It’s been long presumed by many scientists the world over that modern-day sponges—the humble and porous sea creatures made famous by a certain geometric cartoon character that lives inside a undersea pineapple—belong to the oldest lineage of animals. These simple animals appear almost amorphous in shape, have no discernable internal organs, and seemingly lack the basic body symmetries most other animals in the family tree exhibit.
Thought to be the first animals to ever branch off the family tree hundreds of millions of years ago, their traces are among the first features in old rock paleontologists look for when finding the oldest evidence of life on Earth. Admittedly, it is a challenge: being invertebrates with no shells or exoskeletons, these simple animals don’t possess any “hard” body parts like bones that can easily be left behind after their deaths to fossilize over eons of time. (Bones wouldn’t appear in the fossil record until much later; for a similar reason, ancient cartilaginous fish, like ancient sharks, are only identifiable from their fossilized teeth—the parts of their body most resistant to the geological stresses induced by fossilization.) Yet, somehow, there are fossil impressions of ancient invertebrates, like jellyfish, that posses no “hard” body parts to speak of; these instances, however, are very rare occurrences, and require very specific conditions that these animals must be in at the time of their deaths before fossilization can occur—and even then, the best that we can get as fossils are impressions of their body parts locked in place in sedimentary rock (or entire bodies, if we get extremely lucky).
For this reason, scientists often have to resort to other techniques like molecular clock dating (MCD) to determine when these kinds of animals first appeared on Earth. MCD involves examining the rate of change of an animal’s DNA and/or proteins due to mutations alongside comparisons of a target’s DNA with those of other modern organisms; doing so gives a relative estimate of when the animal first appeared.
This, then, presents a dilemma: it’s one thing to determine when a kind of animal first appeared using MCD; finding the evidence for these animals in rocks of the same age as that determined by MCD, however, are an entirely different matter. Oftentimes, looking for rocks dated to an age range estimated by MCD yields empty rocks with no fossils to speak of. This, then, leads to two possibilities: either the age determined by MCD is wrong; or the dates from MCD were actually roughly correct, but fossilization of the animal in question would be a rare occurrence due to their invertebrate nature. Other times, scientists do find fossils in rocks relatively close to the dates determined by MCD, yet other scientists scrutinize whether or not the “fossils” in question were really fossils at all. Such is the nature of science and research—to seek and find, then doubt if you do.
A Rare Occurrence
This dilemma would find itself in the middle of recent scientific discussions once again right in the middle of 2021: a couple of rocks found in the Northwest Territories of Canada, discovered back in the 1990s but only recently reexamined, might just contain the oldest evidence yet of animal life, according to recent findings in a study submitted to the journal Nature.
In the study, author Elizabeth Turner, a sedimentary geologist at the Laurentian University, Sudbury, Canada, described finding rocks from the said area a few decades ago. At the time, Turner was on the search for features related to ancient cyanobacteria. Like most geologists do, they slice the rocks into thin cross-sections, then examine them under the microscope to search for features and shapes that are otherwise impossible to see with the naked eye.
Turner found impressions in the rock that seemed too “sophisticated” to be created by cyanobacteria: in fact, the impressions appeared closer to a spongin network, a network of “skeletal” collagen proteins recognizable as the main body component of modern keratose demosponges, more commonly known as horny sponges. She shelved the odd findings soon after, as they weren’t the focus of her studies at the time. Decades later, she would rediscover these findings while looking for more evidence of these ancient fossil reefs; by that time, other researchers have already published findings of similar structures that they attribute to ancient sponges, further strengthening her suspicions on the identity of whatever it was that left behind those “fibrous” networks in the rock. Said Turner: “If you look at the body of a fossil sponge microscopically, it has this characteristic microstructure, which was described and characterized and fully affiliated with the spongin skeleton in modern keratose demosponges. […] And it’s the identical structure to what I have.”
The fact that these fibrous networks that appear identical to those of ancient fossil sponges aren’t too surprising, considering they’ve already been found multiple times before; what’s striking to Turner, however, was the age of the rock that these structures were found in. Now, she was studying ancient cyanobacteria, so she knew that the rocks were definitely old. Dating the rocks revealed them to be around 890 million years old—a full 350 million years older than the then oldest evidence of animals ever found: ancient mollusks, worms, and arthropods, from the Cambrian explosion.
