Notes-Part-2-Class-12-Biology-Chapter-5-Origin and Evolution of Life -Maharashtra Board

Chapter-5-Origin and Evolution of Life

Maharashtra Board-Class-12th-Biology-Chapter-5


Topics to be learn - Part-1

  • Origin of life (Protobiogenesis)
  • Chemical Evolution
  • Organic Evolution
  • Darwinism
  • Mutation Theory of Life (Self-assembly theory of origin of life)
  • Modern synthetic theory of evolution
  • Mechanism of organic evolution

Topics to be learn in Part-2 :

  • Hardy-Weinberg’s principle
  • Adaptive radiation
  • Evidences of organic evolution
  • Speciation
  • Geological time scale
  • Human Evolution

Hardy-Weinberg’s principle :

  • Hardy and Weinberg were two scientists who proposed a concept of genetic equilibrium popularly known as Hardy-Weinberg principle or equilibrium.
  • This principle states that gene, allele or genotype frequencies remain the same from generation to generation unless disturbed by factors like mutation, non-random mating, genetic drift, etc.
  • For explaining the concept of equilibrium they assumed that there are two alleles located at a single locus (A and a).
  • Their respective frequencies are p and q.
  • The frequency of genotype AA is p, for 2Aa is 2pq and for aa is q.
  • The equilibrium equation is p2 + 2pq + q2 = 1
  • It says that if sum total of gene frequencies is 1, then sum total of genotype frequencies is also equal to 1. When the equilibrium is disturbed then only evolution occurs.

Adaptive radiation :

  • Adaptive radiation is the process of evolution which results in transformation of original species to many different varieties.
  • The well-known example of adaptive radiation is Darwin’s Finches. When Charles
  • Darwin went on his voyage to Galapagos, islands, he noticed finches which is a variety of small birds.
  • According to Darwin’s observations, the American main land species of finches was the original one which must have migrated to the different islands of Galapagos.
  • Since environmental conditions here were different, they adapted in various ways to the differing environmental conditions of these islands.
  • Original bird had a beak suited for eating seeds, but the changed feeding pattern has changed the shape of beaks too. Some birds also show altered beaks for insectivorous mode. Thus, this demonstrated adaptive radiation.
  • Adaptive radiation in Australian Marsupials is also Well studied. In Australia, there are many marsupial mammals who evolved from common ancestor.
  • Adaptive radiation leads to divergent evolution.

Evidences of organic evolution :

The theory of organic evolution states that the present day complex organisms have originated from earlier simpler forms of life.

The process of evolution is supported by evidences provided by various branches of biology such as : Palaeontology, comparative anatomy, embryology and molecular biology.

(i) Palaeontology : The study of ancient life with help of fossils is called palaeontology.

  • The study of fossils provides the most convincing and direct evidence of evolution.
  • Fossils are formed in sedimentary rocks, amber (yellowish fossils resin), ice, peat bogs etc.
  • During fossilization, the primitive forms of organisms occupy the older, lower layers and the advanced forms occupy the upper, more recent layers of the earth.

Types of fossils :

Types of fossils :

There are four main types of fossils : actual remains, moulds, casts and compressions.

  • Actual remains : The most common type of fossil is actual remains in which the plants, animals and human bodies are seen embedded in permafrost of arctic or alpine snow. Due to severe cold temperature, the bodies remain preserved in the actual state, E.g., Fossil of Woolly Mammoth in Siberia. Many insects and smaller arthropods remained embedded and thus preserved in amber or hardened resin.
  • Moulds : Hardened encasements formed in the outer parts of organic remains of animals or plants form moulds. The organisms later decays leaving cavities or the impression in permanent form. E.g. Footprints.
  • Casts : Casts are hardened pieces of mineral matter which is deposited in the cavities of moulds.
  • Compressions : A thin carbon film indicates the outline of external features of ancient organism, but other structural details are not seen.


Significance of Palaeontology :

  • It is useful in reconstruction of phylogeny.
  • It helps in studying various forms and structures of extinct animals.
  • It provides record of missing link between two groups of organsims.
  • It helps in the study of habits of extinct organisms.

A connecting link (missing link) :

A connecting link (missing link) : A connecting link is an intermediate or transitional state between two systematic groups of organisms.

  • It bears characters common to both these groups on either side of its position. Thus it represents an evolutionary line.
  • Connecting links are also called a missing link.
  • Archaeopteryx, the extinct bird is a connecting link between Reptiles and Aves.
  • Seymouria is a connecting link between Arnphibia and Reptilia.
  • Ichthyostega is a connecting link between Pisces and Amphibia.


(ii) Morphology (comparative anatomy) : Morphology deals with study of external structures while, anatomy deals with study of internal structures.

From comparative study of morphology and anatomy we can understand the evolutionary aspects in the form of homologous, analogous and vestigeal organs.

