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Class 12-science NCERT Solutions Biology Chapter 6 - Evolution

Evolution Exercise 126

Solution 1

  • Joshua Lederberg and Esther Lederberg showed the genetic basis of adaptations in bacteria by culturing bacterial cells by their plating experiment.
  • Lederberg took an agar plate and inoculated bacteria on it by taking a dilute suspension. He thus obtained the master plate of bacteria.
  • He then took a sterile velvet disc mounted on a wooden block and gently pressed it on the master plate. Many bacteria stuck to it.
  • He then pressed this velvet on new agar plates and obtained new master plates inoculated with bacteria which were exact replicas of the original master plate.
  • He again took the impression from the master plate and applied it on another agar plate containing antibiotic penicillin.
  • He observed that few colonies were able to grow on the agar plate and were said to be penicillin-resistant, while the others that did not grow were said to be penicillin-sensitive.
  • According to Darwin’s view, there were a few bacteria carrying mutant genes in the original suspension of bacteria which had the ability to survive the action of penicillin and form colonies. These mutations had arisen by chance and had not been induced by penicillin.
  • This reveals the phenomenon of natural selection where the mutant variety of bacteria had the selective advantage to overcome the effect of penicillin rather than the non-mutant type.

Solution 2

Few new fossil discoveries or controversies about evolution found from newspapers and popular science articles:

  • Large-feathered dromaeosaur: A recent fossil study described a large, feathered dinosaur Zhenyuanlong. It had asymmetrical feathers probably as an adaptation for flight but was likely too large to fly. The discovery of this dinosaur illustrates that feathers probably evolved for reasons of sexual display or insulation rather than purely for powered flight.
  • Monster scorpion: A 1.7-metre long 460 million-year-old sea scorpion was discovered in the remains of an ancient ocean in Iowa. It had a strangely shaped head and paddle-like appendages. It is the oldest sea scorpion ever discovered.
  • 540 million-year-old brain: It is a common prenotion that brain tissue, like most soft tissues, does not fossilise. Recent studies have described a fossilised shrimp-like creature which had preserved brain tissue. This amazing discovery of a brain reveals clues about the evolution of the shape of the central nervous system, which is typically impossible to trace in the fossil record.
  • Challenge to human evolution: A 14,000-year-old thigh bone belonging to a mysterious human species was discovered in southwest China. Analysis of the remains suggests a separate group of prehistoric ancient humans different from the Neanderthals or Homo sapiens. This ancient group lived until as recently as the last Ice Age, which ended about 12,000 years ago.

Solution 3

A species is a group of similar individuals differing from the members of other species. They interbred freely, produce fertile offspring, share a gene pool and are relatively stable. The species is the smallest unit of classification.

Solution 4

During human evolution, the ancestors of humans went through several changes with respect to brain size, skeletal features and dietary preferences as listed in the table below:

Ancestors in human evolution

Brain size


Skeletal features

Dietary preferences



Large brain (size unknown)

Large canines and incisors, square molars

  • Arms and legs of the same length.
  • Arboreal, knuckle-walker.

Soft fruits and leaves



Small brain (size unknown)

Small canines, flattened molars

  • Walked erect on the feet.
  • Had a short face.

Hard nuts and seeds

Australopithecus afarensis


500 cm3

Small canines and incisors

  • About 1.05 metres high.
  • Mainly a terrestrial creature with bipedal locomotion.

Mostly fruits and leaves rather than seeds and other hard plant material

Australopithecus africanus

350–450 cm3

Small canines

  • Low forehead, protruding face, lack of chin.
  • Probably not taller than 4 feet but walked upright.

Essentially fruits but hunted with stones

Homo habilis

650–800 cm3

Small canines

  • About 1.5−1.8 metres tall.
  • Bipedal and moved erect.

Probably did not eat meat

Homo erectus

800–1,200 cm3

Small canines

  • About 5.5 feet tall with a bowl-shaped pelvis.
  • Foot was arched to support body weight, and the grasping ability of the foot was completely lost.

