First life form on earth was a

Options

A. cynaobacterium
B. chemoherotroph
C. autotroph
D. photoautotroph.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks about the first life form on Earth and provides four options: cyanobacterium, chemoheterotroph, autotroph, and photoautotroph.

2. **Exploring Theories of Origin of Life**: It is important to consider the widely accepted theory of chemical evolution, which suggests that life originated from simple organic molecules formed from inorganic substances on primitive Earth.

3. **Formation of Biomolecules**: Before any living organism could form, various organic substances such as nucleic acids, proteins, and lipids had to be created. These biomolecules eventually aggregated to form primordial structures resembling living cells.

4. **Identifying the First Life Form**: The first cellular life forms were likely simple and not capable of complex processes like photosynthesis. Among the options provided, a chemoheterotroph is a type of organism that obtains its nutrients from organic substances, which aligns with the idea that the first life forms fed on the organic materials present in their environment.

5. **Eliminating Other Options**:
- **Cyanobacterium**: These are photosynthetic organisms and likely evolved later, as photosynthesis is a complex process.
- **Autotroph**: This term refers to organisms that produce their own food, which would not apply to the first life forms.
- **Photoautotroph**: Similar to autotrophs, these organisms rely on sunlight for energy, which is unlikely for the first life forms.

6. **Conclusion**: Based on the reasoning above, the first life form on Earth was most likely a chemoheterotroph, as it would have been able to utilize the organic materials available in its aquatic environment.

**Final Answer**: The first life form on Earth was a **chemoheterotroph**.

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