The origin of the eukaryotic cell, with its compartmentalized nature and generally large size compared with bacterial and archaeal cells, represents a cornerstone event in the evolution of complex life on Earth. In a process referred to as eukaryogenesis, the eukaryotic cell is believed to have evolved between approximately 1.8 and 2.7 billion years ago from its archaeal ancestors, with a symbiosis with a bacterial (proto-mitochondrial) partner being a key event. In the tree of life, the branch separating the first from the last common ancestor of all eukaryotes is long and lacks evolutionary intermediates. As a result, the timing and driving forces of the emergence of complex eukaryotic features remain poorly understood. During the past decade, environmental and comparative genomic studies have revealed vital details about the identity and nature of the host cell and the proto-mitochondrial endosymbiont, enabling a critical reappraisal of hypotheses underlying the symbiotic origin of the eukaryotic cell. Here we outline our current understanding of the key players and events underlying the emergence of cellular complexity during the prokaryote-to-eukaryote transition and discuss potential avenues of future research that might provide new insights into the enigmatic origin of the eukaryotic cell.

真核细胞的起源，具有区室化的性质，并且与细菌和古细菌细胞相比通常尺寸较大，代表了地球上复杂生命进化的基石事件。在被称为真核发生的过程中，真核细胞被认为是在大约 1.8 至 27 亿年前从其古菌祖先进化而来，其中与细菌（原线粒体）伙伴的共生是一个关键事件。在生命之树中，将所有真核生物的第一个共同祖先与最后一个共同祖先分开的分支很长并且缺乏进化中间体。因此，复杂真核特征出现的时间和驱动力仍然知之甚少。在过去的十年中，环境和比较基因组研究揭示了有关宿主细胞和原线粒体内共生体的身份和性质的重要细节，使得能够对真核细胞共生起源的假设进行批判性的重新评估。在这里，我们概述了我们目前对原核生物向真核生物转变过程中细胞复杂性出现的关键参与者和事件的理解，并讨论了未来研究的潜在途径，这些途径可能为真核细胞的神秘起源提供新的见解。