Ancient Chemical Fossils Strengthen Case for Sponges as Earth’s First Animals
CAMBRIDGE, MA – New research published in PNAS provides compelling chemical evidence that sponges where likely the earliest animals on Earth, potentially appearing hundreds of millions of years ago. Scientists at MIT have re-analyzed fossil sterols found in Oman‘s Marinoan rock formations, bolstering a theory previously challenged by the scientific community.
For years, pinpointing the origins of animal life has been hampered by the scarcity of customary fossils from the Ediacaran period. Researchers have turned to “chemical fossils”-molecular signatures preserved in ancient rocks-to understand early life forms. A 2009 study, involving MIT geobiologist Roger Summons, initially identified an abundance of C30 sterols, fatty organic compounds found in the cell membranes of complex organisms, within the Omani rock. These sterols are produced by modern demosponges but not by simpler,pre-sponge organisms.
Skeptics suggested the sterols could have non-biological or algal origins. However, Lubna Shawar, MIT organic geochemist and lead author of the new study, and her team have now strengthened the case with the discovery of C31 sterols, derived from the same gene responsible for C30 sterol production in sponges.
“It’s very unusual to find a sterol with 30 carbons,” Shawar stated. “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.”
to further validate their findings, the team simulated the fossilization process in a laboratory setting, using eight types of synthetic C31 sterols subjected to conditions mimicking millions of years within Earth’s crust.Two of the synthetic samples precisely matched the ancient C31 sterol remnants.
“It’s a combination of what’s in the rock, what’s in the sponge, and what you can make in a chemistry laboratory,” explained Summons. “You’ve got three supportive, mutually agreeing lines of evidence, pointing to these sponges being among the earliest animals on Earth.”
Shawar added,”These special steranes were there all along.It took asking the right questions to seek them out and to really understand their meaning and from where they come.”
The research team now plans to analyze additional geological samples in the search for further evidence of early animal ancestors.
