What Is the Significance of the Genesis Block in the Context of the Longest Chain Rule?

The genesis block is the very first block of the blockchain (Block 0) and is hardcoded into the software. It serves as the common, immutable starting point for all subsequent blocks.

Every block must ultimately trace its lineage back to the genesis block. While the longest chain rule applies to all blocks after the genesis block, the genesis block itself is the anchor that establishes the initial, undisputed base for the chain competition.