In bitcoin, a blockchain always starts at the unique genesis block. Beyond that point there can be forks, and only when it is clear which branch will grow fastest (or if it has grown enough), we can know for sure (or almost) if a block will end up in the actual blockchain.
Is it necessary to have a genesis block? I could imagine that without, when bitcoin was launched, miners would just have started mining blocks, and as soon as one was mined, the chain would grow from there. If another miner found another block 0, the blockchain could be extended from there as well just as with any other fork (note that this would be equivalent to having an abstract genesis block that doesn't need to have any properties that can be treated as the parent of any block 0).
Is this just an accidental feature of the initial design of bitcoin, or is there a deep reason for the existence of an eternal genesis block?