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New Chemical Fossils Strengthen Case That Sponges Were Earth’s Earliest Animals

An MIT-led PNAS study links rare C31 steranes in Ediacaran rocks to demosponge sterols using three converging tests.

Some of the first animals on Earth were likely ancestors of the modern sea sponge, according to MIT geochemists who unearthed new evidence in very old rocks.
Colorful sea sponges line the ocean floor. (© denissimonov - stock.adobe.com)
A rare chemical indicator suggests sponges may have arrived as the first animals on Earth.
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Overview

  • Researchers report in PNAS that Ediacaran-age rocks contain abundant C31 steranes alongside earlier C30 detections, reinforcing a sponge origin for these biomarkers.
  • Analyses of drill cores and outcrops from Oman, western India, and Siberia revealed the rare C31 signatures in sediments older than 541 million years.
  • Some living demosponges were found to produce C31 sterols that are biochemical precursors of the fossil C31 steranes identified in the ancient rocks.
  • Laboratory synthesis of eight candidate C31 sterols followed by simulated burial showed that only two produced exact matches to the ancient steranes, arguing against abiotic formation.
  • The team frames the combined rock, modern biochemistry, and lab chemistry evidence as authentication of demosponge biomarkers and plans wider global sampling to refine the timing of early animal evolution.