Ancient steroids establish the Ediacaran fossil Dickinsonia as one of the earliest animals

The enigmatic Ediacara biota (571 million to 541 million years ago) represents the first macroscopic complex organisms in the geological record and may hold the key to our understanding of the origin of animals. Ediacaran macrofossils are as “strange as life on another planet” and have evaded taxonomic classification, with interpretations ranging from marine animals or giant single-celled protists to terrestrial lichens. Here, we show that lipid biomarkers extracted from organically preserved Ediacaran macrofossils unambiguously clarify their phylogeny. Dickinsonia and its relatives solely produced cholesteroids, a hallmark of animals. Our results make these iconic members of the Ediacara biota the oldest confirmed macroscopic animals in the rock record, indicating that the appearance of the Ediacara biota was indeed a prelude to the Cambrian explosion of animal life.The study is funded by Australian Research Council grants DP160100607 and DP170100556 (to J.J.B.), Russian Foundation for Basic Research project 17-05-02212A (A.I. and I.B.), and the Max-Planck-Society (C.H.). I.B. gratefully acknowledges an Australian Government Research Training Program stipend scholarship, and B.J.N. acknowledges a Geobiology fellowship of the Agouron Institute

Pyrolytic formation of alkylsteranes - Assigning geological orphans to their biological parents

Steranes alkylated at position C-3 occur in significant concentrations in many geological samples (e.g. Fig. 1). However, biological equivalents are not known from any living organisms and the formation pathway remains equally enigmatic, rendering them some of the most prominent orphan biomarkers. In some geological samples, the presence of sulphur functionalities indicated that the 3-alkyl group was originally functionalised, which together with a dominance of pentyl-derivatives pointed towards origins form C5 sugars. 3-alkylsteranes were therefore inferred to represent an entirely new class of natural products. Classified as putative ‘bacteriosteroids’ they were thought to reflect the bacterial fusion of eukaryotic (dietary) steroids with sugars to yield steroids with hopanepolyol-like side-chains (Dahl al., 1992; 1995). Other hypotheses encompass the likely bacterially mediated alkylation of stereneintermediates (Summons and Capon, 1988) or algal sources (Schaeffer et al., 1993). We simulated the geological maturation of regular 3-hydroxylated sterols by laboratory-based thermolysis and pyrolysis in the presence of carbon-catalysts and observed the formation of significant quantities of C-3 alkylated products that exhibit alkyl chain lengths of up to eight carbon units—similar to distributions in many geological extracts. Co-elution with an extract of the Ediacaran Araras group, previously shown to contain a series of 3β-n-alkyl steranes (Sousa Jr. et al., 2016), reveals that the lower members correspond to geological αααR isomers of steranes that have a straight hydrocarbon chain added to the 3β-position (Fig. 1). Our results show that 3-alkylsteranes readily form via carbon-catalysed geological process acting on regular (3-OH) sterol precursors. Considering that 3-alkylated steroids have never been identified in any living organism, there is thus no reason to assume that any of the geological 3-alkyl steroids have direct biosynthetic origins. Instead, regular sterols or their diagenetic intermediates are likely abiogenically alkylated in proportions that may be related to the diagenetic and catagenetic conditions, as well as the composition of the bitumen, kerogen and mineral matrix

Algal origin of sponge sterane biomarkers negates the oldest evidence for animals in the rock record

The earliest fossils of animal-like organisms occur in Ediacaran rocks that are approximately 571 million years old. Yet 24-isopropylcholestanes and other C₃₀ fossil sterol molecules have been suggested to reflect an important ecological role of demosponges as the first abundant animals by the end of the Cryogenian period (>635 million years ago). Here, we demonstrate that C₃₀ 24-isopropylcholestane is not diagnostic for sponges and probably formed in Neoproterozoic sediments through the geological methylation of C₂₉ sterols of chlorophyte algae, the dominant eukaryotes at that time. These findings reconcile biomarker evidence with the geological record and revert the oldest evidence for animals back into the latest Ediacaran

Genetics re-establish the utility of 2-methylhopanes as cyanobacterial biomarkers before 750 million years ago.

Fossilized lipids offer a rare glimpse into ancient ecosystems. 2-Methylhopanes in sedimentary rocks were once used to infer the importance of cyanobacteria as primary producers throughout geological history. However, the discovery of hopanoid C-2 methyltransferase (HpnP) in Alphaproteobacteria led to the downfall of this molecular proxy. In the present study, we re-examined the distribution of HpnP in a new phylogenetic framework including recently proposed candidate phyla and re-interpreted a revised geological record of 2-methylhopanes based on contamination-free samples. We show that HpnP was probably present in the last common ancestor of cyanobacteria, while the gene appeared in Alphaproteobacteria only around 750 million years ago (Ma). A subsequent rise of sedimentary 2-methylhopanes around 600 Ma probably reflects the expansion of Alphaproteobacteria that coincided with the rise of eukaryotic algae-possibly connected by algal dependency on microbially produced vitamin B12. Our findings re-establish 2-methylhopanes as cyanobacterial biomarkers before 750 Ma and thus as a potential tool to measure the importance of oxygenic cyanobacteria as primary producers on early Earth. Our study illustrates how genetics can improve the diagnostic value of biomarkers and refine the reconstruction of early ecosystems

Putative sponge biomarkers in unicellular Rhizaria question an early rise of animals

The dawn of animals remains one of the most mysterious milestones in the evolution of life. The fossil lipids 24-isopropylcholestane and 26-methylstigmastane are considered diagnostic for demosponges—arguably the oldest group of living animals. The widespread occurrence and high relative abundance of these biomarkers in Ediacaran sediments from 635–541 million years (Myr) ago have been viewed as evidence for the rise of animals to ecological importance approximately 100 Myr before their rapid Cambrian radiation. Here we show that the biosynthesis of 24-isopropylcholestane and 26-methylstigmastane precursors is common among early-branching unicellular Rhizaria—heterotrophic protists that play an important role in trophic cycling and carbon export in the modern ocean. Negating these hydrocarbons as sponge biomarkers, our study places the oldest evidence for animals closer to the Cambrian Explosion. Cambrian silica hexactine spicules that are approximately 535 Myr old now represent the oldest diagnostic sponge remains, whereas approximately 558-Myr-old Dickinsonia and Kimberella (Ediacara biota) provide the most reliable evidence for the emergence of animals. The proliferation of predatory protists may have been responsible for much of the ecological changes during the late Neoproterozoic, including the rise of algae, the establishment of complex trophic relationships and the oxygenation of shallow-water habitats required for the subsequent ascent of macroscopic animals

Bisnorgammacerane traces predatory pressure and the persistent rise of algal ecosystems after Snowball Earth

It remains unclear when and why the world’s oceans, once largely occupied by bacteria, became dominated by photosynthetic algae. Here, using fossil lipids in million year old rocks, the authors show that predation after the Snowball Earth glaciations created the opportunity for a global shift to algal ecosystems