A newly discovered fossil record of steroid molecules, spanning 1.64 billion years, points to ancient organisms in the eukaryotic domain being capable of only early steps in the synthesis of sterol molecules.

The biosynthetic pathways that give rise to molecules called sterols are well established in the scientific literature. These pathways include the modifications (oxidation and cyclization) of a molecule called squalene to form lanosterol and cycloartenol, which are protosterols — precursors of other sterols. Numerous other steps are then needed to make cholesterol and other related sterols (known as crown sterols) that are found in organisms with cells that have a nucleus (eukaryotes); organisms called crown eukaryotes are either living eukaryotic species or extinct eukaryotic species that are descended from the last common ancestor of all living eukaryotes. Under favourable conditions, the carbon backbones of sterols can be preserved in ancient sedimentary rocks as molecular fossils, which are versions of the molecules that arise as a consequence of geological processes. Writing in Nature, Brocks et al.1 report that their exploration of molecular fossils has uncovered an approximately 640-million-year-long period of Earth’s history when sterol biosynthesis had not yet evolved the complex pathways that exist today.

Brocks et al. show that sedimentary rocks dated to between 1,640 million years and approximately 1,000 million years ago (a time frame corresponding to the mid-Proterozoic era), contain abundant molecules, known as aromatic steroids, that are molecular fossils of protosterols, but that these rocks show no signs of molecular fossils of crown sterols. Brocks and colleagues also clearly show that later, around 800 million years ago, during the Tonian period (which spanned 1,000 million years ago to 720 million years ago), the oldest-known molecular fossils of crown sterols (the aromatic hydrocarbons of cholesterol and ergosterol) replaced fossil protosterols as the most abundant fossil sterols.

Nearly all eukaryotes can synthesize one or several crown sterols, and it is estimated that the last eukaryote common ancestor (LECA) had the ability to synthesize them as well2. The absence of molecular fossils of crown sterols during the mid-Proterozoic favours the proposal that the LECA appeared later, between 1,200 million and 1,000 million years ago, rather than earlier, between 1,800 million and 1,600 million years ago2...

...In an essay4 entitled ‘Evolutionary perfection of a small molecule’, Bloch discusses molecules called ursterols, intermediate molecules in the synthesis of crown sterols. He suggests that there were previous organisms in which ursterols were useful end products that had the same role as crown sterols in controlling the fluidity of cellular membranes. It is notable that Brocks et al. show the presence of a type of molecule (an ursterol-derived steroid termed 4,24-dimethyl triaromatic steroid) in approximately 1,300-million-year-old sedimentary rocks, indicating that sterol biosynthesis at that time had already evolved to produce ursterols from protosterols. Another 500 million years separate this molecular fossil of an ursterol from the first occurrence of molecular fossils derived unambiguously from crown sterols.

What organisms are responsible for these ancient protosteroids and ursteroids? The enzymes squalene monooxygenase and oxidosqualene cyclase, which enable the oxidation and cyclization of squalene, respectively, are necessary for the synthesis of protosterols, are ubiquitous among living eukaryotes and were present in the LECA2. These enzymes are not present in another branch of life, archaea, and no archaeal microorganisms synthesize sterols...

a, Relative abundances of aromatic protosteroids (purple and cyan tones) and crown-group steroids (reds, blues and greens), highlighting the transition from a protosterol biota to a ‘crown-sterol biota’ in the Neoproterozoic era. Each horizontal colour bar represents one sample, and grey triangles assign data bundles to geological units 1 to 11 (key provided in Methods). Details on data assembly and geological formations (1–11) are provided in Methods. Ga, billion years ago. b, Phylogenetic tree of the domain Eukarya with black, red and green highlighting crown-group branches. Stem-group branches (purple) are hypothetical only, illustrating the notion that mid-Proterozoic ecosystems may have been dominated by extinct stem forms that did not produce crown sterols. LECA, the last common ancestor of all extant eukaryotes, may have emerged between 1.2 and more than 1.8 Ga. c, Microfossils of early eukaryotes: 1–5, likely crown-group Eukarya (approximately 1.1 to 0.7 Ga); 6–11, microfossils that are possibly or certainly eukaryotic but lack diagnostic crown-group characteristics (1,600 to 1,000 Ma). Detailed information and image credits are provided in Methods. C, Cenozoic era; Cr, Cryogenian period; Ed, Ediacaran period; M, Mesozoic era; P, Palaeozoic era; Palaeo., Palaeproterozoic era; Tn, Tonian period. For images in c: image 1, Melicerion poikilon, a possible testate rhizarian54, image courtesy of S. Porter, reproduced with permission from ref. 54; image 2, Bonniea dacruchares, a testate amoebozoan54, image courtesy of S. Porter, reproduced with permission from ref. 54; image 3, Bangiomorpha pubescens, a likely bangiacean rhodophyte alga55,56, image courtesy of N. Butterfield, reproduced with permission from ref. 57, © The Palaeontological Association; image 4, Proterocladus antiquus, a likely multicellular, benthic, siphonocladalean chlorophyte alga11, image courtesy of Q. Tang; image 5, Ourasphaira giraldae, a likely fungus12, image courtesy of C. Loron; image 6, Trachyhystrichosphaera aimika, a microfossil with diagnostic eukaryotic features, image courtesy of J. Beghin, reproduced from ref. 58 with permission from Elsevier; image 7, Leiosphaeridia jacutica, a microfossil of possible eukaryotic origin, image courtesy of J. Beghin, reproduced from ref. 58 with permission from Elsevier; image 8, Satka favosa, a microfossil with diagnostic eukaryotic features9, image courtesy of E. Javaux, reproduced from ref. 9; image 9, Valeria lophostriata, a microfossil with diagnostic eukaryotic features9, image courtesy of E. Javaux, reproduced from ref. 9; image 10, Tappania plana, a microfossil with diagnostic eukaryotic features1, image courtesy of Y. Leiming, reproduced with permission from ref. 1, © The Palaeontological Association; image 11, Shuiyousphaeridium macroreticulatum, a microfossil with diagnostic eukaryotic features1, image courtesy of Y. Leiming, reproduced with permisison from ref. 1, © The Palaeontological Association.