Homologous organs :

Homologous organs:

Homologous organs are those organs which are structurally similar but functionally dissimilar.


  • Forelimbs of frog, lizard, bird, bat, whale and man are homologous to each other. All the limbs are morphologically similar in construction such as similar limb bones but are dissimilar in function. Frog limbs are meant for hopping, lizard limbs help in crawling, birds and bats fly with the help of forelimbs while whale uses it for swimming and man for handling the objects.
  • Vertebrate heart and brain.
  • In plants, thorns of Bougainvillea and tendrils of Cucurbita represent homology.

The structural similarities between the homologous organs indicate that they have a common ancestry.

Different homologous organs indicate divergent evolution or adaptive radiation.

Homologous organs help in tracing the phylogenetic relationships.


Analogous organs :

Analogous organs :

Analogous organs are similar in function but dissimilar in structural details,

They do not help to trace the relationship in the evolution but help to understand the convergent evolution.

Structural modifications in the organs are due to similar habitat.

Examples :

  • Wing of an insect and wing of a bird both are useful in flight S0 they are functionally similar but are structurally different. Insect wing is formed by exoskeleton expansion while bird wing is the modified forelimb.
  • Eye of the Molluscan octopus and of eye of mammals. They differ in their retinal position, structure of lens and origin of different eye parts, but both perform function of vision.
  • The flippers of penguins (birds) and dolphins (mammals).
  • Sweet potato which is a root modification and potato which is a stem modification, both perform similar function of storing starchy food.


Vestigial organs :

Vestigial organ :

  • Vestigial organs are rudimentary organs which are imperfectly developed and non-functional, degenerate structures.
  • These organs in animals become functionless thus their presence in the body is not required.
  • But they are simply present as they descend down during evolution and continue to exist.
  • In the process of evolution, they may disappear totally.
  • They indicate evolutionary line as they were once functional in the ancestors.

Examples of vestigial organs in human beings :

  • Caecum and vermiform appendix: These are functional in herbivorous animals where they help in cellulose digestion. In humans they are functionless.
  • Nictitating membrane situated in the eyes of humans. It is a remnant of third eyelid.
  • Coccyx or tail vertebrae which shows remnant of tail, wisdom teeth or 3rd
  • These organs indicate that human beings descended from ape like ancestors.


(iii)Embryological Evidences : Different vertebrates have similar development pattern.

(iv) Molecular Biological Evidences :

  • Different organisms have basic similarities in their molecules constituents, and the cellular.
  • All living organisms have the same basic structural and functional unit, i.e. cell.
  • Cell organelles such as endoplasmic reticulum, Golgi bodies, mitochondria, etc. are present in different types of organisms.
  • Proteins and gene performing different functions have the same basic pattern which shows a common ancestry.
  • Catabolic activities of liberating energy, synthesis of macromolecules such as proteins, carbohydrates, nucleic acids, etc. are similar in different organisms.
  • ATP is the common energy currency of all the organisms.
  • All the above facts are called molecular evidences in favour of evolution.

Speciation :

Speciation : The process of formation of a new species from the pre-existing species.

Species : A group of similar organisms that can interbreed and produce a fertile offspring in nature

Types of Speciation :

Types of Speciation

(i) Intraspecific Speciation :

(a) Allopatric speciation : Allopatric speciation is the formation of a new species due to separation of a segment of population from the original population.

  • There is geographical barrier cutting across the species range during such speciation.
  • Allopatric speciation does not have physiological barrier.
  • Migration of individual are also helpful in allopatric speciation.
  • g. African elephant and Indian elephant.

(b) Synipatric speciation : Formation of species within single population without geographical isolation.

  • There is no geographical isolation during sympatric speciation.
  • Sympatric speciation is due to physiological or reproductive isolating barrier.
  • Mutations are helpful in sympatric speciation.
  • g. Cichlid fishes in Lake Victoria.

(ii) Interspecific Speciation :

  • Hybridization : Two different species on crossing may give rise to a new species. E-g- Mule and Hinny.


Geological time scale :

  • Geological time scale is the arrangement of major divisions of geological time into eras, periods and epochs on the time scale.
  • This division is based on the study of fossilized organisms obtained from the different strata of the earth.
  • The characteristic significant events that occurred in the organization of organisms helped the geologists to understand the geological time scale.
  • The major divisions of geological time are called eras.
  • The eras are divided into periods and the periods into epochs.
  • By studying fossils in the earth crust, the evolutionary changes in the organisms have been traced out.

Geological time scale chart :


Human Evolution :

Major changes in evolutionary development of man :

  • Increase in size and complexity of brain and enhanced intelligence with increased cranial capacity.
  • Bipedal locomotion, erect posture.
  • Opposable thumb.
  • Shortening of forelimbs and lengthening of hind limbs.
  • Development of chin. Orthognathous face.
  • Broadening of pelvic girdle, development of lumbar curvature.
  • Articulated speech, art, development of tools, social and cultural development.

