Human Evolution

The evolutionary history of human beings has been traced back to a tree-dwelling, shrew-like ancestral animal, with the process beginning during the Palaeocene epoch, when shrinking forests forced arboreal mammals to begin adapting to a life spent on the ground — a shift in habitat that is thought to have been an important driving force behind the descent to land. In the following chart (Chart 5.17) it can be seen that we are most closely related to the gibbons, chimpanzees and gorillas.

Several major evolutionary trends mark the transition from ape to man: an increase in the size and complexity of the brain, accompanied by markedly enhanced intelligence; a corresponding increase in cranial capacity; the adoption of bipedal locomotion; the development of an opposable thumb; an erect (upright) posture; a shortening of the forelimbs together with a lengthening of the hind limbs; the development of a distinct chin; a broadening of the pelvic girdle; the development of the lumbar curvature of the spine; and, ultimately, social and cultural development — including articulate speech, art and the deliberate making of tools.

The fossil record traces this transition through a sequence of ancestral and related forms (summarised in Table 5.18). Dryopithecus (Miocene epoch, roughly 20-25 million years ago; fossil sites include Lake Victoria in Africa and Haritalyangar in Himachal Pradesh) was distinctly ape-like in appearance and showed a close resemblance to the living chimpanzee, with a comparatively small cranial capacity. Ramapithecus (late Miocene to early Pliocene, roughly 14-12 million years ago; found in the Shivalik Hills of India and also in Kenya) was already more man-like in its features. Australopithecus (late Pliocene to early Pleistocene, roughly 4-1.8 million years ago; found at Taung in South Africa and also in Ethiopia and Tanzania) is regarded as the connecting link between ape and man, and is often described as having had 'a man's body with an ape's brain' — an upright, bipedal gait combined with a comparatively small, ape-sized brain (roughly 900 cc). Homo habilis (late Pliocene to early Pleistocene, roughly 2.5-1.4 million years ago; found at Olduvai Gorge in Tanzania) was nicknamed the 'handy man' because it is credited with making the first tools out of stone, and probably did not eat meat. Homo erectus (middle Pleistocene, roughly 1.5 million years ago; found in Java and near Peking) was more clearly an 'ape-man' in build, is thought to have been omnivorous and to have probably eaten meat, and may have used fire. Neanderthal man (late Pleistocene, roughly 100,000 to 40,000 years ago; found in the Neanderthal valley in Germany) was a heavily built, advanced prehistoric human who used animal hide, buried the dead, and constructed flint tools, with a skull marked by prominent brow ridges and a cranial capacity of roughly 1400 cc. Homo sapiens, modern man, is found across Africa (and subsequently the whole world), developed distinct races over time, and produced cave art as far back as roughly 18,000 years ago.

Over this same evolutionary sequence, cranial capacity increased steadily and the enlargement of the frontal lobe allowed a high forehead to develop; the growth in intelligence made a corresponding physical development necessary, so that body and brain could be used together effectively and productively. Freedom of the forelimbs from any locomotory role, combined with an opposable thumb, allowed much better use of the hands for holding objects and for the development of fine motor skills. Bipedal locomotion and an upright posture, together with stereoscopic vision, together let early humans move about safely on land. Table 5.18 clearly shows this gradual increase in cranial capacity, together with the accompanying changes in skull shape and dentition, across the ancestral human lineage right up to the present day — a journey of evolution that, as the chapter notes, still continues.