Chapter 20. HISTORY OF LIFE ON EARTH
INTEXT QUESTIONS 20.1
1. When did the earth come into existence?
Answer: Earth formed about 4.54 to 5 billion years ago from gas and dust around the young Sun.
2. Why did life not exist on primitive earth?
Answer: Life did not exist on primitive Earth because it was too hot, and the atmosphere contained only certain gases with a reducing atmosphere, which was not suitable for life.
3. What are fossils?
Answer: Fossils are the remains of organisms that lived in the past and are preserved in rocks.
4. What is meant by Cambrain explosion?
Answer: The Cambrian explosion was the time about 600 million years ago when there was a sudden formation of different groups of invertebrates on Earth.
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 ______________
Answer: (i) Dinosaurs became extinct — 144–65 mya
(ii) Human evolution began — 1.5–2 million years ago (mya)
(iii) Flowering plants became dominant on Earth — 57–34 mya
INTEXT QUESTIONS 20.2
1. Who is Charles Darwin? Name his famous book on natural selection.
Answer: Charles Darwin was an English scientist who explained the theory of evolution by natural selection. His famous book on natural selection is On the Origin of Species.
2. Mention his two major contributions.
Answer: The two major contributions of Charles Darwin are:
(i) all organisms related through ancestry
(ii) Mechanism of evolution is Natural Selection.
3. What is the function of Natural Selection?
Answer: The function of Natural Selection is that the fittest individuals in a population survive and reproduce, passing their fit genes to the next generation.
4. What is meant by Neo Darwinism?
Answer: Neo-Darwinism is Darwin’s theory of evolution modified in the light of advances in genetics.
5. Name the evolutionary mechanism that causes organisms to evolve.
Answer: The evolutionary mechanism that causes organisms to evolve is Natural Selection.
INTEXT QUESTIONS 20.3
1. When did human evolution begin?
Answer: Human evolution began about 1.5–2 million years ago (mya) according to the geological time scale referred to in your lesson.
2. Who is ‘Lucy’?
Answer: Lucy is a fossil of an Australopithecus, considered one of the earliest human-like ancestors. The fossils of Lucy were found in African rock deposits.
3. Write the scientific name of Cro-magnon and Neanderthal man
Answer: The scientific name of Cro-Magnon man and Neanderthal man is Homo sapiens.
4. With which group of animals do humans share their immediate ancestors.
Answer: Humans share their immediate ancestors with apes.
5. Name the earliest ancestors of modern day humans.
Answer: The earliest ancestors of modern-day humans were 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.
Answer: Primitive Earth was very hot and had a reducing atmosphere containing gases like methane, ammonia, hydrogen and water vapour. Oxygen gas was absent and today life cannot exist without oxygen.
2. What are the main points of Oparins’theory of Origin of life? Make a five point quiz on it.
Answer: The main points of Oparin's theory are:
(i) Primitive atmosphere: Earth's early atmosphere was reducing (lacked free oxygen) and contained methane, ammonia, hydrogen, and water vapor.
(ii) Abiotic synthesis: Simple organic molecules (amino acids, sugars) formed from inorganic substances using energy from UV rays, lightning, and heat.
(iii) Formation of coacervates: These organic molecules aggregated into colloidal droplets called coacervates, which had a primitive membrane and could absorb substances.
(iv) Primitive metabolism: Coacervates developed simple internal chemical reactions, resembling early metabolism.
(v) Self-replication & evolution: Over millions of years, these systems evolved the ability to reproduce and pass on characteristics, leading to the first primitive cells.
5-Point Quiz on Oparin's Theory
(a) What type of atmosphere did Oparin propose existed on primitive Earth?
Answer: Reducing atmosphere with no free oxygen.
(b) What were the main gases in Oparin's primitive atmosphere?
Answer: Methane, ammonia, hydrogen and water vapor.
(c) What energy sources did Oparin suggest drove the formation of organic molecules?