Probably ate meat

Homo neanderthalensis

1,400 cm3

Heavier than modern teeth, wisdom teeth

  • Flat cranium, sloping forehead, no chin.


Homo sapiens fossils

1650 cm3

Teeth closer together, wisdom teeth

  • Large skull, high forehead.
  • Prominent chin and broad flat face.
  • Sturdy body and less hair.

Ate both plants and animals

Homo sapiens sapiens

1,500 cm3

Strong jaws with teeth closer together, wisdom teeth

  • Slight raising of the skull cap.
  • Thinning of skull bones.

Ate both plants and animals

Solution 5

Yes, animals such as orangutans, chimpanzees, gorillas, elephants and rhesus macaques are self-conscious. They have well-developed facial muscles for expressing facial gestures and communicating with animals bearing similar self-consciousness.

Solution 6

Modern Day

Ancient Fossil

Modern horse (Equus)

Eohippus (=Hydracotherium) - Dawn horse - The first fossil found in the evolution of horse.

Camel (Camelus)

Protylopus - The first ancestor of modern camel.

Modern Elephant (Elephas)

Moeritherium - The ancestor of modern elephant.

Man (Homo sapiens)

Ramapithecus - The oldest of man's ancestors.


Seymouria - The missing link between amphibians and reptiles.


Cyanognathus - The missing link between reptiles and mammals.


Archaeopteryx - The missing link between reptiles and birds.


Cyanognathus - The missing link between reptiles and mammals.

Apes and mammals

Dryopithecus - The common ancestor of apes and mammals.


PropliopithecusThe ancestor of gibbons.

Frogs, toads and salamanders

Some stem amphibians called Labyrinthodontia (e.g. Eryops) gave rise to modern amphibians such as frogs, toads and salamanders.


Gyroceros – The ancestor of the mollusc family.


Belemnite The extinct fossil of cephalopods.

Solution 7

With the help of your books and the Internet, practise drawing diagrams of various plants and animals you have come across. Some diagrams of different plants and animals have been illustrated below for your reference.

Diagrams of animals:

i.Peripatus - Walking worm



ii.Protopterus - African lung fish

iii.Ornithorhynchus - Duck-billed platypus


Diagrams of plants:

i.Stem tendrils of Passiflora

ii.Leaf tendrils of Pisum sativum

iii.Leaf spine of barberry


Solution 8

Adaptive radiation is a process of divergent evolution in which members of the same ancestral species of a large taxonomic group are evolved along different lines in different habitats of the same geographical area.

Darwin's finches of the Galápagos Islands exhibiting a variety of beaks are an example of adaptive radiation. Darwin reported that finches found on different islands of the Galápagos Islands with varied environmental conditions differed with respect to bill size and shape due to different feeding habits. However, they were closely related to one another and had evolved from a common ancestral seed-eating ground finch living in a particular geographical area of the South American mainland. Later, these finches radiated to different geographical areas and adapted differently in their feeding habits, developing different kinds of beaks.

Solution 9

No, we cannot call human evolution as adaptive radiation. In human evolution, brain size, skeletal structure, dietary preference and social and cultural evolution occurred, while in adaptive radiation, the origin, basic structure and development of the organs remain same, only morphological changes occur.

Solution 10

The evolutionary history of horse has been traced from the fossils discovered from the tertiary rocks in North America. The ancestry of horse is tabulated below:

Scientific name

Common name


Age (in million years)

Height (in cm)





Dawn horse



(Fox size)

4 (1 splint finger)

3 (2 splint toes)

Short crowned


Intermediate horse



60 (Pony size)

3 (1 splint finger)

3 (1 splint toe)

Short crowned


Ruminating horse



100 (Ass size)

3 (1 splint finger)

3 (1 splint toe)

Long crowned


Pliocene horse




Only 3rd finger (2nd & 4th digits as splint bones)

Only 3rd toe

(2nd & 4th digits as splint bones)

Long crowned


Modern horse




Only 3rd finger (2nd & 4th digits as splint bones)

Only 3rd toe

(2nd & 4th digits as splint bones)

Long crowned

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