Question

Arrange the landmarks in the evolution of life on the earth.
A. Formation of oceans and continents.
B. Diversification of multicellular eukaryotes.
C. Appearance of protists, the first eukaryotes.
D. Appearance of endosymbionts (mitochondria).
E. Appearance of aerobic bacteria development of O\(_2\) - rich atmosphere.
Choose the correct answer from the options given below :

• A. A, D, C, B, E
• B. D, B, C, A, E
• C. C, D, B, A, E
• D. A, E, C, D, B

Hint:

A simple evolutionary timeline mnemonic: Abiotic (Oceans) \(\rightarrow\) Prokaryotes \(\rightarrow\) Oxygen Crisis \(\rightarrow\) Eukaryotes (Nucleus then Mitochondria) \(\rightarrow\) Multicellularity.

Answer 1

The Correct Option is D

Step 1: Concept:
The question asks for the correct chronological sequence of major geological and biological milestones during the evolutionary history of Earth.

Step 2: Step-by-step Explanation:

• First Event (A): Earth formed roughly 4.5 billion years ago. As the molten planet cooled, water vapor condensed to form the primordial oceans and early tectonic plates formed the continents. This abiotic geological event forms the foundation for life and happens first.

• Second Event (E): Early life consisted entirely of anaerobic prokaryotes. Around 2.4 billion years ago, a massive evolutionary leap occurred with the evolution of photosynthetic cyanobacteria. They produced oxygen as a metabolic byproduct, leading to the "Great Oxidation Event," thereby establishing an O\(_2\)-rich atmosphere and paving the way for aerobic respiration.

• Third Event (C): With oxygen readily available, more complex single-celled organisms evolved. These were the first primitive eukaryotes, broadly categorized as protists. They possessed distinct membrane-bound nuclei, differentiating them from their prokaryotic ancestors.

• Fourth Event (D): According to the widely accepted Endosymbiotic Theory (proposed by Lynn Margulis), these early ancestral eukaryotic cells engulfed free-living aerobic bacteria (alpha-proteobacteria). Instead of digesting them, a symbiotic relationship was formed. These engulfed endosymbionts eventually evolved into modern mitochondria, granting the host cells immense energy-generating capabilities.

• Fifth Event (B): Armed with massive energetic advantages provided by mitochondria, eukaryotic cells eventually began congregating and cooperating, leading to the massive diversification of complex multicellular eukaryotes (like early plants, fungi, and animals).

• Thus, the chronological order is: Oceans (A) \(\rightarrow\) Oxygen atmosphere (E) \(\rightarrow\) First Eukaryotes (C) \(\rightarrow\) Mitochondria endosymbiosis (D) \(\rightarrow\) Multicellularity (B).

Step 3: Final Answer:
The correctly arranged sequence corresponds to Option (D).