Life on Earth probably started some 3 billion years ago, give or take several hundred million years. But multicellular organisms, including animals, didn't start to show up until much more recently, within the last 700 million years.

Paleontologists have unearthed an extraordinary diversity of fossils from the period starting about 540 million years ago, a period known as the Cambrian explosion. But fossils from before this time are scant or peculiar, making it difficult to pinpoint what type of creature was the first in the animal line.

But new genetic and paleontological evidence assembled by MIT researchers may finally offer an answer.

So what was Earth's first animal? It turns out, it was probably a simple sea sponge, reports Phys.org.

The clues didn't come from fossils in the traditional sense, but rather from traces of certain molecules found in ancient rocks — molecular fossils, if you will. Basically, when an animal dies and decays, it leaves evidence of its existence in the form of biomarkers and chemicals, even when it doesn't fossilize. So, theoretically, scientists can study ancient rocks and look for biosignatures unique to certain kinds of animals, even in the absence of fossils.

Previous research in 1994 had identified one chemical in particular, 24-isopropylcholestane (or 24-ipc for short), in high amounts in Cambrian and slightly older rocks. This substance is a lipid molecule, or sterol, a modified version of cholesterol, and it's known to be produced by sponges and a few other organisms alive today.

Then, in 2009, another research team confirmed the presence of 24-ipc in 640-million-year-old rock samples from Oman. The sheer age of these rock samples means they could very well represent traces of the first animals to evolve on Earth.

To assemble the puzzle of what kinds of animals might have produced this 24-ipc, researchers turned to genetic analysis. They surmised that if they could identify the gene responsible for making 24-ipc and find the organisms that carry this gene, they could trace when the gene evolved in those organisms.

The gene they identified, it turns out, is found in just the right form in both sponges and some types of algae. Researchers then performed genetic analysis to determine whether sea sponges or algae had evolved this gene first. The results were definitive: it was the sponges. Even more telling, the genetic analysis revealed a rough date for when the gene likely first appeared among sponges: 640 million years ago.

The pieces of the puzzle fit together perfectly, offering a compelling case that the sponge — or at least some manifestation of a sponge-like creature — was the first animal to inhabit the planet.

Think about that the next time you're scrubbing yourself down in the shower.

"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," said David Gold, a postdoc in MIT's Department of Earth, Atmospheric and Planetary Sciences.

The results have been published in the Proceedings of the National Academy of Sciences. Gold is the lead author on the paper, along with senior author and EAPS Professor Roger Summons.