MCQ Detailed View

The earliest forms of life on Earth were heterotrophs, simple microorganisms that could not produce their own food. Instead, they relied on consuming organic molecules available in the early Earth’s oceans. Scientists believe life began about 3.5 to 4 billion years ago, shortly after the planet cooled enough for liquid water to exist.In the primordial Earth environment, the atmosphere lacked oxygen and was rich in gases like methane, ammonia, hydrogen, and water vapor. Energy from lightning, volcanic activity, and ultraviolet radiation triggered chemical reactions that produced organic compounds such as amino acids. These molecules accumulated in the oceans, forming what scientists call the “primordial soup”.

The first heterotrophic organisms:

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  Were unicellular and very simple in structure.

  
  


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  Obtained energy by breaking down organic molecules already present in their environment.

  
  


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  Were anaerobic, meaning they lived without oxygen since early Earth’s atmosphere had no free oxygen.

  
  


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  Reproduced asexually, leading to more organisms over time.

  
  

Later in evolution, autotrophs (like early cyanobacteria) appeared. These organisms could make their own food through photosynthesis or chemosynthesis, gradually releasing oxygen into the atmosphere. This change led to the development of more complex life forms, eventually allowing aerobic organisms (oxygen-breathing species) to evolve.

Other options explained:

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  Autotrophs (Option A): Evolved after heterotrophs, capable of producing food from sunlight or chemicals.

  
  


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  Saprotrophs (Option C): Organisms that feed on dead matter, developed later in the food chain.

  
  


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  None of these (Option D): Incorrect, as evidence strongly supports heterotrophs as the first life forms.

  
  

Thus, the correct answer is Option B: Heterotrophs, as they were the first organisms on Earth, utilizing available organic molecules for survival before autotrophs evolved.