Eukaryogenesis—the process by which the eukaryotic cell emerged—has long puzzled scientists. It has been assumed that the fossil record has little to say about this process, in part because important characters such as the nucleus and mitochondria are rarely preserved, and in part because the prevailing model of early eukaryotes implies that eukaryogenesis occurred before the appearance of the first eukaryotes recognized in the fossil record. Here, I propose a different scenario for early eukaryote evolution than is widely assumed. Rather than crown group eukaryotes originating in the late Paleoproterozoic and remaining ecologically minor components for more than half a billion years in a prokaryote-dominated world, I argue for a late Mesoproterozoic origin of the eukaryotic crown group, implying that eukaryogenesis can be studied using the fossil record. I review the proxy records of four crown group characters: the capacity to form cysts as evidenced by the presence of excystment structures; a complex cytoskeleton as evidenced by spines or pylomes; sterol synthesis as evidenced by steranes; and aerobic respiration—and therefore mitochondria—as evidenced by eukaryotes living in oxic environments, and argue that it might be possible to use these proxy records to infer the order in which these characters evolved. The records indicate that both cyst formation and a complex cytoskeleton appeared by late Paleoproterozoic time, and sterol synthesis appeared in the late Mesoproterozioc or early Neoproterozoic. The origin of aerobic respiration cannot as easily be pinned down, but current evidence permits the possibility that it evolved sometime in the Mesoproterozoic.

真核发生——真核细胞出现的过程——长期以来一直困扰着科学家。人们认为化石记录对这一过程几乎没有提及，部分原因是细胞核和线粒体等重要特征很少被保存下来，部分原因是早期真核生物的流行模型表明真核发生发生在真核生物出现之前。化石记录中发现的第一个真核生物。在这里，我提出了一个与广泛假设不同的早期真核生物进化场景。我主张真核生物冠群起源于中元古代晚期，而不是冠群真核生物起源于古元古代晚期，并在原核生物占主导地位的世界中保留了超过 5 亿年的生态次要成分，这意味着真核生物可以使用化石记录。我回顾了四个冠群特征的代理记录：形成包囊的能力，由包囊结构的存在所证明；刺或幽门证明了复杂的细胞骨架；由甾烷证明的甾醇合成；和有氧呼吸——因此线粒体——由生活在有氧环境中的真核生物所证明，并认为可以使用这些代理记录来推断这些特征进化的顺序。记录表明，包囊形成和复杂的细胞骨架出现在古元古代晚期，甾醇合成出现在中元古代晚期或新元古代早期。有氧呼吸的起源无法轻易确定，但目前的证据表明它有可能在中元古代的某个时候进化出来。