitive to color are found in the retina of the eyes.
11. Olfactory sense - These are less developed
12. Hearing- The sense of acoustic efficiency is also less developed.
13. Breeding capacity - In case of humans, the birth rate was lowered and complete loss of poly embrony. With the development of menstruation, breeding cycle evolved in woman, while the male developed perennial fertility.
14. Loss of body hair - Due to gradual reduction of outdoor living and absence of hunting habts, body hair reduced and although present are in vestigial form only. The cooling system of the body was stressed due to hunting in hotter climates. Eventually heat loss will happen because of loss of body hair.
15. Social and cultural organization - in case of evolution of humans, the development of language has been seminal. Languages help in conveying the expression of feelings and ideas. It is the means of active communication among individuals.
All the dissimilarities between modern apes and humans indicate that both took different paths in the evolution process although there is a common ancestor. Apes continue to live in the trees and the ancestors of humans returned back to the ground.
Molecular biology within the past decade has significantly helped in appreciating the evolution of Primates. The comparative studies regarding the substantial number of proteins available in humans, primates and mammals refer to the intense relationship between humans and higher apes.
Likewise even in the DNA structure, presence of hemoglobin, transferring and albumin molecules of chimpanzees, gorilla and humans always indicate a high level of relationship between humans and higher apes. Therefore it can be held that the stem ancestor for the order primate, as also the location and time of the origin of the Order can be obtained from the information stated above.
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