Way before humans, sharks, or dinosaurs, the sea sponge was very likely the first animal on Earth. That's according to a PNAS study out of MIT concluding that a molecule in 640 million-year-old rocks came from the simple creature. Assuming the researchers are right, that means the multi-celled organisms were around at least 100 million years before most animal groups blossomed during what's known as the Cambrian explosion, reports MIT News. The few fossils from the pre-Cambrian era are difficult for paleontologists to evaluate, so researchers looked to molecule traces left behind in rocks by decaying creatures. One molecule in particular, a modified version of cholesterol called 24-isopropylcholestane, kept cropping up, the same one produced by some sea sponges and algae today.
Using what Discovery calls "evolutionary-tree detective work," the researchers were able to rule out algae as the source. “We brought together paleontological and genetic evidence to make a pretty strong case that this really is a molecular fossil of sponges,” says one of the scientists. “This is some of the oldest evidence for animal life.” The find isn't so much a bombshell as further confirmation of a widely held theory that life originated with sea sponges. And it opens the door to yet more questions about this earliest stage of animal life, including precisely what the organisms looked like and what kind of environment allowed them to thrive. Another biggie as stated by the MIT researcher: "Why is there this big gap in the fossil record?" (Sea sponges may have helped create other animals.)