A new study by researchers at the Massachusetts Institute of Technology (MIT) suggests that sea sponges may have been the first animals to inhabit the Earth. The team conducted a study that used genetic analyses to reveal that sea sponges are the source of a unique molecule found in 640-million-year-old rocks. These rocks predate the Cambrian explosion, the period when animal life exploded onto the planet, by a great deal of time.
"We brought together paleontological and genetic evidence to make a pretty strong case that this really is a molecular fossil of sponges," David Gold, who participated in the research, said in a press release. "This is some of the oldest evidence for animal life."
While many paleontologists believe that modern animal groups "exploded" onto the Earth during the Cambrian period in a fairly short period of time, fossils that predate this explosion shed doubts on this hypothesis and raise questions regarding the first animals on Earth. In order to shed light on this mystery, Gold and his team examined molecules that have survived in ancient rocks even after animal life was washed away.
"There's a feeling that animals should be much older than the Cambrian, because a lot of animals are showing up at the same time, but fossil evidence for animals before that has been contentious," Gold said. "So people are interested in the idea that some of these biomarkers and chemicals, molecules left behind, might help resolve these debates."
Previous research identified the sterol 24-isopropylcholestane, or 24-ipc, in 640-million-year-old rock samples, which represents the oldest evidence of animal life. Now, Gold and his team looked through the genomes of around 30 different organisms in order to identify the sterols that they produce and the genes associated with them.
Their comparison led to the identification of one gene, sterol methyltransferase (SMT), that is responsible for producing unique sterols, depending on the number of SMT genes in the organism. Furthermore, they found that sea sponge and algae species that create 24-ipc possess an extra copy of SMT compared to their close relatives.
Using these findings to map the gene relationships onto the evolutionary tree, they found that sea sponges evolved an extra SMT copy around 640 million years ago, earlier than algae, potentially making them the earliest life form on Earth.
"This brings up all these new questions: What did these organisms look like? What was the environment like? And why is there this big gap in the fossil record?" Gold concluded. "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."
The findings were published in the Feb. 24 issue of the Proceedings of the National Academy of Sciences.
