Two apparently irreconcilable models dominate research into the origin of eukaryotes. In one model, amitochondrial proto-eukaryotes emerged autogenously from the last universal common ancestor of all cells. Proto-eukaryotes subsequently acquired mitochondrial progenitors by the phagocytic capture of bacteria. In the second model, two prokaryotes, probably an archaeon and a bacterial cell, engaged in prokaryotic endosymbiosis, with the species resident within the host becoming the mitochondrial progenitor. Both models have limitations. A search was therefore undertaken for alternative routes towards the origin of eukaryotic cells. The question was addressed by considering classes of potential pathways from prokaryotic to eukaryotic cells based on considerations of cellular topology. Among the solutions identified, one, called here the "third-space model", has not been widely explored. A version is presented in which an extracellular space (the third-space), serves as a proxy cytoplasm for mixed populations of archaea and bacteria to "merge" as a transitionary complex without obligatory endosymbiosis or phagocytosis and to form a precursor cell. Incipient nuclei and mitochondria diverge by division of labour. The third-space model can accommodate the reorganization of prokaryote-like genomes to a more eukaryote-like genome structure. Nuclei with multiple chromosomes and mitosis emerge as a natural feature of the model. The model is compatible with the loss of archaeal lipid biochemistry while retaining archaeal genes and provides a route for the development of membranous organelles such as the Golgi apparatus and endoplasmic reticulum. Advantages, limitations and variations of the "third-space" models are discussed. REVIEWERS: This article was reviewed by Damien Devos, Buzz Baum and Michael Gray.

中文翻译：

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在基质中进行分工，而不是吞噬作用或共生，作为真核细胞起源的途径。

在对真核生物起源的研究中，有两种看似不可调和的模型。在一个模型中，线粒体原核生物是自所有细胞的最后一个共同祖先自发出现的。真核生物随后通过吞噬细菌捕获获得了线粒体祖细胞。在第二种模型中，两个原核生物，可能是古细菌和细菌细胞，参与原核内共生，而居住在宿主内的物种成为线粒体祖细胞。两种模型都有局限性。因此，进行了寻找到真核细胞起源的替代途径的研究。通过基于细胞拓扑的考虑来考虑从原核细胞到真核细胞的潜在途径的类别来解决该问题。在确定的解决方案中，其中一个称为“ 讨论了“第三空间”模型的限制和变化。审阅者：这篇文章由Damien Devos，Buzz Baum和Michael Gray审阅。