LIFE × EVOLUTION
Protosterols and the Lost Eukaryotes
~1.64 billion to 800 million years ago
Proterozoic · Paleoproterozoic · Statherian
Protosterols, intermediate molecules in the making of the sterols of cell membranes, abound in rocks from 1.64 billion to 800 million years ago, where they are the only trace of an early biota, or set of living organisms.
Protosterols are the intermediate compounds of the pathway, or chain of chemical reactions, by which cells build sterols. Sterols are molecules that stiffen the membrane of every complex cell. Rocks of the mid-Proterozoic, the middle of a geological eon, are rich in protosterols. Chemists passed over these compounds for decades while they searched for the finished sterols. Protosterols are molecular fossils, molecules of ancient organisms preserved in rock. They reveal a biota that left no body fossils, the preserved remains of bodies. Producing any sterol requires oxygen, so the organisms that made protosterols lived in water that already contained some oxygen. These organisms were bacteria, or deep-branching eukaryotes, complex-celled organisms whose lineages separated early from the others, or both. They dominated the ocean from about 1.64 billion years ago until roughly 800 million years ago. Modern eukaryotes then displaced them, in an expansion led by the red algae.
Why it matters
Two kinds of evidence contradicted each other. Genes placed the common ancestor of all complex cells far back in the Proterozoic eon. Rocks showed almost no trace of complex cells for the billion years that followed. The gap came from what chemists were looking for, the finished sterols, and not from an absence of life.
Dating & uncertainty
Protosterols were predicted in the 1960s, and Brocks and colleagues (2023) recognised them in rocks only in 2023. Whether protosterols come from bacteria or from deep-branching eukaryotes is unsettled. The answer determines what share of these molecular fossils belongs to the early history of complex cells.
Earth history as one day
15:19:498 h 40 min before midnight
if Earth’s 4.54 billion years were compressed into a single day