A team of scientists led by University of California, Riverside’s Professor Gordon Love has found the oldest evidence yet of animal life, dating back 100 million years before the famous Cambrian explosion.
Rather than searching for conventional body fossils, Professor Love and colleagues have been tracking molecular signs (biomarkers) of animal life as far back as 660-635 million years ago (Neoproterozoic Era).
In ancient rocks and oils from Oman, Siberia, and India, the team found a steroid compound produced only by sponges, which are among the earliest forms of animal life.
“We have been looking for distinctive and stable biomarkers that indicate the existence of sponges and other early animals, rather than single-celled organisms that dominated the earth for billions of years before the dawn of complex, multicellular life,” said Alex Zumberge, a doctoral student at the University of California, Riverside.
The biomarker the researchers identified is a steroid compound named 26-methylstigmastane (26-mes).
It has a unique structure that is currently only known to be synthesized by certain species of modern sponges called demosponges.
“This steroid biomarker is the first evidence that demosponges, and hence multicellular animals, were thriving in ancient seas at least as far back as 635 million years ago,” Zumberge said.
In 2009, the same team reported the first compelling biomarker evidence for Neoproterozoic animals from a different steroid biomarker, called 24-isopropylcholestane (24-ipc), from rocks in South Oman.
However, the 24-ipc biomarker evidence proved controversial since 24-ipc steroids are not exclusively made by demosponges and can be found in a few modern algae.
The finding of the additional and novel 26-mes ancient biomarker, which is unique to demosponges, adds extra confidence that both compounds are fossil biomolecules produced by demosponges on an ancient seafloor.
The study also provides important new constraints on the groups of modern demosponges capable of producing unique steroid structures, which leave a distinctive biomarker record.
The scientists found that within modern demosponges, certain taxonomic groups preferentially produce 26-mes steroids while others produce 24-ipc steroids.
“The combined Neoproterozoic demosponge sterane record, showing 24-ipc and 26-mes steranes co-occurring in ancient rocks, is unlikely attributed to an isolated branch or extinct stem-group of demosponges,” Professor Love said.
“Rather, the ability to make such unconventional steroids likely arose deep within the demosponge phylogenetic tree but now encompasses a wide coverage of modern demosponge groups.”
The study was published online this week in the journal Nature Ecology Evolution.
J. Alex Zumberge et al. Demosponge steroid biomarker 26-methylstigmastane provides evidence for Neoproterozoic animals. Nature Ecology Evolution, published online October 15, 2018; doi: 10.1038/s41559-018-0676-2