Origin of man is one of the most puzzling phenomena of nature. While the fossil records in the case of other animals doubtlessly reveal their ancestry, human fossil records are scanty and full of gaps. There is no written account of our ancestry other than the religious theories that unanimously indicate that Gods descended from heaven and created man in their own image. Man appears to have evolved through natural selection from ape-like creatures that migrated from forests to grasslands and lived as group hunters.
Factors that influenced human evolution
Both Miocene and Pliocene were dry periods of great stress, when sea level went down by about 200 meters and northern and southern hemispheres had huge ice caps. There are indications that fragmentation of dense tropical forests in east Africa initiated the evolution of bipedal hominids from the arboreal apes. The dry period of a few million years put a lot of pressure on the animals living in the shrinking forests and forced them to migrate and adapt to the open grasslands. The evolution of horse, camel, giraffe, and elephant was also triggered by the same factors.
Some groups of apes that were omnivorous and semiterrestrial, faced with intense competition in the forests, started migrating to grasslands. Community living evolved to defend themselves in groups and also to hunt large animals in groups. As the hands were engaged in handling arms and food, they had to walk and run on two legs, giving rise to bipedalism, resulting in the elongation and strengthening of legs. Standing upright in the tropical savannas could also have given them the advantage of scanning horizons for predators as well as potential prey, exposed less of body surface to the perpendicular sun rays and more to the horizontal cooling winds, and freed the hands for carrying stones, sticks, food or infants.
Manipulation of objects by hand put a lot of pressure on brain, leading to its enlargement and increased intelligence. As these migrants continued to live in grasslands and perfected their bipedal locomotion and use of hand for more complex jobs, their brain increased in size and capacity. Later, they started living in caves for protection and probably developed a language for communication. Australopithecus was such a creature, which lived socially in caves, made stone tools and hunted animals. Eating cooked food led to the reduction of canines, shortening of jaw and simplification of teeth. Human evolution is an example of specialization in brain’s ability, achieved due to manipulative skills of hands, strategic group hunting and communication in social groups.
Fossil Records of human phylogeny
Explosive primate radiation took place in the early part of Coenozoic era, in Palaeocene and Eocene epochs. Primitive monkeys and primitive anthropoid apes made their appearance in the Oligocene epoch, about 35 million years ago, thus setting the stage for further hominid evolution. However, fossil records depicting human evolution are incomplete and fragmented and do not give a clear linear picture of human phylogeny.
Parapithecus was a primitive primate ancestral to man, apes and monkeys. It was very small squirrel-like earliest primitive monkey having tarsier-like appearance. The jaw was conical, the two halves converging at an angle of 33 degrees. These creatures were probably adapted for arboreal mode of life and had opposable thumb, forwardly directed eyes and reduced snout.
Propliopithecus fossils were discovered from Fayum deposits in Egypt that comprised of a lower jaw with teeth. The jaw is smaller and more pointed than that of a gibbon, prognathous and deep. Canines were smaller and bunodont grinders had 5 bulbous cusps as in apes and man.
Limnopithecus first discovered by Hopwood in 1933 from Kenya is represented by several fragments of mandibles, teeth and limb bones. The dentition is gibbon-like but limb bones are unspecialized and a combination of monkeys and gibbons.
Pliopithecus is represented by several well preserved fossils from Egypt and Europe. It shows affinity with pongids but the mandibular symphysis is longer and more prosimian type. A shallow simian gap is present. Body is gibbon-like in the morphology of pelvis, vertebrae and sternum. Limb bones are surprisingly primitive, resembling those of prosimians while the general body proportions are like those of monkeys.
Dryopithecus (=Sivapithecus) (=Proconsul) was ancestral to Orangutan, chimpanzee and gorilla and resembled gibbon in stature. Most of the fossils are represented by jaw fragments and teeth, with few exceptions such as a humerus and an ulna from France and a femur from Germany. The three genera, Dryopithecus, Sivapithecus and Proconsul have been placed in the subfamily Dryopithecinae. Arms and legs were of same length and posture was semi-erect. Skull lacked the well-developed crests and massive ridges characteristic of modern apes. Dental arch was parabolic and dentition more man-like but canines were larger and lower premolar is sectorial. It was a brachiator, swinging with arms on tree branches.
Sivapithecus is believed to be the direct ancestor of Ramapithecus, whose fossils have been recovered from the same deposits in the Siwalik Hills and date from 17 to 8 million years old.
Sugrivapithecus fossils were discovered from Siwalik Hill by Lewis in 1934 and are represented by fragments of jaw and teeth. The small size of teeth and canines and simplified molars suggest a transitional stage to hominid type of dentition.
Gigantopithecus remains have been recovered from Siwalik Hills. Reduction of front teeth and canines shows hominid tendencies but the jaw was massive and premolars and molars were large. Jaw allowed grinding sideways motion. It was larger than gorilla, a terrestrial herbivore and lived in open grasslands.
Lufengpithecus fossils recovered from China reveal a highly sexually dimorphic hominoid, having distinctly smaller females and considerably larger male of the size of a chimpanzee.
Sahelanthropus tchadensis. A cranium, jaw fragments and several teeth were discovered from Chad in Africa by Michel Brunet et al. (2002). The 6-7 million year old skull resembles that of a chimpanzee from the posterior side but on the front side is Australopithecine in character.
Ramapithecus (=Kenyapithecus)(=Bramapithecus) fossils of fragments of an upper jaw and teeth were found in 1932 from Haritalaya Nagar in Himachal Pradesh in Siwalik Hills in India by G.Edward Lewis. In Kenya L.S.B. Leakey (1955) discovered a few teeth and jaw fragments of the same species. Canines were small and grinders had low cusps but coated with thick layer of enamel. Brahmapithecus is represented by lower jaw only. Face was short and jaw allowed sideway motion. Dentition was human and palate arched. Incisors and canines were small, permitting lateral chewing. Grinding teeth were large and broad with thick enamel coating, suggesting herbivore diet of grass, seeds, roots and perhaps raw meat.
Ramapithecus fossils show advancement in morphology over Sivapithecus that brings it closer to Australopithecus. They probably originated in Africa and later migrated to Eurasia.