[USA] According to a study by researchers at the Massachusetts Institute of Technology, there are about 8.7 million species of animals on Earth, give or take 1.3 million. But in the beginning there could be only one. That animal was very likely the simple sea sponge,
Based on new genetic tests, the team of scientists can say with confidence that molecules produced by sea sponges have been found in 640-million-year-old rocks. These rocks significantly predate the Cambrian explosion, the period 540 million years ago in which most animal groups took over the planet, suggesting that sea sponges may have been the first animals.
The researchers write in the study, which was published Friday in the Proceedings of the National Academy of Sciences, that their testing provides “the oldest evidence for animal life.”
“We brought together paleontological and genetic evidence to make a pretty strong case that this really is a molecular fossil of sponges,” explains David Gold, a post-doctoral researcher in MIT’s Department of Earth, Atmospheric, and Planetary Sciences (EAPS), in a press release.
Paleontologists have long struggled to determine which type of animal was the first to the evolutionary line. While they have unearthed a large number of fossils from the start of the Cambrian explosion, the fossils that are known from before then are peculiar in many respects.
EAPS Professor Roger Summons has spent more than two decades searching for the animal kingdom’s extended evolutionary tree. His lab has been looking for clues in molecular fossils, trace amounts of molecules that have survived in ancient rocks long after the rest of an animal has decayed.
ALGAE VS. SEA SPONGES
Finding a very large number of fossils dating back to roughly 540 million years ago, paleontologists realized that that they had all evolved from single celled organisms in a very short matter of time. This is known as the Cambrian explosion – an event that set in motion the eventual appearance of humans.
However, pre-Cambrian fossils are very strange, making it exceedingly difficult to ascertain the type of animal to which they belonged. Some of these fossils are in the form of molecules captured in ancient rocks, preserved almost intact after all this time.
So far, the theory had been that those molecules, which were dated to be far older than the Cambrian explosion, either belonged to algae or to sea sponges, as they were the most likely to predate other forms of multicellular life. But there really wasn’t any way to determine which one it was.
URANIUM AND CHOLESTEROL
That is, until the development of more modern ways of dating. Back in the ‘90s, a team of researchers including the lead author of the current study found traces of 24-isopropylcholestane, a form of cholesterol which is basically an ancient lipid. These traces were found in the aforementioned fossils.
Flash forward to 2009, when a team finally finished analyzing the rocks and determined they were around 640 thousand years old – the oldest evidence of animal life to date. This was determined by using modern dating techniques that use uranium-lead.
The “battle” raged on, as both algae and sponges are known to produce the 24-isopropylcholestane lipid, but the scientists still didn’t know which came first. The next part of the study consisted of identifying the gene responsible for generating the lipid, finding the organisms which carried it, and then trace when it evolved in them.
Finding the sterol methyltransferase gene, or SMT, responsible for the lipid, and present in both sponges and algae, the team went on with the dating techniques, and finally uncovered the truth – sea sponges developed the gene way before algae even appeared, some 640 million years ago.
The so-called “‘sponge biomarker hypothesis” was first hypothesized in 1994 and partially confirmed in 2009. It focuses on 24-isopropylcholestane, a lipid molecule, or sterol, that scientists have found in unusually high amounts in Cambrian and slightly older rocks. Genetic testing by Dr. Gold adds “a further layer of evidence supporting” the theory that sponges or their ancestors might be their source, Dr. Summons said in the release.
The results of the study provide strong evidence that sea sponges appeared on Earth 640 million years ago, much earlier than any other life form.
“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 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.
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