Q.77 One of the most remarkable features of evolution is the formation of amnion and allantoin. This
appeared for the “first time” in evolutionary time scale in
(A) reptiles
(B) birds
(C) fishes
(D) humans
The amnion and allantois are key extraembryonic membranes that first evolved in reptiles, marking a pivotal shift to terrestrial reproduction.
Correct Answer
The correct option is (A) reptiles. These structures appeared for the first time in the evolutionary timescale among amniotes, with reptiles as the earliest group.
What Are Amnion and Allantois?
The amnion forms a fluid-filled sac around the embryo, protecting it from desiccation and mechanical shock. The allantois, derived from the hindgut, handles waste storage and gas exchange, enabling development independent of water.
Option Analysis
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(A) Reptiles: Correct, as reptiles were the first amniotes (~340 million years ago) to develop these membranes, allowing shelled eggs for land-based reproduction.
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(B) Birds: Incorrect; birds inherited these from reptilian ancestors and retained them in hard-shelled eggs, but did not originate them.
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(C) Fishes: Incorrect; fishes lack these membranes, relying on aquatic gill-breathing and lacking shelled eggs.
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(D) Humans: Incorrect; humans (mammals) evolved from synapsid amniotes, so these structures predate mammals by millions of years.
Evolutionary Significance
These membranes freed vertebrates from water dependency, enabling reptiles to colonize dry habitats. Birds and mammals adapted them further, but reptiles marked their debut.