Turner defends that the structures were far too complex for ancient cyanobacteria to create (these cyanobacteria created the ancient reefs that she was studying at the time); according to her, it also made sense to find these sponges—should they actually be sponges—in ancient cyanobacterial reefs: these cyanobacteria created oxygen underwater, which the sponges might have delightfully bathed themselves in. These cyanobacteria might have also shed off polysaccharides from their cell walls, nourishing these suspected ancient sponges. These, according to her, cemented her position that she just found the oldest remnants of animal life yet.
Suspicions Abound
Not everybody is on board with the results just yet, however. Scientists are pointing out that other biological players, even bacteria, can make similar structures. According to Jonathan Antcliffe, a paleontologist at the University of Lausanne, Switzerland: “What we have is essentially something a bit like a Rorschach inkblot test, where there are some squiggles in a rock. […] Basically, every area of life—bacteria, algae, the fungi, the plants, the animals—they can all make things that look like this.” In fact, a 2014 review of recent findings related to early evidence of sponges done by Antcliffe and his team found out that the oldest, most conclusive findings of ancient sponges are sponge spicules (a sharp-looking microstructure identifiable with sponges) dating back to around 535 million years ago. According to him, no other recent studies have changed his mind on the matter.
Others are saying that even non-biological processes can create such structures. After all, experts often look for what they call biological markers in fossils—evidence of biological molecules like cholesterol—that may help cement the presence of previous animal life in fossils. For these findings, however, the evidence presented in support of the case are all visual in nature; much like how invertebrates only leave behind impressions of their bodies once fossilized, these remnants only appear to look like features found in existing sponges.
Future Work
It’s only natural for science to constantly doubt itself; it wouldn’t be as rigorous as it appears to be if scientists just accepted any and all findings willy-nilly. It’s imperative that this constant process of trial and error is upheld to the highest standard with each and every finding researchers publish: after all, if what you find isn’t dependent on time, it must be just as true then as it is now, and it should still remain true in the future. The paleontological dilemma presented by these findings are just one of many aspects that showcase how rigorous science truly is: the truth won’t always remain the absolute truth, especially in light of new evidence that says otherwise.
At the end of the day, more work needs to be done on findings of such importance as these. Scientists like Turner will need to find more ways of proving that what she really found were in fact ancient sponges; only then will she be able to truly claim to have found the oldest evidence of animals to date. After all, if true, her ancient sponge remnants would be the closest scientists would have gotten to the “actual” age of sponges determined by MCD—certainly beating out the results that scientists like Antcliffe stand by. In the words of Turner herself: “If I’m correct in my interpretations of this material, animals emerged long before the appearance of traditional animal fossils—they had a long prehistory.”
Bibliography
- Bobrovsky, I. (2020, November 24). Ancient sponges or just algae? New research overturns chemical evidence for the earliest animals. The Conversation. Retrieved September 3, 2021, from https://theconversation.com/ancient-sponges-or-just-algae-new-research-overturns-chemical-evidence-for-the-earliest-animals-150635
- Kozlov, M. (2021, July 28). Sponge-like fossil could be Earth’s earliest known animal. Nature. Retrieved September 3, 2021, from https://www.nature.com/articles/d41586-021-02066-9
- Wei-Haas, M. (2021, July 28). 890-million-year-old sponge fossil may be the earliest animal yet found. National Geographic. Retrieved September 3, 2021, from https://www.nationalgeographic.com/science/article/890-million-year-old-sponge-fossil-may-be-the-earliest-animal-known
- Weisberger, M. (2021, July 28). Oldest animal life on Earth possibly discovered. And it’s related to your bath sponge. LiveScience. Retrieved September 3, 2021, from https://www.livescience.com/oldest-animal-life-fossils-sponges.html
- Turner, E. C. (2021). Possible poriferan body fossils in early Neoproterozoic microbial reefs. Nature, 1–5. https://doi.org/10.1038/s41586-021-03773-z
