Steroid fossils dating back 1.6 billion years suggest a vanished realm of microorganisms

Remnants of primitive eukaryotes have been discovered in ancient sedimentary rocks through laboratory analysis of rock samples from around the world, according to an article in Nature. These findings, revealing the lost world of eukaryotes that dominated aquatic ecosystems 800 million years ago, were made possible through the discovery of protosteroids, primitive compounds produced in the formation of steroids. The majority of these molecules were likely produced by primordial eukaryotes, which include animals, plants, algae and fungi. Steroids are an integral part of cell membranes and do not degrade easily, thus remnants can be identified in sedimentary rocks as molecular fossils. While physical and molecular eukaryote fossils dated to 800 million years ago have already been found, physical fossils originating further back in time have become scarce, making protosteroid discoveries crucial in filling gaps in the fossil record.

Some researchers had speculated that primordial eukaryotes lacked in numbers to leave behind detectable steroid fossils. Bacterial dominance was also considered a possibility due to the scarcity of eukaryote fossils. However, the discovery of the protosteroids suggest that these microorganisms were actually present and abundant in the ancient waters of oceans and lakes. The researchers have proposed that the ability to create steroid precursors gave primordial eukaryotes an advantage in thriving in the low-oxygen environment of early Earth.

“If true, [this study] suggests that we may be able to examine the stepwise evolution of eukaryotes at [an] unprecedented level of detail,” says evolutionary biologist Yosuke Hoshino of the GFZ German Research Centre for Geosciences in Potsdam, who was not involved in the study. “This is such a great opportunity to understand the evolution of complex life, which biologists have always dreamed of.”

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