Being a scientist is much like being a lawyer. You painstakingly gather evidence to support an argument — though one rooted in the scientific method, not courtroom conjecture.
According to new research published Wednesday in the journal Nature, one scientist may have gathered enough evidence to successfully make her case: The earliest animals on Earth might be even older than researchers previously realized.
To get to this conclusion, Elizabeth Turner, a professor at Laurentian University’s Harquail School of Earth Sciences, had to travel to a remote area of northwest Canada accessible only by helicopter. It was there that she accessed 890-million-year-old rock samples taken from the mountains. Long ago, these mountains were ancient reefs.
Based on her analysis, Turner suggests the samples are actually the fossils of ancient sponge bodies. If she’s is correct, these otherwise ordinary-seeming Canadian rocks would represent the oldest body-fossil evidence of animals known so far.
How they made the discovery — Turner collected the samples between 1992 and 2018, examining the rock specimens under a specialized microscope.
Turner’s findings are compatible with molecular research that “suggests sponges emerged around this time.”
“The earliest animals to emerge evolutionarily were probably sponge-like,” Turner, the only author of the study, tells Inverse. “This is not surprising, given that sponges are the most basic type of animal both today and in the fossil record.”
How can you tell the difference between a rock and an ancient sponge? The presence of certain structures in the rocks, known as vermiform microstructures, clued her into the possibility that she was looking at one of the earliest animals on Earth.
These microscopic features were previously associated with algae, but recent research suggests these fossilized structures actually represent certain types of horny keratose sponges, which are used today to make bath sponges.
When the sponge bodies decay after death, they form “sponge mummies” comprised of cement-filled microtubules. This strange mummification process leads to the formation of vermiform microstructures — evidence of ancient animal life.
What they found — Turner has four key conclusions based on her work:
- Animals emerged “evolutionarily long before the appearance of traditional animal fossils around 540 million years ago,” she says. If the fossils Turner found are indeed 890-million-year-old sponges, then they are 350 million years older than the next-oldest sponge fossils.
- Sponges are likely among the earliest animals to emerge on Earth. This was known; this study supports it.
- These early sponges lived in environments similar to those that modern sponges inhabit, suggesting ancient sponges found suitable homes that would support their kind for hundreds of millions of years.
- The evolution of sponges occurred before two major Earth events that were previously thought necessary for the formation of diverse animal life. The first was the Neoproterozoic or the “great” oxygenation event which brought oxygen to Earth’s atmosphere and deep oceans sometime between 540-800 million years ago. The second was a turbulent Ice Age period known as the Cryogenian glaciations (635-720 million years ago), when much of Earth was covered in glaciers.
Why it matters – Hidden within these 890-million-year-old rock samples are possible evidence of the earliest multicellular animal life on Earth. The finding could change how scientists perceive the evolution of life.
We typically think of the Phanerozoic era — the current geologic era in which complex plant and animal life emerged on Earth — as beginning around 540 million years ago. This finding potentially pushes that timeline back by roughly 300 million years.
Uncovering these ancient potential sponge samples could help us better understand the kind of environment into which the earliest animals on Earth emerged. For example, the reefs on which these sponges were found would typically contain oxygen-producing cyanobacteria, which could support complex animal life.
However, these sponges emerged millions of years before enough oxygen emerged to support complex animal life. Therefore, the study implies that these earliest animals on Earth were able to thrive in low-oxygen environments — unlike most modern animals.
What’s next — As exciting as these findings are, they’re still preliminary, Turner says.
Further research will be necessary to build her case that these ancient sponges were really the earliest forms of fossilized animal life on Earth, while also connecting their evolution to modern sponges.
Along with finding more physical evidence of these sponges, Turner aims to use certain biomarkers, or “fossil lipids that are preserved in rocks and can be distinctive to particular organism types,” to gather more evidence.
Could animal life on Earth go back even further — perhaps a billion years or more?
“If sponge-like organisms were present then and did not require the Neoproterozoic oxygenation event to emerge evolutionarily, how much farther back might they have been present?” Turner asks.
Abstract: Molecular phylogeny indicates that metazoans (animals) emerged early in the Neoproterozoic era, but physical evidence is lacking. The search for animal fossils from the Proterozoic eon is hampered by uncertainty about what physical characteristics to expect. Sponges are the most basic known animal type; it is possible that body fossils of hitherto-undiscovered Proterozoic metazoans might resemble aspect(s) of Phanerozoic fossil sponges. Vermiform microstructure, a complex petrographic feature in Phanerozoic reefal and microbial carbonates, is now known to be the body fossil of nonspicular keratosan demosponges. This Article presents petrographically identical vermiform microstructure from approximately 890-million-year-old reefs. The millimetric-to-centimetric vermiform-microstructured organism lived only on, in and immediately beside reefs built by calcifying cyanobacteria (photosynthesizers), and occupied microniches in which these calcimicrobes could not live. If vermiform microstructure is in fact the fossilized tissue of keratose sponges, the material described here would represent the oldest body-fossil evidence of animals known to date, and would provide the first physical evidence that animals emerged before the Neoproterozoic oxygenation event and survived through the glacial episodes of the Cryogenian period.