Scientists have discovered the oldest molecular sign of animal life, but it didn’t come from a pile of prehistoric bones. While the vast majority of the ancient animals we know about come from fossils left over from the Cambrian explosion — the period when animal life rapidly diversified for the first time — new research shows there were bizarre animals living long before then, but they didn’t leave behind bones or bits. As the authors of the new Nature Ecology & Evolution study show, the evidence they left behind was chemical.

In the new paper, an international team of scientists report the discovery of a biomarker left behind by members of the Animalia kingdom between 660 and 635 million years ago, making it the oldest evidence ever discovered. The biomarker, detected in ancient rocks and oils from Oman, Siberia, and India, is a steroid compound named 26-methylstigmastane, which today is only known to be synthesized by a species of modern sponges called demosponges.

“This steroid biomarker is the first evidence that demosponges, and hence multicellular animals, were thriving in ancient seas as far back as 635 million years ago,” first author Alex Zumberge, Ph.D. said Monday. Zumberge is a doctoral student in Earth sciences at the University of California, Riverside.

sponge
A freshwater sponge in Columbia River in Washington State. This sponge is a demosponge.

Today the Demospongia make up the planet’s most diverse sponge group. All of the nearly 8,000 species of demosponges worldwide are brightly colored invertebrates that reproduce both sexually and asexually. While sponges, like all animals, have a certain type of skeleton that gives their bodies a shape, a sponge’s skeleton doesn’t usually leave behind a recognizable body fossil. That’s why in the hunt for signs of ancient sponge life, Zumberge and his colleagues focused on finding distinctive and stable biomarkers instead of fossils.

The spectacularly old date of the steroid biomarker is important because it demonstrates that animals lived at least 100 million years before the Cambrian explosion, which occurred 540 million years ago. For a long time, scientists widely believed that any organisms that lived prior to the Cambrian explosion were simple individual-celled creatures. Now it’s becoming increasingly clear that there were animals bobbing around in ancient seas before the Cambrian explosion — and that evidence of some of the oldest animals may come from chemicals, not bones or preserved flesh. The oldest known animal fossil, essentially a 558 million-year-old ribbed oval, was identified in September by the bits of organic material its body left behind.

These new findings strongly suggest that demosponges floated in Neoporterozoic marine environments and even existed as far back as the Cryogenian period, which spanned 720 to 635 million years ago. These demosponges may not have had eyes or spines, but they were animals that were adaptable enough to have descendants thriving today.

Abstract: Sterane biomarkers preserved in ancient sedimentary rocks hold promise for tracking the diversification and ecological expansion of eukaryotes. The earliest proposed animal biomarkers from demosponges (Demospongiae) are recorded in a sequence around 100 Myr long of Neoproterozoic–Cambrian marine sedimentary strata from the Huqf Supergroup, South Oman Salt Basin. This C30 sterane biomarker, informally known as 24-isopropylcholestane (24-ipc), possesses the same carbon skeleton as sterols found in some modern-day demosponges. However, this evidence is controversial because 24-ipc is not exclusive to demosponges since 24-ipc sterols are found in trace amounts in some pelagophyte algae. Here, we report a new fossil sterane biomarker that co-occurs with 24-ipc in a suite of late Neoproterozoic–Cambrian sedimentary rocks and oils, which possesses a rare hydrocarbon skeleton that is uniquely found within extant demosponge taxa. This sterane is informally designated as 26-methylstigmastane (26-mes), reflecting the very unusual methylation at the terminus of the steroid side chain. It is the first animal-specific sterane marker detected in the geological record that can be unambiguously linked to precursor sterols only reported from extant demosponges. These new findings strongly suggest that demosponges, and hence multicellular animals, were prominent in some late Neoproterozoic marine environments at least extending back to the Cryogenian period.