👀 Here is the very first animal on Earth

The earliest forms of animal life on our planet continue to fascinate scientists, who are seeking to unravel the mystery of their origins. A team of geochemists from MIT recently provided new clues by analyzing extremely ancient rocks.

The study published in the Proceedings of the National Academy of Sciences reveals the discovery of specific "chemical fossils" in rocks dating back more than 541 million years. These molecular markers, called stera nes, represent the geologically stable form of sterols, essential compounds found in the cell membranes of complex organisms. The researchers identified particular stera nes with 30 and 31 carbon atoms, which they linked to a class of marine sponges called demosponges. These filter-feeding organisms, which were soft and lacked siliceous skeletons at that time, would thus have left their chemical signature in ocean sediments.



The work builds on previous research conducted in 2009, where the same team first identified 30-carbon stera nes in rocks from the Ediacaran period. This period, extending from approximately 635 to 541 million years ago, precedes the Cambrian explosion where multicellular life diversified dramatically. To confirm the biological origin of these compounds, the scientists analyzed modern samples of demosponges, synthesized sterols in the laboratory, and reproduced the geological processes these molecules underwent over millions of years.

The simultaneous presence of C₃₀ and C₃₁ stera nes in the ancient rocks, combined with their detection in contemporary sponges and their controlled synthesis in the laboratory, forms a convincing body of evidence. Only two of the eight synthesized sterols exactly matched the fossil compounds, ruling out the hypothesis of production by non-biological geological processes. This triple verification strengthens the idea that demosponges were among the first animals to evolve on Earth, long before the majority of modern animal groups.

These discoveries open new perspectives for tracing the history of primitive life. The researchers now plan to extend their investigations to other regions of the world, hoping to further refine the timeline of the appearance of the first animal forms. The authentication of these biomarkers provides a valuable tool for distinguishing biological signals from contamination or abiotic processes in the study of ancient rocks.

Marine sponges: architects of animal evolution

Modern demosponges represent one of the most diverse classes of sponges, with over 8,000 species recorded in all the world's oceans. These simple organisms possess a basic cellular organization without true tissues or organs, but exhibit remarkable molecular complexity.

Their active filtration system, which allows them to capture nutrient particles from the water, constitutes one of the earliest forms of animal feeding. Their skeleton, when present, is composed of silica spicules or spongin, a fibrous protein that gives them their characteristic elasticity.


Biochemically, demosponges produce a unique range of secondary metabolites, including the sterols studied by the researchers. These particular membrane compounds appear to have been conserved throughout evolution, making modern sponges living witnesses of very ancient life forms.

Their position at the base of the animal evolutionary tree makes them valuable models for understanding the origin of fundamental cellular mechanisms, such as intercellular communication and tissue differentiation that characterize more complex animals.