Consider the following events: 1) First reptiles 2) First insects 3) Shelled animals 4) First mammals Arrange the above events in the chronological order of their origin on the Earth, and select the correct answer from the codes given below:

The correct order is 3, 2, 1, 4, so the answer is C. Shelled animals came first. Hard-shelled organisms such as brachiopods and early molluscs appear in the Cambrian, about 540 million years ago, during the Cambrian explosion. Insects came next, in the Devonian (around 410 million years ago), with wingless forms like Rhyniognatha as the earliest known. Reptiles evolved from amniote ancestors in the late Carboniferous, roughly 315 million years ago. Mammals, descended from synapsid lineages, appear in the Late Triassic, around 225 million years ago.

Why the others fail: option A puts insects before shelled animals, ignoring the Cambrian. Option B places reptiles before insects, which reverses the Devonian and Carboniferous sequence. Option D puts mammals before reptiles, which reverses the synapsid-to-mammal lineage.

Core concept: the geological time scale and the sequence of evolution of life. For Mains, this links to the Cambrian explosion and to biodiversity and evolution topics in GS-I.

Trap: aspirants often assume insects came after reptiles because land animals seem later, or they forget that shelled animals sit at the very bottom of the sequence.