The title of "Earth's First Animal" may be awarded to sea sponges, American scientists say. A new study from the Massachusetts Institute of Technology (MIT) reveals that sea sponges were present even before most animals appeared millions of years ago.
New genetic analyses confirm that a mysterious molecule found in rocks that are about 640 million years old are from sea sponges. This means that sea sponges were already present before the Cambrian explosion, a period that began about 540 million years ago, when most ancient animals were reported to appear.
"We brought together paleontological and genetic evidence to make a pretty strong case that this really is a molecular fossil of sponges," said David Gold of the MIT. He said that this is one of the oldest evidence of animal life.
Molecular Fossils Provided Ancient Clues
The researchers analyzed molecular fossils, which are small quantities of molecules found in prehistoric rocks that are still present today. They focused on the lipid molecule 24-isopropylcholestane, or 24-ipc, a modified version of cholesterol produced by some modern sea sponges and algae types.
To identify which organism made the molecule 640 million years ago, Gold and his team had to identify the gene that makes 24-ipc, determine which organisms carry the gene and track down when the gene evolved. They studied the genomes of 30 organisms such as algae, fungi, plants and sea sponges.
Results show that both algae species and sea sponge have the same number of copies of the gene sterol methyltransferase (SMT) to produce 24-ipc. Through genome analysis and the mapping of an evolutionary tree using fossil record data, the researchers found that sea sponges were already producing the molecules long before algae did.
The research topic was first pursued in 2009 by a team led by Gordon Love from the University of California at Riverside. Love and his colleagues analyzed 640-million-year-old rock samples in Oman and confirmed the presence of 24-ipc. The discovery represented the oldest evidence for animal life.
New Questions Emerge
Though a milestone discovery has been established, this brings up more questions about sea sponges that lived millions of years ago. The discovery may spur curiosity among scientists on how these organisms looked like and what their environment was like.
The groundbreaking discovery shows that there is still much to discover about animal life and evolution. Molecular fossils may serve as valuable clues to fill in those gaps.
The study was published in the journal Proceedings of the National Academy of Sciences.
Photo: Tam Warner Minton | Flickr