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Evolutionary Trend In Hominidae – Geology Notes – For W.B.C.S. Examination.
Numerous studies have been made to ascertain the origin and evolution of humans. The fossils which have been so far unearthed indicate that man has originated somewhere in the plains of Asia, Africa and Europe from some non-human beings, similar to proconsul — the prehistoric African anthropoid apes — in late Miocene period.Continue Reading Evolutionary Trend In Hominidae – Geology Notes – For W.B.C.S. Examination.
The exact course of evolution is still not known due to paucity of fossil records. The following steps are being discussed that help in tracing out the human ancestry and the evolution of humans (Homo sapiens).
a. Bipedal locomotion with erect posture, straightening knees, flattened feet, and shortening of toes.
b. Perfect erect posture is due to the development of S-shaped curvature in the vertebral column.
c. Broadening of iliac bones of pelvic girdles and position of acetabulum.
d. Freeing of forelimbs from ground for manipulation. The forelimbs with opposable pollex (thumb) are used in grasping, and other mechanical functions.
e. Loss of hairs over the body except sole, palm and parts of facial region.
f. Orthognathous face (vertical or vertically placed skull).
g. Reduction in size of canines and incisors.
h. There is a reduction of jaw with smaller chewing apparatus.
i. Dental arch is smooth and semicircular.
j. Reduction in size of eyebrow ridges and simian shelf.
k. Loss of opposability of great toe (hallux) in hind limb.
l. Development of chin.
m. Increase in cranial capacity, 1390 cc with brain to body weight ratio being highest 1: 46 as compared to 1: 250 in apes. The average size of the brain of human beings is the highest as compared with the brains of apes and human beings.
n. Reduction of nozzles.
o. Loss of jaw power.
p. Development of articulate speech and sharp memory.
Pre-primate ancestry:
The sequences are as follows:
Sarcopterigian fishes (Lobe-finned fishes) —> Labyrinthodont amphibians —> Cotylosaur reptiles —> Therapsid reptiles —> Primitive mammals —> Jurassic pantotheres —> Mammals of Order Insectivora.
Primate ancestry:
The earliest primates appeared among the late Cretaceous under the order of placental mammals, differing probably only a little with the insectivores from which they arose.
2. Closest Relatives of Humans:
Among primates the Chimpanzee and Gorilla are judged to be man’s closest relatives on the basis of their structural, serological, biochemical as well as chromosomal and molecular level of similarities.
A. Structural similarities:
(i) More or less bipedal locomotion,
(ii) Semi-erect or erect posture,
(iii) Absence of tail in both cases,
(iv) Fewer lumbar vertebrae,
(v) Presence of brow ridges,
(vi) Grasping pollex in both cases,
(vii) Menstrual cycles in females.
B. Serological similarity:
AB blood group is found in both apes and humans.
C. Biochemical similarities:
(i) Haemoglobin analysis between chimpanzees and humans indicates that one amino acid is different in between two forms,
(ii) Serum tests indicate protein homology to be maximum in between humans and chimpanzee, then less maximum between gorilla and humans and, at last less similarly between the humans and Asiatic apes [Holoock (Hylobates), and Orangutang (Pongo)].
D. Cytological and Molecular homology:
The fossil record of living African apes is not known. For comparison in between African apes and human beings we must rely on cytological and molecular studies. From molecular studies it is known that gorillas were diverged from the Chimpanzee and human beings from a common ancestor about 8-10 million years ago. Again, Chimpanzee and humans were isolated about 6 million years ago.
At present the differences are:
(i) DNA content in diploid cells is similar,
(ii) Hybridization of non-repeated DNA sequences of chimpanzees and humans indicate 98% similarity,
(iii) Banding pattern between the chromosomes of apes and humans shows that maximum chromosomes of humans are similar with apes except 3 and 6 chromosomes where little differences occur,
(iv) Human beings have 46 chromosomes while apes have 48.
Due to anatomical differences between apes and humans, they are placed in different families.
