Question

The first organisms were
A) Primitive Eukaryotes
B) Aerobic bacteria
C) Prokaryotic chemoautotrophs
D) Photosynthetic

Answer 1

Hint: If life started somewhere sometimes on the earth, the first organisms must have needed some food to satisfy energy needs. Food either can be made from raw material present in nature (Producers) or already available food can be taken (Consumers).

 Complete answer: According to the actual evidence, the first simple organisms (prokaryotes) which appeared on earth about 3.8 billions of years ago and evolved in completely anoxic conditions. Initially, they were chemoautotrophs that used $CO_2$ (Carbon Dioxide) as a carbon source and oxidized organic substrates.

>Like all living things, prokaryotes need energy and carbon. They meet these needs in a variety of ways. They may get energy from chemical compounds(Chemo) they may get carbon from CO2 (Carbon Dioxide) (Autotrophs). Chemoautotrophs are cells that break down in organic molecules to supply energy for the cell and use $CO_2$ (Carbon Dioxide) as a carbon source. Chemoautotrophs include prokaryotes that break down hydrogen Sulphide and ammonia.
>As life on Earth started to undergo evolution and become more complex the simpler type of cells called prokaryotes underwent several changes over a long period of time to become Eukaryotes which states that primitive eukaryotes were not the first organisms that existed on earth.
>Aerobic bacteria were found in organically poor sediments up to 101.5 million years ago. So this also proves that aerobic bacteria was also not the first organism to exist on Earth
>Photosynthetic organisms were found on Earth 3.4 billion years ago. That proves that they were not the first organisms to exist on the Earth.

Thus, the correct answer is C i.e., Prokaryotic Chemoautotrophs.

Note: All known Chemoautotrophs are prokaryotes belonging to the Archaea or Bacteria domains. They have been isolated in different extreme habitats associated with deep biosphere. This form of energy conservation is considered one of the oldest on earth. These micro-organisms are of astrobiological interest because they could develop in extreme conditions existing in different extraterrestrial planetary bodies like Mars or Europa.