There is a specific moment in Earth’s history in which most animals took over the planet, known as the Cambrian Explosion, which took place approximately 540 million years ago. While sea sponges have been widely thought to be the first animal on Earth, recent genetic evidence suggests that the more complex comb jelly might claim this title. But new research by MIT scientists may side with the sea sponge. They looked at a molecule found in rocks that are 640 million years old, and confirmed that it was originally produced by sea sponges, 100 million years before the Cambrian Period began. The evidence suggests that sea sponges may indeed have been the first animals to inhabit Earth. Very few fossils exist from well before the Cambrian explosion, making it extremely difficult to determine what the first animals were like. So researchers at MIT have been looking for the answer in molecular fossils, which are trace amounts of molecules that have survived in ancient rocks, long after the actual animal has decayed away. The researchers focus on 24-isopropylcholesterol, or 24-ipc for short, a lipid molecule that is a modified version of cholesterol. Because this molecule has been found in rocks that are 640 million years old, and is produced today by sea sponges, it could be the key to unlocking evidence of the oldest animal life. In this study, the scientists analyzed genes and lipids across a large group of organisms, including multiple types of plants, fungi, and sponges, trying to determine how many times the ability to produce 24-ipc has evolved. They identified a gene responsible for this molecule, and discovered that the number of gene copies each species has can predict what kind of molecules they can produce. They found that only two organisms – sea sponges and algae – have enough copies of this gene to produce 24-ipc. No matter how they manipulated the timing of the evolutionary tree, the researchers found that sea sponges evolved the extra copy of the gene much earlier than algae, and they did so around 640 million years ago the same period in which 24-ipc was found in rocks. Their results provide strong evidence that the simple sea sponge, and thus animals, appeared on Earth much earlier than fossils currently suggest.