First, the Sponge
Sponge fossils predate Cambrian explosion.
New genetic analyses led by MIT researchers confirm that sea sponges are the source of a curious molecule found in rocks that are 640 million years old. These rocks significantly predate the Cambrian explosion, the period in which most animal groups emerged 540 million years ago—suggesting that sponges may have been the first animals to appear on Earth.
“We brought together paleontological and genetic evidence to make a pretty strong case that this really is a molecular fossil of sponges,” says David Gold, a postdoc in the lab of Earth, Atmospheric and Planetary Sciences professor Roger Summons. “This is some of the oldest evidence for animal life.”
In 1994, having found unusual amounts of 24-isopropylcholestane, or 24-ipc, in Cambrian and slightly older rocks, Summons’s lab speculated that sponges or their ancestors may have been the source of these molecular fossils. In 2009, the team used precision uranium-lead dating techniques developed by EAPS professor Samuel Bowring to confirm the presence of 24-ipc in 640-million-year-old rock samples from Oman.
Upon examining the genomes of more than 30 organisms, Summons and Gold found that sea sponge and algae species that produce 24-ipc have an extra copy of a single gene, which produces sterol methyltransferase, or SMT.
Using evidence from the fossil record, they determined that sea sponges evolved the extra copy of the SMT gene much earlier than algae, around 640 million years ago—the same time period in which 24-ipc was found in rocks.
Their results provide strong evidence that sea sponges—and thus animal life—appeared on Earth much earlier than previously thought.