Chapter 14 : Human Evolution - Selina Solutions for Class 10 Biology ICSE

(c) Lamarck

(d) Darwin

(b) Moth

(a) Ramapithecus

Neanderthal man and modern man exhibit different characteristics, and hence, they are considered two distinct species. Below are the characteristic differences between Neanderthal man and modern man:

(a) Use and disuse:

Parts of the body which are used extensively become larger and stronger, while those which are not used deteriorate.

(b) Inheritance of acquired characters:

An organism could pass its modifications to its offspring.

Three vestigial organs found in humans are wisdom teeth, vermiform appendix and pinna.

Biston betularia is a classical example of 'natural selection'.

(a)The fossil history of humans is fragmentary.

(b)The first remarkable human fossil was that of Homo habilis.

(c)Evolution is an ever continuing process. 

Charles Darwin proposed the idea of natural selection. Darwinism or Theory of Natural Selection is based on some facts:

(i) Overproduction:

• All organisms have the capacity to reproduce at a very high rate.
• However, organisms cannot survive by reproduction alone.
• Due to lack of food and space, offspring soon begin to die.
• Some are eaten by predators, while some get destroyed due to adverse environmental conditions.

(ii) Struggle for existence:

• Overproduction of organisms results in a struggle for existence among organisms.
• The struggle is to obtain food, space and mate.

 (iii) Variation:

• Progeny of the same parents are not exactly alike. Such differences are known as variations. The variations may be harmful or advantageous.

(iv) Survival of fittest:

• In the struggle for existence, organisms that develop new favourable characteristics will survive in the long run. This idea is called 'Survival of the fittest'.
• Organisms which survive will transmit favourable characters to their offspring.
• These characters get accumulated and give rise to new species.

• The best example is industrial melanism showing the effect of industrial pollution on the moth population on a nearby tree.
• Before industrialisation, light-coloured lichen used to grow on trees and moth predators could not spot white moths easily, while dark moths were lesser in the moth population. However, due to industrial pollution, lichens could not grow on trees and it became difficult for predators to spot dark moths on the dark background of the stem bark, and thus, the population of white moths became less than that of dark moths.
• This process is termed industrial melanism. Before industrialisation, white moths were better adapted towards nature, but after industrialisation, dark-coloured moths were more fit towards the changed environmental conditions.

Australopithecus averaged about 120 cm tall, and the cranial capacity ranged from 450-600 cm³. It had a low forehead, protruding face, lack of chin and low brain capacity. Teeth resembled man because the dental arc was a smoothly rounded parabola, and a simian gap was absent which justifies a human ancestor.  

Correct sequence through which modern man evolved:

1. Australopithecus

2. Homo habilis

3. Homo erectus

4. Neanderthal man

5. Cro-Magnon

6. Homo sapiens

• According to the theory of inheritance of acquired characters, 'the changes in structure or function of any organ acquired during the life-time of an individual in response to changes in the surrounding environment are inherited by offspring and keep on adding over a period of time'. 
• This theory states that characters are acquired by animals in two ways:

- Effects of environment

- Use and disuse of body parts

• For example, the long neck of giraffe is explained by Lamarck on the same principle. Giraffe, which lived in the dry and arid deserts of Africa, tried to reach the foliage high up on the trees to eat them as there was no vegetation on the ground. In the process, its neck and forelegs got stretched a bit and this was inherited by the next generation. Then, in the next generation, the same efforts continued. Gradually, through many successive generations, we got a giraffe having a long neck and long forelegs.

Organs which are found in reduced or rudimentary condition and do not perform any function in the possessor are called vestigial organs or non-functional organs. They help in understanding the history of evolution and continuity of life.

1. Wisdom teeth: They are the last molars which appear last at the age of 17-20 years.

2. Vermiform appendix: It projects from the blind end of the caecum and is a functionless organ in humans. It is helpful in herbivorous mammals.

3. Pinna: It is the lobe-like part of the external ear which humans cannot move as in other mammals. As it is poorly developed, it is a vestigial organ.

Stage A: Australopithecus

Stage B: Homo sapiens sapiens

(a) Contrasting characters between Australopithecus and  Homo sapiens sapiens: 

(b) Stages of human evolution in their correct sequence:

Australopithecus ® Homo habilis ® Homo erectus ® Neanderthal man ® Cro-Magnon man ® Homo sapiens sapiens

(c) Characteristic features of stage B (Homo sapiens sapiens):

• Bipedal locomotion with four reversed curves in the spine

• Forehead steep, reduced brow ridges

(a) Industrial melanism

(b) Common name: Peppered moth

Scientific name: Biston betularia

(c) Reasons for changes in the two figures:

• Before the Industrial Revolution, a thick growth of white-coloured lichen covered the trees. As a result, the light-coloured moths were camouflaged and survived under this cover, while the dark-coloured moths were easily spotted by predators.

• After the Industrial Revolution, pollution resulted in a decline in the growth of lichens. The tree bark got exposed due to the absence of lichens. As a result, dark-coloured moths now got an advantage of a dark background, were camouflaged and survived, while the light-coloured moths were easily picked by predators.

• This showed that in a mixed population, those moths which could adapt to the changing environment after the Industrial Revolution survived and increased in number, while the ones which could not adapt were slowly wiped out from the population.

(d) (i) Natural selection

During the struggle for existence, only those individuals which have advantageous variations survive while the ones which lack these variations are wiped out. Nature selects only those variations which are suitable for existence. This process is called natural selection.

(ii) Charles Darwin