Chapter 15, Problem 1SQ

Place these events in the history of life on Earth in order.

1. a. accumulation of O₂ in Earth’s atmosphere
2. b. colonization of land by plants and fungi
3. c. diversification of animals (Cambrian explosion)
4. d. origin of eukaryotes
5. e. origin of humans
6. f. origin of multicellular organisms
7. g. origin of prokaryotes

Summary Introduction

To arrange:

The given events in history of life on the Earth in order.

Introduction:

In the starting phase of formation of the Earth, our planet was filled by reducing gases that did not supported life at the time of evolution of the Earth. Evolution of life took place in large seas and oceans in the form of certain micro-organisms like archeabacteria and coacervates. After this event, there was production of oxygen gas and thereon the atmosphere of the Earth became amicable for supporting life and hence life started.

Answer to Problem 1SQ

Correct sequence:

The theory of origin of life has not been proved till date. It is just a hypothesis. The hypothesis of origin of life can be explained according to the following sequence of events:

(g) Origin of prokaryotes

(a) Accumulation of oxygen in the Earth’s atmosphere.

(d) Origin of eukaryotes.

(f) Origin of multi-cellular organisms.

(c) Diversification of animals (Cambrian explosion)

(b) Colonization of land plants and fungi

(e) Origin of humans.

Explanation of Solution

The reason for the above sequence is as follows:

First, the life started in sea as the aggregation of organic molecules which then gave rise to micro-organisms. The first to be evolved on the Earth was archeabacteria, then bacteria, and other micro-organisms evolved after them. First, the micro-organisms survived and gave out reducing gases that were prevalent at that time. Then, these micro-organisms started giving out oxygen so in this way there was the evolution and the accumulation of oxygen started.

As the O₂ gas started accumulating, there was the evolution of single-cellular eukaryotic organisms. As the eukaryotic organisms become in good quantity, they began to grow in groups and in this way there was the evolution of multicellular organisms. After this, there was evolution of plants and then plants and fungi became to grow in groups or colonies.

Next, evolution tree was the evolution and development of animals. After this there was evolution of different types of animals. In this way, the diversification of animals took place. Finally, the most highly evolved animal humans evolved.

Conclusion

The correct order of evolution was the evolution of prokaryotes, the accumulation of oxygen in the Earth’s atmosphere, origin of eukaryotes, origin of multicellular organisms, colonization of land plants and fungi, diversification of animals (Cambrian explosion) and finally at last, origin of humans took place.