Chapter 20: History of Life On Earth

NIOS Class 10 Science Chapter 20 Question Answers

INTEXT QUESTIONS 20.1

1. When did the Earth come into existence?

Ans: 4.5 to 5 billion years ago.

2. Why did life not exist on the primitive Earth?

Ans: Life did not exist on primitive Earth because its atmosphere was filled with harmful gases such as methane, ammonia, and hydrogen cyanide, along with minor gases like carbon dioxide and carbon monoxide. There was no oxygen to sustain life. Moreover, the atmosphere was too hot.

3. What are fossils?

Ans: Fossils are remains of plants, animals, and lower living beings that lived in the past.

4. What is meant by the Cambrian explosion?

Ans: The Cambrian explosion refers to the time 600 million years ago, when sudden or explosive diversification of life into multicellular organisms with a variety of body plans and lifestyles, of all those invertebrates and higher plant groups took place. 

5. From the geological time scale, find out the time in million years ago(mya) when:

(i) dinosaurs became extinct ______________

(ii) human evolution began ______________

(iii) flowering plants became dominant on earth ______________

Ans:

(i) 144-65 million years ago 

(ii) 1.5-2 million years ago (mya) 

(iii) 57-34 million years ago 

INTEXT QUESTIONS 20.2

1. Who is Charles Darwin? Name his famous book on natural selection.

Ans: Charles Darwin was the founder of the theory of evolution, which revolutionized our understanding of life and its diversification on Earth. His famous book on natural selection is, Origin of Species, published in 1859.  

2. Mention his two major contributions.

Ans: According to his theory of evolution, 

(i) All organisms related through ancestry

(ii) The mechanism that causes diversification of species from ancestors is ‘Natural Selection’.

3. What is the function of Natural Selection?

Ans: Fittest individuals in a population survive and reproduce to leave the fit genes in the next generation. Organisms with slightly less survival value will probably perish first, leaving the fittest to pass on their genes to the next generation. Therefore, the best adapted individuals were selected by nature to survive and leave offspring for the next generation.

4. What is meant by Neo-Darwinism?

Ans: While Darwin established the occurrence of heritable variations, he was unaware of the principles of genetics developed by Mendel before he published his book. Neo-Darwinism is the incorporation of Mendelian genetics into Darwinian theory later by evolutionary biologists.  

5. Name the evolutionary mechanism that causes organisms to evolve.

Ans: Natural Selection

INTEXT QUESTIONS 20.3

1. When did human evolution begin?

Ans: 1.5 to 2 million years ago

2. Who is ‘Lucy’?

Ans: The fossils of an australopithecine, Australopithecus, found in African rock deposits, which is deemed the first human ancestor, are named ‘Lucy’.

3. Write the scientific names of Cro-Magnon and Neanderthal man

Ans: They were both Homo sapiens.

4. With which group of animals do humans share their immediate ancestors?

Ans: Apes

5. Name the earliest ancestors of modern-day humans.

Ans: Australopithecus

TERMINAL EXERCISES

1. What were the primitive conditions on Earth? Tell your friend/cousin/colleague. Then ask your friend to name the gas which was absent without which today no life can exist.

Ans:   The primitive Earth had its atmosphere made of lethal gases such as methane, ammonia, hydrogen cyanide, carbon dioxide and carbon monoxide. It was uninhabitable as it lacked the most essential gas, oxygen, that sustains life. 

2. What are the main points of Oparin’s theory of the Origin of life? Make a five-point quiz on it.

Ans: According to Oparin’s theory of the Origin of Life, 

• Life originated in the shallow seas where important organic compounds, such as amino acids that are the building blocks of life, were present in high concentrations.  
• This provided the necessary ingredients for the emergence of life.  
• These might have formed from inorganic substances that were washed down with torrential rains.
• It formed the primordial soup in which life originated. 

3. Mention Darwin’s two major contributions to evolutionary ideas.

Ans:   According to his theory of evolution, 

• (i) All organisms are related through ancestry
• (ii) The mechanism that causes diversification of species from ancestors is ‘Natural Selection’.
• (iii) He observed that all organisms tend to produce more offspring than can survive. 
• (iv) Since most offspring do not survive, all organisms must be going through a struggle for survival, being eaten, suffering from disease and competition.
• (v) The struggle for existence caused a large number of individuals to die.
• (vi) Nature selects the fittest individuals of the population.

4. Write a note on NeoDarwinism.

Ans:  While Darwin established the occurrence of heritable variations, he was unaware of the principles of genetics developed by Mendel before he published his book. Neo-Darwinism is the incorporation of Mendelian genetics into Darwinian theory later by evolutionary biologists.  

5. List the five major events during the geological time period beginning from the origin of life. You may begin with the origin of animals.

Ans:  

• Origin of animals 
• First vertebrates (jawless fishes)
• First amphibians and insects
• Origin of mammalian orders
• Ice ages: Humans appear

6. State major trends and stages of human evolution. Do you think humans are still evolving? Write five sentences to justify your response.

Ans:  The trends of human evolution are towards 

• (i) bipedal gait or walking on two legs and
• (ii) acquiring a large brain.

The other part, write in your own words. 

7. Earlier groups of animals became extinct due to natural happenings. Today, how is it that wild animals have become endangered and are heading towards extinction?

Ans:   The reasons for the extinction of wild animals are many, including,

• Human activity that compromised their existence.
• Change in habitat, like losing their natural habitat through degradation or human activities.
• Overexploitation
• Pollution
• Climate change
• Natural adaptations

8. Write a ten-sentence conversation between your father and yourself justifying the need for conservation of animals living in our forest.

Ans:   Write in your own words.

Additional Study Materials

• Chapter 19 Classification of Living Organisms