The differences are:
(i) Humans have an erect, bipedal posture, and it is due to the “S”-shaped curvature of the vertebral column and broadened pelvis, and position of the acetabulum. In apes, the posture is semi-erect and pelvis is elongated,
(ii) The jaws of apes are either rectangular or “U” shaped but in human beings, it is bow-shaped,
(iii) In apes larger incisors, “U”-shaped dental arch and lower premolars with cutting edges. In humans the dental arch is a smoothly rounded parabola and small incisors,
(iv) There is a simian gap among apes in between incisors and canine on each side of the jaw. Simian gap or diastema is absent in humans,
(v) The palate of the humans is arched but in apes it is flatter between the parallel rows of cheek teeth,
(vi) As compared with human beings, the forelimbs of apes are longer but, in human beings, the legs are larger and stronger than arms,
(vii) The body of apes has a thick growth of hair but, in humans body hair is much reduced and development of large number of sweat glands may be correlated with this,
(ix) The skull of gorilla possesses brow ridges and large sagittal and nuchal crests which increase the area and help the attachment of jaw muscles. In humans such crests are not found,
(x) The ape’s brain differs in size and organization from the human’s. In human brain parietal, frontal and temporal areas of the cortex are enlarged and more convoluted than the ape’s brain. Apes are less intelligent, with less cranial capacity as compared with humans. Cranial capacity of Orangutan and Chimpanzee is 400 cc, and 550 cc in Gorilla, whereas in humans it is in between 1400 – 1450 cc.
(xi) In humans, neck is long and the position of foramen magnum is at the rear of the brain-case. In apes, the neck is short and the position of the foramen magnum is more ventral than humans.
3. Ancestry of Humans [Early Evolution of Apes and Hominids]:
There is no clear-cut idea about the ancestry of Hominidae. Different opinions prevail regarding the origin of Hominidae.
According to one group the earliest known ape is Aegyptopithecus which had a small brain, a tail and feet and hands that indicate an arboreal quadrupedal life. A robust ulna also indicates that they were canopy dweller. By Miocene time it could have led to the genus Dryopithecus (subfamily Dryopithecinae).
As early as middle Miocene times, 15 to 25 million years ago, Dryopithecus (= Proconsul) led an arboreal life and lived in Africa, Europe and Asia. Fossils of Aegyptopithecus were collected from Fayum in Egypt and Dryopithecus from Kenya. The skull of Dryopithecus was lightly built than that of modern man. There were no brow ridges.
Small incisors were like those of modern man. Canines were large. The molars had 5 cusps, arranged in Y-pattern. They were probably the common ancestors of the present day ape (Pongidae) and humans (Hominidae) .
According to another group of scientists, the Eocene prosimians (Lemurs and Tarsiers) gave rise to Oligocene ancestors of Old World Primates, i.e., Parapithecus. It was a squirrel- sized creature, known from a single lower jaw and upper jaw fragments, found at Fayum in Egypt, and could have been the ancestors both of Old World monkeys and of apes (Fig. 10.85).
Earliest Hominids:
In late Miocene and early Pliocene, 14-8 million years ago, there lived in the Siwalik Hills of India and Pakistan, Ramapithecus (= Shivapithecus). Related fossils from Maboko Island and Fort Ternan in Kenya, Hungary and Greece are Kenyapithecus.
They are closer than any other known ape-like forms to the ancestry of Hominidae. Their fossil features indicated that they walked on legs and dentition was similar to modern man. The fossil features also suggest that they lived at the side of the forests and on the open lands.
Ardipithecus Ramidus:
Recently some fragments with four teeth of a new species of fossil hominid, as old as 4.4 million years old, were collected from the sediments of Ethiopia and named Ardipithecus ramidus (formerly called Australopithecus ramidus).
The features of A. ramidus indicate that they were bipedal, had human-like arms and incisiform canines. By judging the features it is assumed that they were the first ape-like hominid ancestor known.
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