The earliest form of life in animal kingdom is the

1. Analyze the question The question asks which of the listed vertebrate classes appeared first in evolutionary history. Remember that "earliest form of life" here is limited to the options given, which are all vertebrate classes.

2. Explain the concept Vertebrates evolved in a definite order, which is shown in the fossil record:

• Pisces (fish) came first, about 500 million years ago, and lived in water.
• Amphibia came next. They were the first to move onto land, but they still depend on water for breeding.
• Reptilia followed, with dry, scaly skin and shelled eggs that suit life on land.
• Aves (birds) and Mammalia came later. Birds evolved from reptiles.

3. Step-by-step solution

• Aves evolved from reptiles, so reptiles came before birds.
• Reptiles evolved from amphibians, so amphibians came before reptiles.
• Amphibians evolved from fish-like ancestors (lobe-finned fishes), so fish came before amphibians.

The order is therefore: Pisces → Amphibia → Reptilia → Aves.

4. Answer The earliest of the four groups is Pisces, which is option C.

5. Exam tip Use the memory phrase "P-A-R-A-M" (Pisces, Amphibia, Reptilia, Aves, Mammalia) to remember the order of vertebrate evolution. It also matches the move from water to land, and from simple to more complex body systems. Whenever JAMB asks which vertebrate class is "earliest" or "most primitive", think of fish first.