The first animal to appear on Earth was probably the humble sea sponge, American scientists believe.

According to researchers at the world-famous Massachusetts Institute of Technology (MIT), sea sponges pre-date the Cambrian explosion, an evolutionary period that began around 540 million years ago, during which a large number of new animals appeared.

The MIT team made the discovery by analysing molecular fossils - trace amounts of molecules found in ancient rocks that survive to this day.

One rare molecule in particular, 24-Isopropylcholesterol, or 24-IPC, has been found in rocks that are 640 million years old. It's also known to be produced by sea sponges today.

In an effort to follow up this lead, the team, led by David Gold, tried to find out which gene was responsible for producing 24-IPC, what organisms carry the gene, and when it had evolved in those organisms.

By analysing the genes of around 30 different organisms, the team narrowed it down to a single gene, known as sterol methyltransferase (SMT), which can cause an organism to produce 24-IPC if it contains the correct number of copies.

The researchers found that both sea sponge and algae species have the right number of copies of SMT to produce 24-IPC, and by analysing these genes and producing evolutionary trees based on the fossil record, they found that sea sponges were producing the molecule long before algae.

The scientists believe they reached this milestone around 640 million years ago - the same period in which the rocks containing 24-IPC were formed.

This study provides strong evidence that sea sponges appeared on the Earth much earlier than any other animals.

However, David Gold believes the discovery brings up new questions.

The ugliest animals on earth Show all 12 1 /12 The ugliest animals on earth The ugliest animals on earth blobfish.jpg A blobfish The ugliest animals on earth pignosed-turtle.jpg The pig-nosed turtle The ugliest animals on earth british-bat#.jpg A British bat The ugliest animals on earth toad.jpg A Titicaca water frog The ugliest animals on earth pubic-lice.jpg Pubic lice The ugliest animals on earth axolotl.jpg An axolotl The ugliest animals on earth lizard.jpg Stephen Fry has tipped the greater short-horned lizard for the prize The ugliest animals on earth prommonkey.jpg A proboscis monkey The ugliest animals on earth dung.jpg A flightless dung beetle The ugliest animals on earth eel.jpg A European eel The ugliest animals on earth kakapo.jpg The kakapo - a critically endangered giant parrot The ugliest animals on earth slug.jpg A Dromedary jumping slug

"What did these organisms look like? What was the environment like [640 million years ago]? And why is there this big gap in the fossil record?" he said.