A genesis block is the first block of a blockchain, the one from which every later block descends. It has a block height of 0 and, unlike every other block, no parent: the field that would hold the previous block's hash is filled with zeros. Its contents are written directly into the network's software, so every node starts from the same agreed point.
Every blockchain, test network and testnet has its own genesis block, and its hash works as a fingerprint of the network itself. The most famous is Bitcoin's, created by the pseudonymous Satoshi Nakamoto in January 2009, which carries a newspaper headline in its first transaction. Ethereum's genesis block, created in July 2015, is notable for allocating the coins sold in the network's 2014 crowdsale.
A genesis block in plain terms
Think of the founding entry in a family tree. Every person in the tree can trace their line back to that one ancestor, but the ancestor has no parents listed. Everyone who keeps a copy of the tree starts from the same founder, which is how they know they are describing the same family.
A genesis block plays that role. It is not discovered through the normal process; it is chosen by the people who launch the network and printed into the software. Each new block names its parent, that parent names its own, and so on, until the chain reaches the genesis block. Two copies of a blockchain that begin from different genesis blocks are simply different families.
Bitcoin's genesis block
The genesis block of the Bitcoin blockchain carries a timestamp of 3 January 2009, 18:15:05 UTC. Inside its coinbase transaction, the field that miners may fill with arbitrary data, Nakamoto placed the text "The Times 03/Jan/2009 Chancellor on brink of second bailout for banks", the front-page headline of the London newspaper that day. The line serves as proof that the block was not created before that date, and it is widely read as a comment on the bank rescues of the 2008 financial crisis; Nakamoto did not explain it.
The block's coinbase created the first 50 BTC block reward, but because of the way the original software handled the genesis block, that output was never added to the database of spendable coins, so those 50 coins cannot be spent. The next block, block 1, has a timestamp of 9 January 2009, around the time the first version of the Bitcoin software was released to the public. Over the years, people have sent small amounts of bitcoin to the genesis block's address as a tribute, and these transfers are visible in any block explorer.
Ethereum's genesis block
The Ethereum blockchain went live on 30 July 2015 with the Frontier release. Its genesis block contained no ordinary transactions. Instead, it set the starting balances of the network's accounts, about 72 million ether in total, made up mostly of the ether bought by participants in the 2014 sale and amounts allocated to early contributors and the Ethereum Foundation. This distinguishes Ethereum from Bitcoin, where no coins existed until miners began earning them.
A starting allocation of this kind is often called a premine. On many networks launched since, the genesis block is where the founders, investors and treasury receive their initial tokens, which makes the genesis file a key document when analysing a project's total supply and distribution.
What the genesis block does
A genesis block has several practical functions:
- It anchors validation. A full node checks every block by following the chain back to the hard-coded genesis block; a chain that leads anywhere else is rejected.
- It identifies the network. The genesis hash, together with network identifiers, lets nodes avoid connecting to a different chain by mistake. This is one reason testnets and private networks each define their own genesis block.
- It sets the starting state. On account-based chains it records initial balances and sometimes initial smart contract code or validator lists.
- It fixes the starting parameters. Values such as the initial difficulty or gas limit are written into it, giving the consensus rules a known starting point.
Forks and new networks
When a chain splits through a hard fork, both resulting networks keep the same genesis block and the same history up to the split. Bitcoin Cash and Ethereum Classic therefore share their genesis blocks with Bitcoin and Ethereum. Protection against a replay attack has to come from other measures, such as chain identifiers, because the shared origin alone does not separate the two networks.
A brand-new network, by contrast, creates its own genesis block from scratch. Projects that copy another chain's code (for example, many early alternatives to Bitcoin such as Litecoin) change the genesis block so that their network can never be confused with the original. The genesis block has also become a cultural symbol: 3 January is marked by Bitcoin enthusiasts as "Genesis Day", and the embedded headline is often quoted in debates about central banks and money.