# Doctrina: Blockchain 5.0 for Artificial Intelligence
## 1 Introduction  
The convergence of blockchain technology and artificial intelligence (AI) marks a significant juncture in the evolution of digital innovation. Blockchain, originally conceived as the underlying architecture for cryptocurrencies, has transcended its initial confines to become a transformative force with wide-reaching applications. On the other hand, AI, characterized by its capacity to harness data-driven insights for problem-solving, is reshaping industries and societies. Blockchain, at its core, is a decentralized, distributed ledger technology known for its immutability, transparency, and robust security features. AI, conversely, encompasses a wide spectrum of machine learning and deep learning techniques that enable computers to learn and make decisions autonomously. When united, these technologies have the potential to revolutionize the way data are managed, shared, and utilized in AI applications. Blockchain technology emerged as a radical innovation underpinning the first decentralized cryptocurrency, Bitcoin [1]. It has since expanded beyond its initial financial confines, morphing into a foundational technology for a multitude of sectors. Several studies trace the blockchain evolution into five different stages [2, 3, 4, 5]. The general overview of the five pivotal stages is shown in Figure 1. The five pivotal stages are as follows:  
* Blockchain 1.0--Introduction of cryptocurrencies. This step symbolizes the genesis of digital currencies, which goes back to 2008, with Bitcoin paving the way for a decentralized financial paradigm. The primary utility of this era was to facilitate peer-to-peer transactions without the oversight of central authorities, a concept that expanded to other cryptocurrencies such as Litecoin [6] and Dogecoin [7]. Blockchain 2.0--Use of smart contracts. The release of a new form of blockchain with the capabilities of storing and executing programs and instances of source codes, Ethereum [8], marked the transition to the new era. With the possibilities of saving and executing programs directly on immutable and public blockchains, new opportunities for business, research, and applications opened. For addressing all possibilities, the alternatives for Ethereum were also introduced, such as Hyperledger [9] and Solana [10]. * Blockchain 3.0--Decentralized applications. Having the possibility of executing programs on distributed ledgers, the decentralized applications (dApps) have been rising that interact with blockchain while performing real-world services. Within the scope of this revolution, decentralized finance [11; 12], gaming [13; 14], social media [15], supply chain [16], identity verification and authorization services [17; 18], healthcare [19; 20], real estate [21], and several other fields [22; 23; 24; 25] benefited from the decentralized services. * Blockchain 4.0--Integration of off-chain computing with on-chain transactions. Alongside the dApps, the new questions related to blockchain's data-handling capabilities have risen, leading to the fourth revolution of blockchain. The introduction of Oracle services [26; 27] played a pivotal role in revolutionizing blockchain technology with the possibilities of interacting with the real world. With this possibility, Fetch.ai [28], SingularityNET [29], and similar solutions have been introduced that perform the heavy, dependency-oriented computing part of--chain and saving transactions on blockchain and managing the flow using smart contracts. In this step, almost all real-life applications, including machine learning and AI services, could be integrated with blockchain, but with certain limitations. In addition to these, blockchain and big data integration [30; 31; 32; 33] have also emerged as part of the fourth step revolution. * Blockchain 5.0--Convergence with artificial intelligence. It represents the next evolution of distributed ledger technology, characterized by its native integration with artificial intelligence.