Recently discovered fossils belonging to ancient sponges might be the earliest known remnants of an animal body and pre-date other sponge fossils by 350 million years.
Elizabeth Turner, a professor of paleontology and sedimentary geology at Laurentian University in Ontario, discovered what she believes are possibly the fossilized structures of sponges that once existed in reefs millions of years ago. They were found in rock samples in northwestern Canada.
Little is known about the earliest days of animal life’s emergence on Earth because the fossil record is sparse. While scientists have used genetic evidence to suggest that sponges first appeared between 541 million and 1,000 million years ago during the early Neoproterozoic era, the lack of fossilized sponges has created a knowledge gap. Turner’s discovery may help fill that gap and provide a glimpse into the earliest marine animal life on Earth.
“I serendipitously came across a few very rare examples of the material during my unrelated PhD research, long ago, on fossil microbial reefs,” Turner said. “When I became a professor and had my own grants, I was able to return to the field sites and collect more material so that I had a more robust collection to work from.”
What she found in the ancient rock samples were fossilized structures that resembled the skeletons like those that exist within horny sponges — the kind you use for a bath sponge. Horny sponges, also called modern keratose demosponges, have a skeleton with three dimensional branching made of a tough organic substance called spongin.
The branched networks of tube-shaped structures were covered by mineral calcite crystal. These also appeared similar to structures found in calcium carbonate rocks that were likely created when horny sponge bodies decayed.
“This organic skeleton is very characteristic and there are not known comparable structures,” said Joachim Reitner, a professor in the University of Goettingen’s department of geobiology who reviewed Turner’s study ahead of publication.
Life as an ancient sponge
The ancient sponges lived in “shadowy nooks and crannies” on and below large reefs made from water-dwelling bacteria that were photosynthesising, or converting light energy into chemical energy.
“They may well have hunkered down and lived a sweet life without having to evolve much for a few hundred million years,” Turner said.
The “oxygen oasis” and potential food sources produced by the bacteria would have been a Goldilocks spot for the sponges.
These sponges appeared 90 million years before events thought necessary to support the appearance and diversification of animal life.
About 800 million years ago, Earth’s oxygen levels increased during what scientists refer to as the Neoproterozoic oxygenation event, when there was a substantial boost in the amount of oxygen in the oceans and atmosphere.
The sponges may have been tolerant of low oxygen levels, so what was provided by the bacteria could have been enough.
Then, there were also the Cryogenian glacial episodes, during which much of Earth likely experienced severe ice ages between 635 million and 720 million years ago. The sponges weren’t likely affected by this either.
The finding in Turner’s paper “is a milestone in the understanding” of the animal family tree and reveals that the origin of it was much earlier than previously thought, Reitner said.
The animal life timeline
Many fossils can be dated back to the Cambrian explosion, a period 540 million years ago when diverse animal life flourished on the planet, according to David Bottjer, professor of Earth sciences, biological sciences and environmental studies at the University of Southern California. Bottjer was not involved in this study.
“The fossil record of animal life is something that many people are interested in and in particular, when animals first evolved on Earth,” he said. “The big discussion has been, was there animal life in a significant way in the pre-Cambrian before the Cambrian explosion or not.”
Researchers use two data sources to address these, he said. One is the fossil record. Then, there are molecular clocks, which are used to help scientists estimate how evolution has occurred in different organisms over time. The idea is that DNA evolves at a constant rate within organisms and genetic evidence can fill the gaps where the fossil record is sparse or entirely bare.
“This molecular clock approach says that animals were on Earth a good deal earlier than the Cambrian,” Bottjer said.
Previously, many scientists believed spicules — the mineralized skeletal structure of sponges — were needed to show the existence of sponges. But some sponges don’t have mineralized skeletons.
If complexly structured and diverse fossils are appearing in the rock record during and before the Cambrian period, it suggests that animals had to evolve ahead of that, Turner said.
This also suggests a time when animals existed, but were not well preserved or preserved at all — prior to skeletons, shells and exoskeletons, Turner said. She understands that the earlier timeline suggested by her research “may be a little alarming for some researchers.”
Bottjer thinks Turner has authored “a very well-founded paper.” He also acknowledges that her findings will certainly ignite discussion among researchers.
“I tell my students that all pre-Cambrian fossils are disputed by somebody because it’s very early in evolution,” Bottjer said. “These animals commonly don’t have all the features that they evolve later on so it can be a little tricky. I look forward to seeing what the response is, but it’s I think it’s a very strong case.”
What isn’t surprising is that the earliest evolutionary animals were probably sponge-like, Turner said. Sponges are simple, basic animals with a long history in the fossil record, and prior research suggests they likely appeared around this time.
Investigating the distant past
Now, Turner wants to investigate when sponges actually emerged if they were already present 890 million years ago.
“The early evolution of animals remains murky, regardless of what I have to offer in this paper,” she said. “What is needed is a lot of really focused work on very thoughtfully chosen rocks of the same age, as well as younger and older rocks, to look for other possible physical evidence of early animals — sponge-like and possibly other more complex types of animals.”
She also suggests keeping an open mind during the search. When it comes to early animal life, researchers really have no idea what they will find.
Turner also said the current academic standard and research grant system is less supportive of the type of time-consuming work, often resulting in unpublishable findings, that will be required to thoroughly understand and investigate the rock record of early animal evolution.
“If we find ourselves looking for something that looks familiarly animal-ish, we will fail, because the familiar is too complex for the earliest animals,” she said. “We need to do some real soul-searching about what early animal preservation might have been like.”
“If we’re going to find things on other planets, it’s probably going to be more like the pre-Cambrian stuff than something that is a million years old and has mammoth bones in it,” Bottjer said.