Sea sponges, widely considered the first animals on Earth, now have some extra ammunition to back up that belief.

A molecule present in 640-million-year-old rocks comes from a simple sea sponge, according to genetic analysis done by Massachusetts Institute of Technology (MIT) researchers. The time frame is well before the Cambrian Explosion 540 million years ago, in which most animals groups sprang up and took hold in a relatively short geologic stretch.

That makes the humble sea sponge a likely bearer of the title “first animal on Earth.”

That’s according to findings appearing in a paper just published in the journal Proceedings of the National Academy of Sciences by lead author David Gold and senior author Roger Summons, both from MIT.

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True to its name, the Cambrian Explosion produced no shortage of fossils. But very few pre-Cambrian samples exist. That paucity has made it difficult for scientists to determine which animal was first.

The MIT researchers decided to look for the elusive first creature by studying molecular fossils –- trace molecules still present in ancient rocks after the animal that left them has long since decayed into nothingness.

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Gold and Summons focused on a particular molecule: 24-isopropylcholestane, or 24-ipc. It had been found, through prior research, in the 640-million-year-old rocks.

They suspected the molecule was from a sea sponge, as some modern sponges are known to produce it. But, the rub was that some forms of algae also make 24-ipc. So they had to prove the molecule came from a sea sponge, while ruling out algae.

The researchers first identified the gene that made the molecule, observing that sea sponges and algae have an extra copy of the gene.

Then, after some evolutionary-tree detective work, they traced the extra-copy genetic behavior back to its oldest evolutionary carrier: the sea sponge. It turned out the sponge had evolved the extra gene copy well before algae, sometime around -- you guessed it -- 640 million years ago.

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“We brought together paleontological and genetic evidence to make a pretty strong case that this really is a molecular fossil of sponges,” said Gold in a statement. “This is some of the oldest evidence for animal life.”

“This brings up all these new questions,” Gold added. “What did these organisms look like? What was the environment like? And why is there this big gap in the fossil record? This goes to show how much we still don’t know about early animal life, how many discoveries there are left, and how useful, when done properly, these molecular fossils can be to help fill in those gaps.”