Researchers from MIT and other institutions have discovered chemical fossils possibly left by ancient sponges on rocks dating back over 541 million years. These fossils consist of a distinctive type of sterlan, a stable variant of sterols found within the cell membranes of complex organisms. The team linked these sterlans to a category of sea sponges known as demosponges.
It highlights the picture representation of the ancient Stellan timeline, highlighting important compounds and their possible biological sources. Image credit: Shawar et al. , doi: 10.1073/pnas.2503009122.
“While I cannot precisely describe what these creatures looked like, I can assert they inhabited the ocean, had soft bodies, and likely lacked a silica skeleton,” stated MIT professor Roger Sammons.
In 2009, the researcher discovered the first chemical fossil believed to have originated from ancient sponges.
The team examined rock samples from outcrops in Oman and found an abundant sterlan they deduced to be a remnant of 30 carbon (C30) sterols—a rare steroid form attributed to ancient sea sponges.
Stellan was identified in very old rocks formed during the Ediacaran era (635-541 million years ago).
This era preceded the Cambrian period, which was marked by a sudden global explosion of complex, multicellular life forms.
The findings imply that ancient sponges may have existed far earlier than most multicellular organisms, potentially being one of the first animals on Earth.
Nevertheless, following the publication of these findings, alternative hypotheses emerged regarding the origin of C30 sterlan, suggesting that these chemicals could arise from other biological sources or non-organic geological processes.
The current study bolsters the initial hypothesis that ancient sponges produced this chemical record, as the researchers found new chemical fossils within the same promelat rock that were almost certainly biogenic.
Similar to previous studies, they searched for chemical fossils in rocks dating back to the Ediacaran period.
Samples were collected from drill cores and outcrops in Oman, West India, and Siberia, with analyses focused on the signatures of geologically stable sterols present in all eukaryotes (including plants, animals, and organisms with nuclear membranes).
“Without sterols or comparable membrane lipids, you cannot be classified as a eukaryote,” Professor Sammons remarked.
The chemical fossil identified in 2009 was 30-carbon sterols.
Additionally, the team deduced that these compounds could be synthesized due to distinct enzymes encoded by genes prevalent in demosponges.
“Finding sterols with 30 carbons is quite rare,” noted Dr. Lubna Shawar, a researcher at Caltech.
In this study, scientists concentrated on the chemistry of these compounds, observing that genes from the same sponge can produce even scarcer sterols with 31 carbon atoms (C31).
Upon analyzing rock samples of C31 sterlan, they discovered it was rich in the aforementioned C30 sterlan.
“These unique sterlans have been present all along,” Dr. Shawar remarked.
“We had to inquire the right questions to uncover them and truly comprehend what they signify and their origin.”
The researchers additionally procured samples of modern demosponges to examine for C31 sterols.
They determined that it is indeed a biological precursor of C31 sterlan found in rocks, observed in several species of contemporary demosponges.
Going further, they chemically synthesized eight different C31 sterols as reference materials to verify chemical structures.
The molecules were subjected to conditions simulating how sterols transform during deposition, burial, and pressurization over millions of years.
They found that two sterol-only products closely matched the structure of C31 sterols located in ancient rock samples.
The evidence from both substances strongly indicates that these compounds were created by living organisms rather than random non-biological processes.
Moreover, these organisms are likely ancestors of demosponges and still possess the capability to produce this set of compounds.
“It’s a blend of what’s present in the rock, what’s within the sponge, and what’s demonstrated in the lab,” explained Professor Sammons.
“Three supportive and concordant pieces of evidence strongly suggest these sponges are among Earth’s earliest animals.”
“This study illustrates how to authenticate biomarkers and confirm that the signals arise from life forms rather than contamination or abiogenic chemistry,” Dr. Shawar stated.
New Results were published this week in Proceedings of the National Academy of Sciences.
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Lubuna Shawar et al. 2025. Chemical characterization of C31 sterols from the sponge and Neoproterozoic fossil star counterpart. PNAS 22 (41): E2503009122; doi: 10.1073/pnas.2503009122
Source: www.sci.news
