Researchers at MIT have uncovered new chemical clues in extremely old rocks that suggest some of the earliest animals on Earth were likely ancestors of modern sea sponges.
Reporting in the Proceedings of the National Academy of Sciences, the team describes the discovery of “chemical fossils” preserved in rocks more than 541 million years old. These chemical fossils are traces of biological molecules once produced by living organisms that were later buried, altered, and locked into sediment for hundreds of millions of years.
The newly identified molecules belong to a group called steranes, which are stable remnants of sterols such as cholesterol that form part of the cell membranes of complex life. By analyzing their structure, the scientists linked these steranes to demosponges, a major group of sea sponges. Today, demosponges appear in many shapes, sizes, and colors and live throughout the world’s oceans as soft filter feeders. Their ancient relatives were likely similar in being soft bodied marine organisms.
“We don’t know exactly what these organisms would have looked like back then, but they absolutely would have lived in the ocean, they would have been soft-bodied, and we presume they didn’t have a silica skeleton,” says Roger Summons, the Schlumberger Professor of Geobiology Emeritus in MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS).
The presence of these sponge specific chemical signatures strengthens the case that ancestors of demosponges were among the first animals to evolve, emerging well before most other major animal groups.
The research team includes lead author Lubna Shawar, a former MIT EAPS Crosby Postdoctoral Fellow who is now a research scientist at Caltech, along with Summons and colleagues Gordon Love of the University of California at Riverside, Benjamin Uveges of Cornell University, Alex Zumberge of GeoMark Research in Houston, Paco Cárdenas of Uppsala University in Sweden, and José Luis Giner of the State University of New York College of Environmental Science and Forestry.
Revisiting a 2009 Discovery in Precambrian Rocks
This work builds on a study the group first published in 2009. At that time, they analyzed rocks from an outcrop in Oman and detected an unusually high concentration of steranes derived from 30-carbon (C30) sterols. These rare steroid molecules appeared to originate from ancient sea sponges.
The rocks dated to the Ediacaran Period, which lasted from about 635 million to 541 million years ago, just before the Cambrian Period when complex multicellular life rapidly diversified. The earlier findings suggested that sponges existed long before the Cambrian explosion and may have been among the planet’s earliest animals.
Not everyone agreed. Some researchers proposed that the C30 steranes might have been produced by other organisms or even formed through nonbiological geological processes.
The new study adds weight to the sponge hypothesis. The team identified another distinctive chemical fossil in the same Precambrian rocks that is highly likely to have come from living organisms rather than from chemistry alone.
Rare Sterols and the Search for Early Animal Life
As in their previous investigations, the researchers examined Ediacaran age rocks collected from drill cores and outcrops in Oman, western India, and Siberia. They searched for steranes, which are stable versions of sterols found in all eukaryotes (plants, animals, and any organism with a nucleus and membrane-bound organelles).
“You’re not a eukaryote if you don’t have sterols or comparable membrane lipids,” Summons says.
Sterols share a core structure made of four connected carbon rings. Different organisms modify that structure by adding carbon side chains and other chemical groups, depending on the genes they carry. In humans, cholesterol contains 27 carbon atoms, while plant sterols typically contain 29.
“It’s very unusual to find a sterol with 30 carbons,” Shawar says.
The earlier research had identified a 30-carbon sterol tied to a specific enzyme encoded by a gene common in demosponges. In the new analysis, the team realized that the same gene could also produce an even rarer 31-carbon sterol (C31). When they reexamined their rock samples, they detected abundant C31 steranes alongside the previously identified C30 forms.
“These special steranes were there all along,” Shawar says. “It took asking the right questions to seek them out and to really understand their meaning and from where they come.”
Laboratory Tests Confirm Biological Origin
To confirm the source, the scientists studied living demosponges and found that some species produce C31 sterols, the biological precursors of the C31 steranes preserved in rock. They then synthesized eight different C31 sterols in the laboratory to serve as reference compounds. After subjecting these molecules to conditions that mimic burial and geological transformation over millions of years, they compared the results with the ancient samples.
Only two of the eight synthesized sterols transformed into compounds that matched the C31 steranes found in the rocks. The absence of the other six products indicates that the molecules were not created by random chemical reactions in the environment.
Together, evidence from rock chemistry, modern sponges, and laboratory experiments supports the conclusion that the steranes originated from living organisms. Those organisms were most likely early ancestors of demosponges, which still retain the ability to produce similar compounds today.
“It’s a combination of what’s in the rock, what’s in the sponge, and what you can make in a chemistry laboratory,” Summons says. “You’ve got three supportive, mutually agreeing lines of evidence, pointing to these sponges being among the earliest animals on Earth.”
“In this study we show how to authenticate a biomarker, verifying that a signal truly comes from life rather than contamination or non-biological chemistry,” Shawar adds.
Expanding the Search for the First Animals
Now that C30 and C31 sterols appear to be reliable indicators of ancient sponges, the researchers plan to examine rocks from other parts of the world. So far, the samples indicate that these sponges lived during the Ediacaran Period. With additional material, the team hopes to pinpoint more precisely when some of the earliest animals first emerged.
This research was supported, in part, by the MIT Crosby Fund, the Distinguished Postdoctoral Fellowship program, the Simons Foundation Collaboration on the Origins of Life, and the NASA Exobiology Program.
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