Biology · Evolution · NEET
The order goes from simple to complex: organic monomers form first (amino acids, sugars, nitrogen bases from inorganic gases), then these join into organic polymers (proteins and nucleic acids), then polymers aggregate into protobionts (the first non-cellular self-replicating capsules), and lastly DNA-based genetic systems arise. Remember it as monomers to polymers to protobionts to DNA. This is the exact order NEET 2016 tested.
Monomers form first. A monomer is a single small unit like one amino acid or one sugar. Polymers are made by joining many monomers, so polymers cannot exist before their monomers. In chemical evolution, inorganic gases give monomers first, then monomers link into polymers like proteins and nucleic acids.
The protobiont comes after polymers. A protobiont is a bag-like aggregate of polymers (like proteins and nucleic acids). So the polymers must exist first, then they collect together to form protobionts. A common wrong answer puts protobionts first, which reverses the real order.
The DNA-based genetic system is the last step in this sequence, after protobionts. Early life is thought to run on simpler self-replicating molecules first, and a stable DNA-based genetic system came later. So DNA-based systems are step four, not step one.
NEET 2016 confirmed the first organisms were non-green and anaerobic (they lived without oxygen). The first autotrophs were chemoautotrophs that made food from inorganic sources and never released oxygen. Oxygen-releasing photosynthetic life came only later, after cellular life appeared about 2000 million years ago.
Which of the following is the correct sequence of events in the origin of life? (i) Formation of protobionts (ii) Synthesis of organic monomers (iii) Synthesis of organic polymers (iv) Formation of DNA-based genetic systems
Following are two statements regarding the origin of life: (i) The earliest organisms that appeared on the earth were non-green and presumably anaerobes. (ii) The first autotrophic organisms were the chemoautotrophs that never released oxygen. Which option is correct?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Use MPPD: Monomers, Polymers, Protobionts, DNA-based systems. It runs from the smallest and simplest to the most complex, with the genetic system built last.
Protobionts are the first non-cellular, membrane-bounded aggregates of organic polymers that could self-replicate their molecules. They come after polymers and before true cells. Cellular life appeared only about 2000 million years ago.
The Oparin-Haldane view is a form of abiogenesis at the chemical level: life arose slowly from non-living organic molecules. Do not confuse this with the disproved idea of spontaneous generation. See the next page, biogenesis vs abiogenesis, for the exact difference.
The Miller-Urey experiment supports monomer formation. Passing electric sparks through CH4, H2, NH3 and water vapour produced amino acids, showing organic monomers can form from inorganic gases on early earth.
