Human Evolution

14.1 What is Evolution?

Early Beliefs: Historically, people believed that life was spontaneously created by a supernatural event and remained unchanging from the beginning.
Autogenesis Theory: The scientific focus shifted to the idea that the origin of life came from non-living organic matter through a series of physical and chemical changes.
Definition of Evolution: It is defined as a slow and continuous process where complex forms of life emerge from simpler forms over millions of years due to changing environmental conditions.

Proposed by French biologist Jean Baptiste de Lamarck.
Use and Disuse: Parts of the body that are used extensively become larger and stronger, while unused parts deteriorate. Lamarck used the giraffe as an example, suggesting they stretched their necks and forelimbs to reach high leaves when ground vegetation became scarce.
Inheritance of Acquired Characters: Lamarck believed these modifications (like the stretched neck) were passed on to the offspring generation after generation.

Definition: Remnants of features that served important functions in the organism's ancestors but have ceased to be of any use to the present possessor, persisting only in a reduced form. There are over a hundred such organs in humans.
Wisdom Teeth: The last molars appearing between 17-20 years of age. They are smaller, less complex, and hardly used for chewing today.
Vermiform Appendix: A narrow, worm-like tube projecting from the caecum (large intestine). It is functionless in humans but is fully functional and helpful in digesting cellulose in herbivorous mammals like sheep.
Pinna: The projecting lobe-like part of the external ear. While it functions to gather sound waves in other mammals, humans have poorly developed, functionless ear muscles and cannot move the pinna.

Proposed by Charles Darwin in his 1859 book, "The Origin of Species". Known as the "Father of Evolution".
Overproduction: Organisms produce more offspring than can possibly survive (e.g., oysters laying 60-80 million eggs, paramecium dividing rapidly). Despite this, population sizes remain relatively constant.
Struggle for Existence: Because space and food are limited, there is intense competition (both within the same species and between different species) to survive.
Variation: No two individuals are exactly alike. These variations are adaptations to the environment. Beneficial variations help an organism succeed, while harmful ones lead to its elimination.
Survival of the Fittest (Natural Selection): Nature selects only those individuals with advantageous variations. The unfit are weeded out. Over many generations, these changes accumulate, leading to the origin of a new species (Speciation).
Example - Industrial Melanism: In Manchester, before the industrial revolution, light-coloured peppered moths (Biston betularia) blended perfectly with tree lichens. Post-revolution, pollution killed the lichens and blackened the trunks with soot. Consequently, dark-coloured moths had a survival advantage, avoiding bird predation, and eventually replaced the light moths.
Neo-Darwinism: Darwin could not explain the source of variations. Modern genetics eventually provided this answer, leading to the modified theory known as Neo-Darwinism.

Carl Linnaeus gave humans the scientific name Homo sapiens. Humans belong to the order Primates alongside monkeys and apes.
Evolution began when forests dwindled due to glaciation, forcing the common ancestors of apes and humans to descend from trees about 15 to 20 million years ago.
Major Physical Changes During Evolution: Bipedal locomotion (walking on two legs), increased cranial (brain) capacity, reduction in canine size (due to omnivorous diet), loss of jaw power, development of a chin, forehead, and brow ridges, erect posture with a lumbar curve, and a reduction in body hair.
Ramapithecus: Considered a prehuman ancestor. Known only from a few teeth and jaw fragments, this early primate walked erect on hindfeet.

Based on fossil records, the stages of human evolution include:

Australopithecus: Short statured (about 120 cm), cranial capacity of 450-600 cm³. Walked nearly straight with a distinct lumbar curve. Had man-like teeth with no simian gap (the space between incisors and canines found in apes). They had a protruding face and no chin.
Homo habilis: The first true man-like ancestor. Height of about 150 cm, cranial capacity 680-735 cm³. Showed true bipedal locomotion, had a less protruding face, but still possessed a hairy body.
Homo erectus: Height 120-150 cm, cranial capacity 800-1125 cm³. First ancestors to hunt animals and use fire. Walked fully upright. They had receding foreheads, projecting jaws, and lacked a chin.
Neanderthal Man: Successors to Homo erectus. Height 160 cm, cranial capacity 1450 cm³. Featured absolute bipedalism, prominent brow ridges, sloping foreheads, and less body hair. Found in Europe, North Africa, and Asia.
Cro-Magnon Man: Appeared about 30,000 years ago, mostly in Europe. Height 180 cm, cranial capacity 1450-1600 cm³. More advanced than Neanderthals; possessed broad faces, prominent chins, lacked eyebrow ridges, and were swift-footed hunters. Made fine stone tools but did not know agriculture. They are the transition point between Neanderthals and modern humans.

Evolved from Cro-Magnon man toward the end of the last Glacial period, first appearing around 11,000 years ago near the Caspian and Mediterranean seas.
Key Characteristics: Perfect bipedal locomotion with four reversed curves in the spine, upright head balancing on the vertebral column, binocular vision, cranial capacity of 1450-1600 cm³ with a highly enlarged cerebrum, prominent chin, steep forehead, and highly reduced body hair.
Cultural Evolution: Evolution shifted from anatomy to culture. Modern man developed logical and syllabic speech, forged metallic tools, cultivated plants, and domesticated animals.
Impact on the Earth: Agriculture led to permanent settlements and thriving civilizations. The scientific revolution further expanded human capabilities, drastically altering the world to benefit mankind, though this has also inadvertently created new survival challenges for humans and other species alike.

Quick Navigation:

Previous Next