The basic building blocks of life have been present on Earth for billions of years, but it was a long time before these pieces organized themselves in such a way that would be considered an animal. According to a report from Discovery News, scientists from MIT have found new evidence suggesting that the sea sponge was likely the first animal to emerge in the history of life on Earth.

A molecule discovered in rock samples more than 640 million years old has been linked to the sea sponge by researchers at the Massachusetts Institute of Technology. This is almost 100 million years before the Cambrian Explosion, the period of time where most animals we know today began to emerge.

The discovery marks the oldest molecular evidence of animal life ever found, earning the sea sponge the title of the world’s first animal. The study was led by David Gold and Roger Summons of MIT, and was published in the journal Proceedings of the National Academy of Sciences.

Finding animal fossils from before the Cambrian Explosion is extremely difficult. Given the abundance of animal fossils from the time period roughly 100 million years alter, it’s hard to even know what to look for as a paleontologist.

The fossil that led to the discovery wasn’t a petrified bone like the massive dinosaur and mammoth fossils we know and love. Scientists were looking for what they call molecular fossils, or minute traces of organic material present in rocks that were left behind by animals millions of years ago. One molecule in particular, 24-isopropylcholestane, or 24-ipc, was discovered in the rock samples in the study, and was subsequently linked to sea sponges.

There was a problem with the study’s methods, however. Certain types of algae produce 24-ipc as well, so the researchers had to prove that the molecule had come from a sea sponge.

They identified a gene that was responsible for creating the molecule, and found that it was present in both algae and sea sponges. After digging deep into the evolutionary tree, however, they confirmed that sea sponges had developed the gene long before algae, roughly 640 million years ago.

A press release from MIT describing the details of the study can be found here.