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The Bitcoin backbone protocol [Eurocrypt 2015] extracts basic properties of Bitcoin's underlying blockchain data structure, such as common prex and chain quality, and shows how fundamental applications including consensus and a robust public transaction ledger can be built on top of them. The underlying assumptions are proofs of work (POWs), adversarial hashing power strictly less than 1/2 and no adversarial pre-computationor, alternatively, the existence of an unpredictable genesis block. In this paper we rst show how to remove the latter assumption, presenting a bootstrapped Bitcoin-like blockchain protocol relying on POWs that builds genesis blocks from scratch in the presence of adversarial pre-computation. Importantly, the round complexity of the genesis block generation process is independent of the number of participants. Next, we consider applications of our construction, including a PKI generation protocol and a consensus protocol without trusted setup assuming an honest majority (in terms of computational power). Previous results in the same setting (unauthenticated parties, no trusted setup, POWs) required a round complexity linear in the number of participants.