user76785: 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?

user7267: The genesis block contains a phrase from a newspaper article "The Times 03/Jan/2009 Chancellor on brink of second bailout for banks" in the coinbase transaction, which proves that the genesis block could not have been created before Jan 3 2009. This proves that there was no pre-mining before that date, assuring fairness in distribution. 
If there was no genesis block, individual miners would need some way to come to consensus about which news paper article to use and whether they meet the criteria of it being printed on that date and this would be extremely impractical. 
But otherwise, I believe it would be possible to have a blockchain without a genesis block. There is nothing that is particularly special about the very first block except for what I mentioned above.

user75001: It is necessary for each block to reference the previous block that it is built upon. Without a genesis block, the miners could not follow the consensus rules when they started mining.

user15552: A genesis block is the starting point.  It's the only block that can't reference a "previous block", which makes it unique, as if you allow any other block to do that, you would have people forking the blockchain all day long.  All blocks in the "blockchain" MUST reference the previous block, which must also reference the previous block, all the way back to the genesis block.  By extension, you could say that all blocks, therefor, directly or indirectly, must reference the genesis block.  This confirms the consensus between all miners/users and gives a reference point to the programmers for if something went wrong somewhere along the blockchain.
The implementation of the Bitcoin Genesis block with the reference to the news article just makes it awesome, showing the intention(s)/desire(s) of Satoshi Nakamoto.
In the Bitcoin blockchain, there must be consensus.  The genesis block is the first point of consensus that all miners can agree upon and then go from there.  The subsequent application of the algorithms control the rest of it to keep all subsequent blocks in "sequence" and "aligned" (if you will).    
Every physical chain must begin with a single physical ring, the genesis block is that first, single ring.  Or you could see it like the foundation of a house; without it what you build will be wobbly and may not even hold up due to not having a solid starting point.  You could also view it as the starting point in a race.  If every miner/user just started wherever they wanted, you'd have no consensus point and total chaos as to what was happening overall.

user208: I think your misunderstanding is in the assumption that the genesis block must be a block.
There is no need for that.
For all we care the genesis block is just a specific hash that is used as hashPrevBlock to identify block number 1. It doesn't actually need to correspond to an actual block. Modern Bitcoin software doesn't actually treat the genesis as a block at all. It's just a piece of data - and its hash is special.
So, no, there is no need for a genesis block. But there is a need for a genesis something - just to identify what the first real block is.

76785: <p>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.</p>

<p>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).</p>

<p>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?</p>
 7267: <p>The genesis block contains a phrase from a newspaper article "The Times 03/Jan/2009 Chancellor on brink of second bailout for banks" in the coinbase transaction, which proves that the genesis block could not have been created before Jan 3 2009. This proves that there was no pre-mining before that date, assuring fairness in distribution. </p>

<p>If there was no genesis block, individual miners would need some way to come to consensus about which news paper article to use and whether they meet the criteria of it being printed on that date and this would be extremely impractical. </p>

<p>But otherwise, I believe it would be possible to have a blockchain without a genesis block. There is nothing that is particularly special about the very first block except for what I mentioned above.</p>
 75001: <p>It is necessary for each block to reference the previous block that it is built upon. Without a genesis block, the miners could not follow the consensus rules when they started mining.</p>
 15552: <p>A genesis block is the starting point.  It's the only block that can't reference a "previous block", which makes it unique, as if you allow any other block to do that, you would have people forking the blockchain all day long.  All blocks in the "blockchain" <strong>MUST</strong> reference the previous block, which must also reference the previous block, all the way back to the genesis block.  By extension, you could say that all blocks, therefor, directly or indirectly, must reference the genesis block.  This confirms the consensus between all miners/users and gives a reference point to the programmers for if something went wrong somewhere along the blockchain.</p>

<p>The implementation of the Bitcoin Genesis block with the reference to the news article just makes it awesome, showing the intention(s)/desire(s) of Satoshi Nakamoto.</p>

<p>In the Bitcoin blockchain, there must be consensus.  The genesis block is the first point of consensus that all miners can agree upon and then go from there.  The subsequent application of the algorithms control the rest of it to keep all subsequent blocks in "sequence" and "aligned" (if you will).    </p>

<p>Every physical chain must begin with a single physical ring, the genesis block is that first, single ring.  Or you could see it like the foundation of a house; without it what you build will be wobbly and may not even hold up due to not having a solid starting point.  You could also view it as the starting point in a race.  If every miner/user just started wherever they wanted, you'd have no consensus point and total chaos as to what was happening overall.</p>
 208: <p>I think your misunderstanding is in the assumption that the genesis block must be a block.</p>

<p>There is no need for that.</p>

<p>For all we care the genesis block is just a specific hash that is used as <code>hashPrevBlock</code> to identify block number 1. It doesn't actually need to correspond to an actual block. Modern Bitcoin software doesn't actually treat the genesis as a block at all. It's just a piece of data - and its hash is special.</p>

<p>So, no, there is no need for a genesis block. But there is a need for a genesis something - just to identify what the first real block is.</p>
