23. Among the following events in history of life
    a. prokaryotic cell
    b. eukaryotic cell
    c. natural selection
    d. organic molecules
    e. self replicating molecule
    Which is the correct chronological order?
    (1) d→e→c→a→b (2) d→e→a→b→c
    (3) e→d→a→c→b (4) d→e→a→c→b

The Stepwise Emergence of Life

The origin of life did not happen in a single leap but rather through a series of incremental steps, each building upon the complexity established by the previous stage. The sequence is best understood as a progression from simple chemistry to complex biology.

1. Formation of Organic Molecules

The journey begins with the formation of organic molecules—carbon-based compounds such as amino acids, nucleotides, sugars, and lipids. These molecules are the building blocks of life and are essential for all biological processes.

• How They Formed:

  • Organic molecules likely arose spontaneously on early Earth under conditions that were very different from today’s environment. The atmosphere was rich in gases like methane, ammonia, hydrogen, and water vapor, but lacked free oxygen.

  • Energy sources such as lightning, ultraviolet radiation, and volcanic heat drove chemical reactions that produced simple organic molecules.

• Significance:

  • The accumulation of organic molecules in the early oceans created a “primordial soup,” providing the raw materials for the next stages of life’s emergence.

2. Emergence of Self-Replicating Molecules

Once organic molecules were present, some of them began to interact in ways that allowed for self-replication—the ability to make copies of themselves.

• How They Emerged:

  • Among the organic molecules, certain nucleotides could link together to form RNA or RNA-like molecules. Some of these molecules developed the ability to catalyze their own replication.

  • This stage is often referred to as the “RNA world,” where RNA served as both genetic material and catalyst.

• Significance:

  • Self-replicating molecules introduced the concept of heredity and variation, setting the stage for natural selection.

3. Origin of Prokaryotic Cells

The next major milestone was the encapsulation of self-replicating molecules within a membrane, forming prokaryotic cells.

• How They Formed:

  • Fatty acids and other lipids in the primordial soup could spontaneously assemble into spherical structures called vesicles. These vesicles trapped self-replicating molecules inside, creating primitive cells.

  • These early cells, known as protocells, were similar to modern prokaryotes—organisms without a nucleus or membrane-bound organelles.

• Significance:

  • The cell membrane provided a barrier between the internal environment and the outside world, allowing for more efficient chemical reactions and protection from the environment.

4. Operation of Natural Selection

With the advent of self-replicating molecules and cells, natural selection could begin to act.

• How It Operated:

  • Variants of self-replicating molecules or cells that were better at copying themselves or surviving in their environment became more common over time.

  • This process led to the gradual improvement and diversification of early life forms.

• Significance:

  • Natural selection is the driving force behind adaptation and evolution, shaping the characteristics of all living organisms.

5. Evolution of Eukaryotic Cells

The final major event in this sequence was the evolution of eukaryotic cells—cells with a nucleus and membrane-bound organelles.

• How They Evolved:

  • Eukaryotic cells likely arose from prokaryotic ancestors through a process called endosymbiosis, in which one prokaryote engulfed another and the two formed a symbiotic relationship.

  • The engulfed prokaryotes eventually became organelles such as mitochondria and chloroplasts.

• Significance:

  • Eukaryotic cells are more complex and versatile than prokaryotic cells, enabling the evolution of multicellular organisms and the incredible diversity of life we see today.

The Correct Chronological Order

Given the above sequence, the correct chronological order of these events is:

1. Organic molecules

2. Self-replicating molecule

3. Prokaryotic cell

4. Natural selection

5. Eukaryotic cell

This sequence is supported by the majority of scientific evidence and is widely accepted in the field of evolutionary biology.

Evaluating the Options

Let’s review the options provided and determine which matches the correct chronological order:

• (1) d→e→c→a→b

  • Organic molecules → Self-replicating molecule → Natural selection → Prokaryotic cell → Eukaryotic cell

  • Incorrect: Natural selection cannot operate before there are cells or self-replicating entities to select upon.

• (2) d→e→a→b→c

  • Organic molecules → Self-replicating molecule → Prokaryotic cell → Eukaryotic cell → Natural selection

  • Incorrect: Natural selection is a process that begins as soon as there are self-replicating entities, long before the evolution of eukaryotic cells.

• (3) e→d→a→c→b

  • Self-replicating molecule → Organic molecules → Prokaryotic cell → Natural selection → Eukaryotic cell

  • Incorrect: Organic molecules must precede self-replicating molecules, not the other way around.

• (4) d→e→a→c→b

  • Organic molecules → Self-replicating molecule → Prokaryotic cell → Natural selection → Eukaryotic cell

  • Correct: This sequence matches the scientific consensus on the order of events in the history of life.

Key Takeaways

• Organic molecules formed first, providing the building blocks for life.

• Self-replicating molecules emerged next, introducing heredity and variation.

• Prokaryotic cells arose when self-replicating molecules were enclosed within membranes.

• Natural selection began to operate as soon as self-replicating entities existed, driving adaptation and evolution.

• Eukaryotic cells evolved later, enabling greater complexity and the rise of multicellular life.

• The correct answer is:

  (4) d→e→a→c→b

Summary Table

Conclusion

The history of life on Earth is a story of increasing complexity, beginning with the formation of organic molecules and culminating in the evolution of eukaryotic cells. The correct chronological order of these key events is: organic molecules → self-replicating molecule → prokaryotic cell → natural selection → eukaryotic cell. This sequence reflects the stepwise process by which life emerged and diversified, as supported by the best available scientific evidence.

In summary, the correct answer is:

(4) d→e→a→c→b