Eukaryotic life appears to have flourished surprisingly late in the history of our planet. This view is based on the low diversity of diagnostic eukaryotic fossils in marine sediments of mid-Proterozoic age (around 1,600 to 800 million years ago) and an absence of steranes, the molecular fossils of eukaryotic membrane sterols^1,2. This scarcity of eukaryotic remains is difficult to reconcile with molecular clocks that suggest that the last eukaryotic common ancestor (LECA) had already emerged between around 1,200 and more than 1,800 million years ago. LECA, in turn, must have been preceded by stem-group eukaryotic forms by several hundred million years³. Here we report the discovery of abundant protosteroids in sedimentary rocks of mid-Proterozoic age. These primordial compounds had previously remained unnoticed because their structures represent early intermediates of the modern sterol biosynthetic pathway, as predicted by Konrad Bloch⁴. The protosteroids reveal an ecologically prominent ‘protosterol biota’ that was widespread and abundant in aquatic environments from at least 1,640 to around 800 million years ago and that probably comprised ancient protosterol-producing bacteria and deep-branching stem-group eukaryotes. Modern eukaryotes started to appear in the Tonian period (1,000 to 720 million years ago), fuelled by the proliferation of red algae (rhodophytes) by around 800 million years ago. This ‘Tonian transformation’ emerges as one of the most profound ecological turning points in the Earth’s history.

真核生命在我们星球的历史上似乎出现得很晚。这一观点基于中元古代（约 1,600 至 8 亿年前）海洋沉积物中诊断性真核化石的多样性较低，并且不存在甾烷（真核细胞膜甾醇的分子化石 1,2 ^）。真核生物遗骸的稀缺很难与分子钟相一致，分子钟表明最后一个真核生物共同祖先 (LECA) 已经在大约 1,200 到 18 亿多年前出现。反过来，干群真核生物形式肯定早于 LECA 出现了数亿年³。在这里，我们报告了在中元古代沉积岩中发现了丰富的原类固醇。这些原始化合物以前一直未被注意到，因为它们的结构代表了现代甾醇生物合成途径的早期中间体，正如 Konrad Bloch ^{4所预测的那样}。原类固醇揭示了一个生态学上突出的“原甾醇生物群”，它在至少 1,640 至大约 8 亿年前的水生环境中广泛且丰富，可能由古代产生原甾醇的细菌和深分支干群真核生物组成。现代真核生物在托尼安时期（1,000 至 7.2 亿年前）开始出现，大约 8 亿年前红藻（红藻）的扩散推动了现代真核生物的出现。这种“托尼安转变”成为地球历史上最深刻的生态转折点之一。