When was the first multicellular complex life born? Most people, being a bit egocentric, were referring to Ediacaran and Cambrian when the first animal life appeared and then diversified. Yet, DNA studies suggest that fungi might have appeared well before animals.
With regard to the fossil record, things are rather rare. No unambiguous evidence of a fungus appears in fossils until after the end of the Cambrian. Some things earlier may have looked like mushrooms, but the evidence was limited to their appearance. The fungi may have been branched at the time suggested by DNA but evolved later in complex multicellular structures. Alternatively, the fossils could be accurate, and the DNA data has something weird about it. Or, finally, we may not have found fossils that are old enough.
A new paper in today Nature strongly advocates for the last option. In this paper, a small team of researchers describes fossils of what appear to be mushrooms that could have up to a billion years. And researchers are backing up the appearance with chemical analysis.
Really, really old
The fossils come from an area of the north coast of the Canadian Arctic. The fossils themselves were discovered by dissolving the minerals that contained them with acid. The disappeared rocks, a vast collection of small fossils floated freely.
Visually, these microfossils resemble a partially deflated balloon with a stem at the base. These rods are connected to a long tube that can connect several structures in the form of a balloon. This is very similar to some modern mushrooms, where the balloon-like structure is a source of spores, while the tubes allow the body to grow and spread within a body. area. An essential feature shared with mushrooms is the fact that the stem that attaches the sphere to the rest of the body branches out at right angles. The structure provided the name of the new genre, Ourasphaira, for tail and sphere; the full name of the species is Ourasphaira giraldae.
That, in itself, is not shocking. What is amazing is the age of these fossils. Although the layer in which they reside has not been dated, the neighboring layers have been. A layer underneath has been identified as having a billion years. On the other side of the stratiographic sandwich, there is an old layer of about 900 million years old. These offer a window of 100 million years during which fossils could have been deposited. But even among the youngest, these fossils are about half a billion years older than the oldest mushroom we've found before.
It is therefore essential to support the claim that it is actually mushrooms. To do this, the researchers looked more closely at the cell wall with the help of an electron microscope and showed that, in the manner of a fungus, the cell wall is bilayered. And chemically, the wall seemed to contain derivatives of chitin, a complex polymer of sugars that forms the cell wall of modern mushrooms. While other organisms produce chitin, an essential component of the insect exoskeleton, for example, many of these organisms have clearly evolved afterwards, and none of them resemble Ourasphaira giraldae.
The presence of chitin is consistent with the resistance of samples to the acid treatment that dissolved the surrounding rock. And an examination of the rock layer revealed that it probably had not been heated to a temperature above 200 ° C since its formation, which is low enough that chitin likely survived intact.
The researchers argue that all this is a compelling argument: Ourasphaira giraldae is really a mushroom, despite its apparent age. They argue that this is part of a group of precursors that ultimately gave rise to dikarya, a subdomain that includes fungi.
What is he doing there?
Maybe the sentence above should read "consistent with apparent age" rather than "despite". As mentioned earlier, studies on the gene content and sequence of fungi have indicated that they originated well before animals, and some work has even suggested an age of around 900 million. ; years. Thus, the data suggest that fossils date from the same period as mushrooms. As such, the results help to anchor molecular data over time, which should improve our estimates of when plants and animals have separated from the world of unicellular eukaryotes.
A key question, though, is what mushrooms were doing there. At present, most mushroom species are of terrestrial origin, but the rocks have deposited in an estuarine environment. It is not clear whether the fossils originate from there or have simply been deposited in the sediments that have formed.
Fungi currently play a key role in contributing to the decomposition of complex tissues of other multicellular organisms, making nutrients and other organic materials available to the ecosystem. It is possible that they have played a similar role in helping to break down the complex structures formed by communities of bacteria before the emergence of animal and plant life. As one half of the symbiotic team that forms lichens, fungi also help to colonize what is bare rock; in this role, they may have played a crucial role in preparing land for invasion by other complex life forms.
To better understand the way of life of the first mushrooms, it will probably be necessary to wait for a way to identify them without dissolving the whole remains of the ecosystem in which they lived.
While the linked document below advances the argument that Ourasphaira giraldae is a mushroom, a previous article describes his discovery, as well as a set of other species of microfossils.
Nature, 2019. DOI: 10.1038 / s41586-019-1217-0 (About DOIs).