Classification of mammals :

Classification of mammals :


Origin and evolution of human being :

Order Primates is divided into two sub-orders :

  • Prosimii : Including lemurs, lorises and tarsiers
  • Anthropoidea : Including New world monkeys—Ceboidea, Old world monkeys-Cercopithecoidea, Apes and Man - Hominoidea.

Hominoidea evolved in Miocene in three separate lines are shown as under :

  • Hyalobatidae — Gibbons
  • Pongidae — Gorilla, Chimpanzee and Orangutan
  • Hominidae — Primates with human characteristics.

 Palaeontological evidences of human evolution:

The available fossils are skulls, mandibles, teeth, bones like humerus, femur and stone implements.

Important stages in the origin of man :

  • Ape like stage : Dryopithecus
  • Men-like stage : Ramapithecus
  • Connecting link between ape and man : Australopithecus.
  • Handy man : Homo habilis
  • Ape man : Homo erectus
  • Advanced prehistoric man : Homo neanderthalensis (Neanderthal man)
  • Modern man : Homo sapiens

Dryopithecus :

Dryopithecus : Dryopithecus is also called proconsul.

  • Leakey discovered the fossils of Dryopithecus, on an island in Lake Victoria of Africa. Also the fossil was found in Haritalyanga in Bilaspur district of Himachal Pradesh.
  • It was a group of apes that lived in Miocene epoch about 20 to 25 million years ago.
  • Several species of Dryopithecus are available, the important among these is African fossil D. africanus.
  • Dryopithecus has a close similarity to chimpanzee and also walked like a modern chimpanzee.
  • The structure of its limbs and wrists show that knuckle walking was lesser in it. It used the flat of its hands like a monkey.
  • It had arms and legs of the same length and had a semi-erect posture.


Ramapithecus :

Ramapithecus :

  • Ramapithecus was on direct line of evolution of man.
  • It was called an ape-man like primate.
  • Its fossils were obtained in the form of teeth and jaw bones in the rocks of Siwalik Hills in India by Lewis and also in Kenya.
  • It existed during late Miocene and early Pliocene epoch about 14 to 12 million years ago.
  • It walked erect on its hind limbs.
  • It had close similarity with chimpanzee.
  • Some scientists believe that Dryopithecus evolved into Ramapithecus.


Australopithecus :

Australopithecus :

  • Australopithecus is considered as connecting link between ape and man.
  • Its fossils were obtained from Toung valley in South Africa, from Ethiopia and Tanzania.
  • It was in late Pliocene or early Pleistocene epoch about 4 to 1.8 million years ago.
  • It was about 4 feet tall. It had prognathous face, with larger jaws. Chin was absent.
  • Lumbar curvature was present.
  • It walked upright.
  • The cranial capacity was about 450 to 600 CC.
  • Therefore, it was called man with ape brain.


Homo habilis :

Homo habilis :

  • Homo habilis is described as Handy man.
  • His fossils were obtained from Olduvai Gorge in Tanzania, Africa.
  • He existed in late Pliocene or early
  • Pleistocene about 2.5 to 1.4 million years ago.
  • He was lightly built.
  • Fossil of lower jaw was obtained which showed that his dentition was more like modern man with small molars.
  • He walked erect. His cranial capacity was 640 to 800 cc.
  • He did not eat meat and made stone tools.


Homo erectus :

Homo erectus :

  • Homo erectus was also known as Java man or Peking Man due to his fossils obtained from these areas.
  • He was also called ape man.
  • He lived in the middle Pleistocene epoch about 1.5 million years ago.
  • He was 5 feet in height with prognathous face, massive jaws, huge teeth and bony eye brow ridges.
  • Chin was absent.
  • He walked erect.
  • The cranial capacity was 900 cc,
  • He was omnivorous and probably used fire and ate meat.


Neanderthal man :

Neanderthal man :

  • The scientific name of Neanderthal man is Homo neanderthalensis. He is described as advanced prehistoric man.
  • It was called Neanderthal man because its first fossil was collected from Neanderthal Valley in Germany by Fuhlrott (1856).
  • It was heavily built and short and had outwardly curved thigh bones.
  • The facial features were as follows : prominent brow ridges, thick skull bones, low and slanting forehead, deep jaw without a chin, etc.
  • Neanderthal man existed in late Pleistocene epoch about 1,00,000 to 40,000 years ago.
  • It was widely spread in Europe, Asia and North America. It became extinct about 25,000 years ago.
  • The cranial capacity of Neanderthal man was about 1400 cc, which was roughly equal to that of modern man. He used hide for dressing.
  • It showed intellectual development in constructing and using flint tools and fire.
  • The Neanderthal men used to bury their dead bodies along with their tools and perform ceremonies.


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