Ever since scientists began to understand evolution, they have argued over the controversial question of what exactly the first animal looked like.
Based on genetic analysis, some say that today’s common sponges (aka demosponges, porifera) are more similar to our original animal ancestor; Others argue that comb jellies better fit the bill.
New research by scientists at the Massachusetts Institute of Technology (MIT) tips the scales in the team’s favor, piecing together chemical clues that suggest these animals emerged during the Neoproterozoic era, more than 541 million years ago.
Unfortunately, the fossil record provides only a very patchy picture of the animals’ early days. It is particularly difficult to trace evidence of soft-bodied animals, like comb jellies and sponges, because they did not have the hard types of materials – bone, cartilage, etc. – which fossilize well.
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Sponges sometimes leave behind biomineralized “spicules,” tiny structures of calcium carbonate or silica, but recent evidence suggests that early sponges lacked these forms. As for comb jellies, their claim to the title is based primarily on genetic analysis, not fossil evidence.
Scientists have had to get creative, scouring Ediacaran oil and sediments for chemical signatures that may support a spongey origin for animal life, without traditional fossils.
“We don’t know exactly what these organisms looked like at the time, but they absolutely would have lived in the ocean, they would have been soft-bodied, and we assume they didn’t have a silica skeleton,” says MIT geobiologist Roger Sumson.
A 2009 study assignment was implicated in an abundance of fossil evidence of sponge-like sterols in the Marinos rock of Oman. These rare C30 Sterols (fatty organic compounds found in the cell membranes of all complex organisms) hint at early sponge activity. Simpler organisms, whose life strategies predate sponges, do not produce the sterols, but modern demosponges do.
“It’s very unusual to find a sterol with 30 carbons,” says the paper’s lead author, MIT organic geochemist Lubna Shawar.
This suggests that ancient sponges may have been the first animals on Earth, appearing much earlier than other multicellular creatures.
Skeptics have argued that the Oman Sterane fossils may have alternative, non-biological or algal origins, but Lubna and his team have now ruled this out with a second chemical ‘fossil’ containing C31. It derives from the same sponge gene that produces c30 sterols, and this same chemistry occurs in some living demosponges.
“In this study, we show how to authenticate a biomarker, verifying that a signal really comes from life rather than contamination or non-biological chemistry,” says Shawar.
The research team simulated the process of sterol fossilization in the laboratory using eight types of synthetic C31 Sterols, processed in a way that mimics hundreds of millions of years in the Earth’s crust. At the end of the simulation, two examples matched the old C exactly31 Sterol remains, further supporting a biological origin.
“It’s a combination of what’s in the rock, what’s in the sponge, and what you can do in a chemistry lab,” Summon says. “You have three mutually agreed upon, favorable lines of evidence pointing to these sponges being among the earliest animals on earth.”
“These special steranes were there all along,” adds Shawar. “We had to ask the right questions to look for them and to really understand their meaning and where they come from.”
Now that they’ve confirmed these ancient chemical signatures for early sponges, the team hopes to sift through other geological samples in search of more of our earliest ancestors.
This research was published in PNAS.
