Question

First life on earth was
a. Chemoautotrophs
b. Autotrophs
c. Photoautotrophs
d. None of these

Answer 1

Hint: Formation of this planet, we currently live, is believed to be $4.5$ billion years ago, whereas, the first trace of life was recorded around $3.7$ billion years ago. The formation of the first single celled organism took place at that time. These organisms were named- bacteria and archaea. From this point, the evolution of species took pace and around $1.7$ billion years ago, the evolution of multicellular organisms took place.

Complete answer:
• Life is believed to be thriving on planet earth for around $3.7$ billion years. The first formed organisms- bacteria and archaebacteria formed a microbial mat under the sea. These organisms started releasing oxygen as a by- product of their metabolism which resulted in the evolution of multi- cellular organisms which released oxygen to survive.
• At the bottom of the aquatic bodies, various types of chemicals were formed at the bed rocks. These first single cellular organisms consumed those chemicals to survive.
• Prokaryotes were the first life forms that occurred at the base of oceans and sea.
• These organisms used inorganic substances which were present at the bed rocks of oceans to form energy.
• This type of mode of nutrition is named as “Chemotroph”.
• There are two types of Chemotrophic organisms- one that can extract energy from the inorganic compounds to synthesize the organic compounds from Carbon Dioxide, called “Chemoautotrophs” and second- those organisms that are not able to form organic compounds on their own, named as “Chemoheterotrophs”.
• Chemotrophs are found at the base of the sea or ocean, mainly where light can’t reach and they evolved to be the type of organisms which were not dependent on the solar energy to make food.
• The first evolved bacterias are believed to be- Chemoautotrophs whereas, chemoheterotrophs were a result of evolution.
• After some time, some bacteria evolved to reach the surface of oceans and started extracting the solar energy to make their own food is called “Autotroph”.
• Most of the phototrophs are autotrophs which use the process of photosynthesis to process their food. Plants, some bacteria and algae fall into this category.

Hence, the correct answer is option (A).

Note: Although the life on planet earth evolved around $3.7$ billion years ago, the first human (Homo habilis), lived around $2.4 - 1.2$ million years ago in Eastern and Southern Africa. As for now, our ancestors have been around for about six million years but the modern form of humans evolved around $200,000$ years ago only. While the civilization built by humans has a history of $6000$ years only and industrialization took place in the early $1800s$.