MIT researchers have discovered molecular fossils of sea sponges in 640 million year old rocks and analyzed them genetically to reach the conclusion.
Sea sponges were likely the first animals to appear on Earth.
MIT researchers have discovered the molecular fossils of sea sponges in 640 million year old rocks. The rocks predate the Cambrian period, an important point in the history of the life on Earth when most major animal groups appear for the first time in fossil record and suggest that sea sponges may have been the first animals living on Earth.
“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 evolutionary period is sometimes called “Cambrian explosion” because many animal groups just ‘exploded’ onto the Earth in a relatively short time span during this period and morphed from single-celled organisms to complex multicellular animals.
Researchers have unearthed extraordinary number of fossils from the period that started around 540 million years ago. But it was difficult to pinpoint exactly which animal was the first to inhabit the Earth since the fossils are very odd and arguable in many respects.
The latest fossil molecules were discovered in the ancient rocks while the animal itself has decayed away a long time ago.
“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,” said Gold. “So people are interested in the idea that some of these biomarkers and chemicals, molecules left behind, might help resolve these debates.”
During genetic analysis, researchers keenly looked at 24-isopropylcholestane, or 24-ipc for short — a lipid molecule to determine to which animal it belongs to.
Many modern-day sea sponges and algae produce 24-ipc but which animal was around to make the molecule 640 million years ago? To find the answer, researchers sequenced the genomes of 30 different organisms including plants, fungi, algae and sea sponges and figured out the organism that carry this gene.
When researchers compared those genomes, they found that sea sponges and algae produce 24-ipc with an extra copy of sterol methyltransferase or SMT. Then, they traced the evolutionary tree of both to see when the copy of gene appeared first.
Researchers found that sea sponges evolved the extra copy of SMT much earlier than algae around 640 million years ago. This is the same period when 24-ipc was found in rocks.
Gold says.“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.”