Ever since scientists began to understand evolution, they've been arguing over the contentious question of what, exactly, the first animal looked like.
Based on genetic analysis, some say today's common sponges (aka demosponges, Porifera) are most like our original animal ancestor; others argue comb jellies better fit the bill.
New research led by scientists at Massachusetts Institute of Technology (MIT) tips the scales in favor of team sponge, piecing together chemical clues that suggest these animals emerged during the Neoproterozoic era, more than 541 million years ago.
Unfortunately, the fossil record only provides a very patchy picture of the early days of animals. It's particularly difficult to track down evidence for soft-bodied animals, like comb jellies and sponges, since they didn't have the kinds of hard materials – bones, cartilage, etc – that fossilize well.
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Sponges do sometimes leave behind biomineralized 'spicules', tiny structures of calcium carbonate or silica, but recent evidence suggests the earliest sponges lacked these forms. As for comb jellies, their claim to the title is based mainly 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 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 MIT geobiologist Roger Summons.
A 2009 study Summons was involved in reported an abundance of fossil evidence of sponge-like sterols in Marinoan rock from 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 ancient sponges may have been the first animals on Earth, appearing much earlier than other multicellular creatures.
Skeptics argued that the Oman sterane fossils may have alternate, non-biological or algal origins, but Lubna and her team have now ruled that out with a second chemical 'fossil' containing C31. It's derived 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 truly comes from life rather than contamination or non-biological chemistry," Shawar says.
The research team simulated the sterol fossilization process in the lab using eight kinds of synthetic C31 sterols, processed in ways that mimic hundreds of millions of years in Earth's crust. At the end of the simulation, two examples exactly matched the ancient C31 sterol remnants, further supporting a biological origin.
"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."
"These special steranes were there all along," Shawar adds. "It took asking the right questions to seek them out and to really understand their meaning and from where they come."
Now that they have 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.