Ask any paleobiologist when the first animals appeared on Earth, and they’ll tell you the same thing: with a few notable exceptions, it was during the Cambrian explosion, 541 million years ago.
Well, not any paleobiologist. A new study published in the journal Nature is turning heads with a radical suggestion: sponges, one of the simplest animal life forms on the planet, may be much, much older than the Cambrian explosion – 350 million years older, to be exact.
Wiggly-looking structures contained in rocks in northwestern Canada resemble the internal scaffolding of sea sponges, except they date back 890 million years. If confirmed, they could be the oldest known animal fossils on Earth.
“If I’m right, animals emerged long, long before the first appearance of traditional animal fossils,” study author Elizabeth Turner told Nature. “That would mean there’s a deep back history of animals that just didn’t get preserved very well.”
Life, uh, finds a way.
The idea that animal life existed before the Cambrian period is controversial, but not new. Paleontologists have two major resources at their disposal to estimate when animals first emerged: the fossil record, and a technique called molecular clock estimation. The former is simple: if we can find a fossil that dates back x million years, then that animal must have been alive at that time. The latter is more abstract: it is based on the idea that evolution happens at a relatively constant pace, and therefore the time since two species shared a common ancestor can be worked out by comparing how different those species are from one another.
The problem comes when these two methods give radically different answers. The majority of molecular clock estimates tell us that the last common ancestor of animal life today evolved more than 100 million years before the earliest definitive fossil record turns up.
In that respect, Turner’s paper is “pretty convincing”, according to paleontologist Amelia Penny, who was not involved in the study. This new estimate “brings the fossil record back into line with the molecular clock estimates,” she told New Scientist.
Turner first found the 890-million-year-old rock samples that would eventually lead to her paper as a PhD student, but it wasn’t until years later, as a professor, that she was able to return to the Little Dal reefs of remote northwest Canada and carry out more robust research. What she found was a potentially shocking discovery: the signature structure of a fossilized sponge.
“This organic skeleton is very characteristic [of sponge fossils],” explained geobiologist Joachim Reitner, who reviewed Turner's study ahead of publication. “[T]here are not known comparable structures.”
But such a big claim will naturally be met with skepticism, and Turner faces some big questions from her peers. Some in the paleontology community say that she has not provided strong enough evidence that the samples show fossils at all, pointing out that there are other potential explanations for the supposed sponge fossils.
“It’s such a big claim that you really have to eliminate all the other possibilities,” geoscientist Rachel Wood explained in Nature. “Microbes, for example, produce weird and wonderful shapes and forms.”
In fact, sometimes such patterns can be created by crystals, Wood pointed out, meaning Turner’s discovery might not even be a fossil at all.
“What we have is essentially something a bit like a Rorschach inkblot test, where there are some squiggles in a rock,” paleontologist Jonathan Antcliffe told National Geographic.
If Turner’s discovery is correct, it would mean that sponges were around at a point in history when the Earth had very low oxygen levels – something which was believed until recently precluded the possibility of animal life. It also means that sponges must have survived throughout the “Snowball Earth” period of 720 to 635 million years ago. Both of these events were previously considered catastrophic for life on Earth.
“It seems almost like a no-brainer,” Turner told National Geographic. “It's time for it to be published and go out to the community for discussion and challenge.”