CAMBRIDGE (CBS) – What was the first animal on Earth? Scientists at the Massachusetts Institute of Technology believe they’ve finally answered that question.

Based on new genetic tests, researchers can say with confidence that molecules produced by sea sponges have been found in 640 million-year-old rocks. That would make the sea sponge the oldest animal on the planet.

An explanation from MIT states that most animals emerged on the planet during the Cambrian explosion, which was “only” 540 million years ago.

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

MIT scientists have been studying this topic for over 20 years, and say there’s been a recent debate over whether the sea sponge or the more complex comb jelly should hold the title of Earth’s first animal. But new evidence appears to tip the scales in the sea sponge’s favor.

There aren’t many fossils around that can show what life was like 640 million years ago, so MIT has been studying molecules in ancient rocks. An unusually high level of a lipid molecule similar to cholesterol has been found in some of these very old rocks, and researchers believe sea sponges were the ones producing it.

See how MIT explains the discovery: 

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