The oldest evidence for found animals

An underwater picture of the species of modern demosponge Rhabdastrella globostellata, which makes the same 26-month-old steroids as those found by researchers in ancient rocks. Credit: Paco Cárdenas

Researchers at the University of California, Riverside, have discovered the oldest evidence of animal life, dating back at least 100 million years before the famous Cambrian explosion of animal fossils.

The study, led by Gordon Love, a professor in the UNR Department of Earth Sciences, was published in Nature Ecology & Evolution. The first author is Alex Zumberge, a PhD student from the Love research group.

Rather than looking for conventional fossils in the body, researchers have studied the molecular signs of animal life, called biomarkers, already 660 to 635 million years ago, in the Neoproterozoic era. In ancient rocks and oils from Oman, Siberia and India, they found a steroid compound produced only by sponges, which are among the oldest forms of animal life.

"Molecular fossils are important for tracking the first animals, because the first sponges were probably very small, did not contain a skeleton, and did not leave well-preserved or readily recognizable body records," said Zumberge. "We were looking for distinctive and stable biomarkers that indicate the existence of sponges and other primitive animals, rather than unicellular organisms that dominated the Earth for billions of years before the dawn of a complex and multicellular life.

The biomarker that they have identified, a steroid compound called 26-methylstigmastane (26-mes), has a unique structure whose synthesis is currently known to be synthesized only by some modern sponge species called demosponges.

"This biomarker of steroids is the first evidence that demosponges, and thus multicellular animals, thrived in ancient seas at least 635 million years ago," Zumberge said.

The work builds on a 2009 study by the Love team, which presented the first convincing biomarker evidence for Neoproterozoic animals from a different steroid biomarker, called 24-isopropylcholestane (24-ipc), from rocks of southern Oman. However, the evidence of the 24 ipc biomarker has proven controversial since the 24 ipc steroids are not exclusively manufactured by demosponges and can be found in some modern algae. The discovery of a new and old 26-month-old biomarker, unique to the demosponges, reinforces the confidence that both compounds are fossil biomolecules produced by demosponges on an ancient seabed.

The study also provides important new constraints to modern demosponga groups capable of producing unique steroid structures, leaving a distinctive biomarker record. The researchers found that in modern demosponges, some taxonomic groups preferentially produce 26 month steroids while others produce steroids of 24 ipc.

"The combined Neoproterozoic record of sterile demosponge, showing 24 and 26 steranes coexisting in ancient rocks, is unlikely to be attributed to an isolated branch or a group of extirpated strains of demosponges," said Love. "On the contrary, the ability to manufacture such unconventional steroids has probably appeared deep within the phylogenetic tree, but now embraces a wide range of modern demosponga groups."