Question

During the course of origin of life what was the sequence of substances which appeared on earth?

• A. Water, oxygen, nucleic acids, enzymes
• B. Amino acids, ammonia, phosphates, nucleic acids
• C. Water, amino acids, nucleic acids, enzyme
  
• D. Ammonia, amino acids, proteins, carbohydrates
• E. Ammonia, amino acids, proteins, nucleic acids
  

Answer 1

The correct option is E Ammonia, amino acids, proteins, nucleic acids

The primitive atmosphere of Earth was reducing atmosphere with little or no oxygen but was rich in water vapour, carbon monoxide, carbon dioxide, nitrogen and hydrogen. High temperature, UV radiations and lightening provided energy for chemical reactions between hydrogen and free oxygen, nitrogen and carbon to form water (present in form of superheated steam due to high earth temperature), ammonia, methane and cyanides. This makes option B incorrect as ammonia appeared before origin of amino acids. Gradual cooling down of earth atmosphere caused condensation of superheated steam into water resulting in rainfall that carried ammonia, methane and cyanides to ocean. High water temperature, absence of oxygen and of living organisms facilitated chemical reactions between above mentioned gases to form simple organic compounds such amino acids, fatty acids, glycerols and nucleotides. Colloidal aggregates of organic compound, called as coacervates, served as site for synthesis of nucleic acids. RNA served as genetic material as well as catalyst in early life forms and catalyzed polymerization of amino acids to form proteins. This makes option D incorrect and option E correct. The first life forms were chemoheterotrophic anaerobes which used inorganic elements as energy source and organic compounds as carbon source. Limited supply of inorganic and organic compounds paved path for origin of non oxygenic chemoautotrophs and then of oxygenic photoautotrophs. The oxygenic photoautotrophic life forms fixed carbon dioxide in form of glucose which makes option C incorrect as glucose appeared much later in evolution. Release of oxygen as by-product during oxygenic photophosphorylation by oxygenic photoautotrophs led to build up of oxygen on earth. This makes option A incorrect. Correct answer is E.