Answer: UV rays, lightning and heat .
(d) What are the microscopic organic droplets called that Oparin considered precursors to cells?
Answer: Coacervates .
(e) According to Oparin, what crucial ability did coacervates eventually develop that led to true cells?
Answer: Self-replication / the ability to reproduce .
3. Mention Darwin’s two major contributions to evolutionary ideas.
Answer: Darwin’s two major contributions to evolutionary ideas are:
(i) shared ancestry and
(ii) natural selection as mechanism of evolution.
4. Write a note on NeoDarwinism.
Answer: Neo-Darwinism is the modern form of Darwin’s theory of evolution. It explains evolution by combining Darwin’s theory of natural selection with the knowledge of genetics. According to Neo-Darwinism, variations arise due to changes in genes and these variations are inherited from one generation to another. Natural selection acts on these variations and helps organisms with favourable traits survive and reproduce. Over many generations, these changes accumulate and may lead to the formation of new species. Thus, Neo-Darwinism explains evolution using both heredity and natural selection.
5. List the five major events during geological time period beginning from origin of life. You may begin with origin of animals.
Answer: Five major events during the geological time period are:
(a) Origin of animals: Different groups of invertebrate animals appeared during the Cambrian explosion.
(b) Origin of fishes: Fishes became the dominant vertebrates in water.
(c) Origin of amphibians and reptiles: Animals started living on land.
(d) Age of dinosaurs : Reptiles, especially dinosaurs, dominated the Earth.
(e) Evolution of mammals and humans: Mammals became dominant after the extinction of dinosaurs, and eventually humans evolved.
6. State major trends and stages of human evolution. Do you think humans are still evolving? Write five sentences to justify your response.
Answer: Major trends and stages of human evolution:
(i) Australopithecus was the earliest human-like ancestor.
(ii) It was followed by Homo habilis.
(iii) Then came Homo erectus.
(iv) Later, Neanderthal man and Cro-Magnon man appeared.
(v) Finally, modern humans, Homo sapiens, evolved.
Yes, humans are still evolving.
(a) Evolution is a continuous process.
(b) Genetic variations continue to occur in human populations.
(c) Environmental conditions keep changing.
(d) Natural selection still acts on certain traits.
(e) Therefore, humans may continue to develop new adaptations in the future.
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.
Answer: Earlier, groups of animals became extinct mainly due to natural causes such as climatic changes, volcanic eruptions and other geological events. Today, many wild animals have become endangered mainly because of human activities. These include deforestation, destruction of habitats, hunting, pollution, industrialisation and excessive use of natural resources. Urbanisation has reduced forest areas and disturbed ecological balance. As a result, many species are unable to survive and reproduce, causing them to move towards extinction.
8. Write a ten sentences conversation between your father and yourself justifying the need for conservation of animals living in our forest.
Answer: A ten-sentence conversation between a father and child justifying the need for conserving forest animals is given bellow:
Myself: Dad, I read that many forest animals are disappearing fast. Why should we really care about conserving them?
Father: That's a great question. Every animal plays a unique role in maintaining the balance of our ecosystem.
Myself: You mean like how tigers control the population of deer and other herbivores?
Father: Exactly! If we lose the top predators, the entire food chain gets disturbed, and it affects even our agriculture.
Myself: So, conserving animals also helps protect our forests, right?
Father: Yes, because many animals help in seed dispersal and pollination, which keeps the forest healthy and provides us with clean air and water.
Myself: But don't they also have value beyond just ecology?
Father: Absolutely. Forests and their animals are a rich source of genetic diversity, which is vital for medical research and developing new medicines.
Myself: I also think we have a moral duty since we are the ones destroying their habitats for development.
Father: Well said. Plus, our forests and wildlife are a part of our natural heritage that future generations have a right to enjoy.
Myself: I completely agree, Dad. We must protect them not just for nature's sake, but for our own survival too.
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