Oceans are the cradle of life on Earth. Giving life a birth date, though, is complicated as we’re still discovering traces of bacteria older than all those we found so far. For now, we think that the oldest one lived at least 3.5 billion years ago, so around one billion years after Earth’s formation. This theory is based on prints that bacteria left on soft mud that turned into rock with time. It is nearly impossible to preserve a bacterium as a “fossil” since they are extremely fragile.

Image: Anomalocaris painting. Reconstruction of what this early predator must have looked like. Image by UNE photos (Creative Commons).

Bacteria are unicellular organisms (their bodies are constituted of only one cell, while the human body is formed by thousands of billions of cells…) and for long, we didn’t find anything between the old bacteria prints and the oldest fossils with shells and skeletons that lived 540 million years ago.

A fossil is the remains of a long dead organism that was buried in mud which transformed into rock (or sediment) over time. Soon, by physical destruction by the mud and the water flow or by decomposition by bacteria, only the skeleton or the shell of the original animal remains. That’s what you can see at the museum, once they’ve been discovered and extracted from the rock.

We know from these fossils that a large amount of species have lived on Earth in the last 600 million years, in the ocean and later (since around 400 million years ago) on the continents, and that their sizes and shapes were very different to one another. In the last few decades, we even discovered a new type of fossils…

In some rare environments, the mud that buries dead organisms can be so fine and the water flow so weak that they don’t harm the remains, and the shape of the skin of the organism can print on the mud and be preserved that way. This is how we could find the Ediacaran fauna in Australia.

In this sort of environment we made an extraordinary discovery: fossils of animals (not bacteria) so old (around 560 million years ago) that they didn’t have any hard parts in their bodies. No shells, no teeth, not a single bone. But these were not the most disturbing characteristic of their anatomy. Their perfect state of preservation allowed palaeontologists to notice another component of modern animals missing: the mouth. Not a single specimen of this strange zoo had any orifice, and we suppose they were able to absorb nutrients from the seawater directly through their skin.

Palaeontologists discovered organisms with the strangest shapes, frozen in time and preserved in this extremely smooth mud turned into rock. Dozens of new fossil species were described and a new extreme limit of the appearance of multicellular organisms (in opposition to unicellular organisms like bacteria) was defined.

But it was just the start of a new series of astonishing discoveries. At Burgess (Canada), new and slightly younger species were found that lived around 540 million years ago, during the Cambrian era. Unlike Ediacara, all species from this fauna had shells (or “exoskeletons”; skeletons outside the organism, unlike humans or other vertebrates but just like molluscs and arthropods). Amongst them, an impressive specimen of the species Anomalocaris presented another major difference with older organisms: it had teeth, and those of a predator. Overall, the Burgess fauna presented a huge amount of different species compared to Ediacara, and we call this rapid increase in the biodiversity the “Cambrian Explosion“. 20 or 30 million years may seem a lot of time, but when dealing with evolution, it’s really not a lot. But for some reason, at this time, life on Earth had rapidly evolved.

Ediacara was important because it was the first time we found multicellular organisms ageing before skeletogenesis (creation of skeleton and other hard parts). Burgess was also important because it was the oldest fauna we found with hard parts. But these two faunas combined made another major discovery: the start of predation. During the Ediacaran era, no organisms had teeth or even mouth, so they were clearly not carnivorous. Anomalocaris was an obvious proof of predation during the time the Burgess rocks were deposited. Because of this, we talk about the “Ediacaran paradise“, when Nature was at peace, without fight for survival.

When we look at other species of Burgess fauna, several produced spines on their back, probably as a defence mechanism against predators like Anomalocaris. This led to the conclusion that hard parts were initially synthesized in order to protect the organism against aggressive species.

Another possible explanation of skeletogenesis is also proposed and focuses on chemistry mechanisms. As the seawater chemistry changed, maintaining an appropriate level of acidity in the organisms became more problematic. Synthesizing shells would have allowed the organism to store acidic molecules from their soft tissues to reach the proper equilibrium. But this doesn’t explain by itself the presence of spines and teeth…

The truth is probably somewhere between the two theories. But anyway, something major happened to living species between 560 and 540 million years ago. Something that changed the shape and the behaviour of all life on Earth, forever.

Vincent Mouchi

Ph.D. student, Department of Geology, Trinity College Dublin

If you want to know more about this topic and have more details on these extraordinary organisms, I would suggest you to read the book “Wonderful Life: The Burgess Shale and the Nature of History” by the palaeontologist Stephen Jay Gould.

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