According to researchers from the Massachusetts Institute of Technology (MIT), the first animal to appear on Earth was probably the sea sponge.
The researchers discovered molecular fossils of sea sponges in 640 million years old rocks and analyzed them genetically to reach the conclusion. These rocks significantly predate the Cambrian explosion, the relatively short event during which most of major animal phyla appeared, indicating that the sponge emerged much before this event.
“We brought together paleontoglical and genetic evidence to make a pretty strong case that this really is a molecular fossil of sponges,” said David Gold, a researcher at MIT’s Department of Earth, Atmospheric and Planetary Sciences. “This is some of the oldest evidence for animal life.”
The genetic analysis suggests that a sea sponge was the source for an unusual molecule found in the 640 million year old rocks. The molecule they focused on is 24-isopropylcholestane, or 24-ipc for short — a modified version of cholesterol.
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 the sea sponge 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 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.
Sea Sponges are multicellular organisms that have bodies full of pores and channels allowing water to circulate through them. Sponges do not have nervous, digestive or circulatory systems. Instead, most rely on maintaining a constant water flow through their bodies to obtain food and oxygen and to remove wastes.