Genesis block

A genesis block is the very first block of a blockchain, block number zero, hardcoded into the network's software rather than mined through the usual chain-building process. Every later block references its predecessor's hash, but the genesis block has no parent: its "previous block hash" field is set to all zeroes, making it the fixed anchor that every other block in the chain ultimately traces back to.

Bitcoin's genesis block was mined by Satoshi Nakamoto on January 3, 2009, with a nonce of 2,083,236,893. Its coinbase transaction embeds the now-famous text "The Times 03/Jan/2009 Chancellor on brink of second bailout for banks," a timestamp and a pointed reference to the financial crisis that motivated Bitcoin's creation. The 50 BTC reward from that block was never added to the spendable coin set due to how the original code handled it, so it can never be moved, even though people still send small "tribute" payments to its address.

Each independent blockchain defines its own genesis block with its own rules. Ethereum's, for example, went live on July 30, 2015, and allocated roughly 72 million ETH across crowdsale contributors and the Ethereum Foundation rather than issuing coins through mining alone. Because a genesis block cannot be altered without invalidating the entire chain built on top of it, it also serves as a practical way to tell a legitimate chain apart from a copycat or fork claiming the same name.