NIOS Class 10 Science Worksheet 20 Questions Answers – History of Life on Earth

This Worksheet 20 contains questions based on Chapter 20History of Life on Earth with complete answers.

This post explains the answers of questions given in the worksheet including explanation, diagrams. These solutions will help you complete your assignments and TMA also.

 

History of Life on Earth Worksheet 20 Solution

Q1. Mention when did the earth come into existence? Why did life not exist on primitive earth?

Answer

The earth came into existence about 4.5 to 5 billion years ago.

Life did not exist on primitive earth because the earth extremely hot and the physical conditions were not suitable for life. The atmosphere contained harmful gases like methane, ammonia and hydrogen cyanide, and oxygen was absent which is essential for most living organisms.

 

Q2. State the significance of the study of fossils in evolution?

Answer

The study of fossils is very important for understand evolution because

   1. Fossils provide evidence of past life: - Fossils are the remains of plants and animals that lived on Earth in the past. They show that life existed millions of years ago.

   2. They help trace the history of evolution: - By studying fossils found in rocks of different ages, scientists can understand how simple organisms gradually evolved into complex forms.

   3. They show the sequence of evolutionary changes: -Fossils help in knowing which organisms appeared earlier and which appeared later in Earth’s history.

   4. They provide proof of extinct species: - Fossils give evidence of organisms like dinosaurs that no longer exist today.

   5. They support the idea of gradual evolution: - Fossils records show that species and higher groups evolved slowly over a long period of time.

So, fossils act as a record of the evolutionary history of life on the Earth.

 

 

Q3. What are the main points of Oparin’s’ theory of Origin of life?

Answer

Oparin’s theory explains how life originated o Earth through chemical evolution. Following are the main points of theory:

   1.  Primitive or early Earth had gases like methane, ammonia, hydrogen and water vapours. There was no oxygen in the atmosphere so the atmosphere was reducing at that time.

   2. Earth was very hot initially and later cooled down. Water vapours condensed and formed oceans and seas.

   3. Simple inorganic substances reacted to form simple organic substances like amino acids due to lighting, head and radiation.

   4. These organic compounds accumulated in the oceans form a thick mixture called “Primordial soup”.

   5. Now simple organic molecules combined to form complex molecules like proteins and nucleic acids.

   6. These complex molecules gradually organised into primitive living cells and form fist life forms on the earth.

So, according to Oparin, life originated from non-living matter through a series of chemical changes in water on the earth.

 

Q4. State the conditions which would have mad the primitive atmosphere of Earth more conducive to the origin of life?

Answer

The primitive atmosphere of Earth conducive to the origin of life due to following conditions-

   1.  Absence of Oxygen- The primitive atmosphere had no oxygen and this reducing atmosphere was responsible for the formation of organic compounds needed for life.

   2. The atmosphere contained gases like methane, ammonia and hydrogen and hydrogen cyanide that played an important role in forming organic molecules.

   3. The earth was very hot and volcanoes released gases and minerals that were used as raw materials in chemical reactions.

   4. Lighting, heat and radiation helped chemical reactions to occur.

   5. Slowly the earth cooled down and water vapours condensed to form rains and oceans. These oceans collected organic compounds to form the primordial soup.

   So, these are the conditions that made the primitive earth favourable for the origin of life.

 

Q5. List the five major events during geological time period beginning from origin of life.

Answer

Following are the five major events during geological time period

    1. Origin of life (about 3.5 billion years ago): - Life first appeared on Earth in the form of simple unicellular organisms like prokaryotes.

    2. Build up of oxygen in the atmosphere (about 2.5 billion years ago):- Photosynthetic organisms released oxygen making the atmosphere suitable for complex life.

    3. Origin of eukaryotes (about 1.5 billion years ago): - Organisms with a true nucleus and more complex cell structure evolved.

   4. Cambrian explosion (about 600 million years ago): - There was a sudden and rapid diversification of multicellular organisms.

   5. Advent and dominance of mammals (about 65 million years ago):- Mammals evolved and diversified and eventually leading to the evolution of humans.

 

Q6. Mention the two major contributions of Charles Darwin. Give salient features of Darwin’s theory of Evolution.

Answer

Two major contributions of Charles Darwin

   1. Common ancestry (Decent with modification) :- Darwin proposed that all living organisms are related and have evolved from common ancestors.

   2. Natural selection as the mechanism of evolution: - He explained that evolution occurs through the process of natural selection, often described as “survival of the fittest”

Salient features of Darwin’s Theory of Evolution

   1. All organisms produce more offsprings that can survive.

   2. Food, space and other resources are limited so all offsprings cannot survive.

   3. Organisms compete with one another to survive because of limited resources.

   4. Individuals of the same species show variations in their characteristics.

   5. Some variations are inherited and passed on to the next generation.

   6. Individuals with useful variations survive e better and reproduce more.

   7. Over many generations, these variations accumulated which lead to formation of new species.

 

Q7. Write a note on (a) Neo-Darwinism (b) Cambrian Explosion.

