One of the earliest events in the molecular evolution of photosynthesis is the structural and functional specialisation of type I (ferredoxin-reducing) and type II (quinone-reducing) reaction centres. In this opinion article we point out that the homodimeric type I reaction centre of heliobacteria has a calcium-binding site with striking structural similarities to the Mn₄CaO₅ cluster of photosystem II. These similarities indicate that most of the structural elements required to evolve water oxidation chemistry were present in the earliest reaction centres. We suggest that the divergence of type I and type II reaction centres was made possible by a drastic structural shift linked to a change in redox properties that coincided with or facilitated the origin of photosynthetic water oxidation.

光合作用分子进化中最早的事件之一是 I 型（铁氧还蛋白还原）和 II 型（醌还原）反应中心的结构和功能特化。在这篇观点文章中，我们指出，日光杆菌的同型二聚体 I 型反应中心具有与 Mn ₄ CaO _{5具有惊人结构相似性的钙结合位点。}光系统簇II。这些相似之处表明，发展水氧化化学所需的大多数结构元素都存在于最早的反应中心。我们认为 I 型和 II 型反应中心的分歧是由于与氧化还原性质的变化相关的剧烈结构转变成为可能的，这种变化与光合水氧化的起源一致或促进了光合水氧化的起源。