The Evolution Of Steroids And Eukaryotes In The Proterozoic

The ecology and diversity of eukaryotic organisms in the mid-Proterozoic ~1,800 to 800 Ma were fundamentally different than later in Earth history. The first fossils with eukaryotic affinity occur in the geological record about 2 billion years ago, but eukaryotic diversity and abundance remain low well into the Neoproterozoic (1,000-542 Ma). The first organism recognized as a member of a known eukaryotic kingdom, the red alga Bangiomorpha, only appears 1,200 Ma ago. In contrast to many younger assemblages, eukaryotes in the mid-Proterozoic predominantly inhabited agitated and well-oxygenated shoreline facies. In sedimentary rocks deposited in deep-water environments, fossils resembling eukaryotes usually remain rare, simple and small.

The distinct ecology and apparently low diversity of early eukaryotes should also have left a distinct biomarker record of eukaryotic membrane lipids. Biomarkers are molecular fossils of natural products. They often retain the diagnostic carbon skeleton of their biological precursors and may endure billions of years enclosed in sedimentary rocks. Eukaryotic membranes contain a suite of distinct sterols with 26 to 30 carbon atoms. In the fossil record, these sterols are often preserved in the form of (C26) to (C30) hydrocarbon steranes. Their relative abundance is often characteristic of specific environments and geological periods in the Phanerozoic. We may expect particularly distinct sterane assemblages in the early history of eukaryote evolution. However, in conflicting contrast to the ecology and evolutionary status of earlier eukaryotes, the distribution of different sterane pseudo-homologues in the mid-Proterozoic is virtually indistinguishable from the Phanerozoic. Although the missing disparity between old and young biomarkers may, in principle, have a biological explanation, it is at least possible that many Proterozoic biomarker assemblages have experienced overprint with petroleum and petroleum products of younger origin. Here we present a methodology to detect traces of younger contaminants that have adulterated complex mixtures of indigenous molecular fossils. Looking through the veil of contamination, we observe sterane assemblages in the interval 1,600 to 700 Ma ago that are distinct from anything observed later in Earth history.