Answer

   1. Neo- Darwinism: - Neo- Darwinism is the modern form of Darwin’s theory of evolution. Darwin explained evolution by natural selection but he did not know how variations arise and are inherited.

   On the basis of Mendel’s laws of genetics, scientists understood that

    i.   Variations arise due to mutation an sexual reproduction.

   ii.  These variations are present in the gene pool or a population.

   

    iii.  Natural selection increase the number of useful genes in population.

    iv.    Over a long period, small genetic changes (microevolution) can lead to formation of new species (macroevolution)

So, Neo-Darwinism combines Darwin’s theory and genetics.

    2. Cambrian Explosion: - The Cambrian explosion refers to a sudden and rapid diversification of life that occurred about 600 million years ago during the Cambrian period.

    i.   Before this time, life mostly existed as simple unicellular organisms.

   ii.  During the Cambrian period, many multicellular organisms with different body structure appeared suddenly.

   

   iii.   Most of the major groups of invertebrates originated during this time.

   iv.  This event marks a major turning point in the history of life on Earth.

The Cambrian explosion was a period of rapid evolutionary change that led to the rise of complex

 

Q8. State the major events in the history of life?

Answer

The major events in the history of life on Earth

i.   Life began in the form of simple unicellular prokaryotic organisms about 3.5 billion years ago.

ii.   Photosynthetic organisms released oxygen which slowly accumulated in the atmosphere and made it possible for complex life about 2.5 billon years ago.

iii.  Cells with a true nucleus and complex structure appeared about 1.5 billion years ago.

iv.  There was a sudden and rapid diversification of multicellular organisms such as invertebrates about 600 million years ago.

v.    Dinosaurs ruled the earth for nearly 150 million years ago and became extinct due to natural cause about 65 million years ago. Mammals appeared.

vi.  Mammals’ diversification and became dominant animal about 65 million years ago.

vii.Evolution of humans began 1.5 – 2 million years ago.

 

Q9. With the help of diagram explain the theory of Oparin and Haldane’s experiments.

Answer

Oparin’s theory

We can explain Oparin’s theory through the following points:

i.    The primitive earth had a reducing atmosphere because there was no oxygen on the earth.

ii.  Earth was extremely hot in the beginning and when it cooled down, water vapour condensed and formed oceans and seas.

iii.   The gases present in the atmosphere reacted due to lightning and heat and formed simple organic compounds like amino acids

iv.  Rain washed these organic compounds into oceans and they collected in high concentration forming a thick mixture called the ‘primordial soup’.

v.    Simple organic molecules combined to form complex molecules like proteins and nucleic acids.

vi.   These complex molecules slowly organised into primitive living cells. This the first life forms originated in water

Haldane’s Experiment

Haldane did not perform a laboratory experiment himself but he proposed how life could have originated on Earth. Later his ideas were experimentally tested by Miller and Urey.

 Haldane suggested that:

i.      The primitive earth had a reducing atmosphere.

ii.   It contained gases like methane, ammonia, hydrogen and water vapour.

iii.  Energy form lightning and heat helped these gases react.

iv.   These reactions formed simple organic molecules like amino acids.

v.   These molecules collected in oceans to form a primordial soup from which life originated.

Stanley Miller and Harold Urey tested Haldane’s idea

They took a closed glass apparatus and filled gases like methane, ammonia, hydrogen and water vapour in side it. Water was heated to produce steam and an electric spark was passed through the gases to stimulate lightning.

The gases were them cooled and condensed. The liquid formed was collected in a trap.

After a few days. The collected liquid contained amino acids that are the building blocks of life.

 

 

Q10. Do you think humans are still evolving. Also state major stages of human evolution to justify your answer?

Answer

Yes, humans are still evolving but not a very obvious physical way.

Biological evolution of humans has perhaps not occurred much after the evolution of moder mam about 50,000 years ago.

Cultural evolution, development of tools, science, technology, language and medicine has been evolved.

Natural selection works on humans and small genetic changes are taking place.

So, humans are still evolving but at the genetic and cultural level but not as physical changes.

Major Stages of Human Evolution

    1. Australopithecus: - is considered the first human like ancestor. Fossils named ‘Lucy’ found in Africa. He walked partly upright.

   2. Homo erectus: - He walked on two legs and had a large brain. Fossils found in many parts of the world.

   3. Neanderthal man (Homo sapiens): - He was more advanced and used tools and lived in groups.

   4. Cro-Magnon man: - He was more intelligent than Neanderthals and used better tools and cave paintings.

   5. Modern man (Homo sapiens sapiens): - This man was evolved about 50000 years ago. He has a well-developed brain and shows advanced cultural development.