Sea sponges appeared on our planet about 640 million years ago, much earlier than any other animal, according to a study published this week in the Proceedings of the National Academy of Sciences.
Tube sponges Callyspongia sp. attracting cardinal fishes, golden sweepers and wrasses. Image credit: Nick Hobgood / CC BY-SA 3.0.
The study, led by Dr. David Gold of the Massachusetts Institute of Technology (MIT), confirms that sea sponges are the source of an unusual molecule found in rocks that are 640 million years old — far before the Cambrian explosion.
“We brought together paleontological and genetic evidence to make a pretty strong case that this really is a molecular fossil of sponges. This is some of the oldest evidence for animal life,” Dr. Gold said.
Scientists have unearthed an extraordinary number of fossils from the time of the Cambrian explosion (circa 521–514 million years ago).
Based on the fossil record, some of them have argued that contemporary animal groups ‘exploded’ onto Earth, very quickly morphing from single-celled organisms to complex multicellular animals in a relatively short geological time span.
However, the fossils that are known from before the Cambrian explosion are peculiar in many respects, making it extremely difficult to determine which type of animal was the first to the evolutionary line.
Dr. Gold, Prof. Roger Summons, also from MIT, and their colleagues have been looking for the answer in molecular fossils.
“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. So people are interested in the idea that some of these biomarkers and chemicals, molecules left behind, might help resolve these debates,” Dr. Gold said.
The team has focused on 24-isopropylcholestane (24-ipc for short), a lipid molecule, or sterol, that is a modified version of cholesterol.
In 1994, scientists first found this molecule in Cambrian and slightly older rocks, and they speculated that sponges or their ancestors might be the source.
In 2009, they confirmed the presence of 24-ipc in 640-million-year-old rock samples from Oman, potentially representing the oldest evidence for animal life.
“It’s known that some modern sea sponges and certain types of algae produce 24-ipc today,” the scientists said. “But which organism was around to make the molecule 640 million years ago?”
To answer this question, Dr. Gold and co-authors sought to first identify the gene responsible for making 24-ipc, then find the organisms that carry this gene, and finally trace when the gene evolved in those organisms.
They looked through the genomes of about 30 different organisms, including plants, fungi, algae, and sea sponges, to see what kinds of sterols each organism produces and to identify the genes associated with those sterols.
“What we found was this really interesting pattern across most of eukaryotic life,” Dr. Gold said.
By comparing genomes, they identified a single gene — sterol methyltransferase (SMT) — responsible for producing certain kinds of sterols depending on the number of copies of the gene an organism carries.
They found that sea sponge and algae species that produce 24-ipc have an extra copy of SMT when compared with their close relatives.
The scientists compared the copies to determine how they were all related and when each copy of the gene first appeared. They then mapped the relationships onto an evolutionary tree and used evidence from the fossil record to determine when each SMT gene duplication occurred.
No matter how they manipulated the timing of the evolutionary tree, they found that sea sponges evolved the extra copy of SMT much earlier than algae, and they did so around 640 million years ago — the same time period in which 24-ipc was found in rocks.
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David A. Gold et al. Sterol and genomic analyses validate the sponge biomarker hypothesis. PNAS, published online February 22, 2016; doi: 10.1073/pnas.1512614113
