Microscopic fossils found in fossil river mud in today’s Canadian arctic have been dated to about a billion years ago and have been identified as nothing less than the earliest known fungus among us.
The microscopic specimens are more than half a billion years older than the previous candidate for “first fungus,” explains Prof. Corentin Loron of Belgium’s University of Liege, with an international team, in Nature.
But actually this primordial ’shroom, named Ourasphaira giraldae, definitely isn’t the first fungus to have evolved. It can’t have been: it’s too advanced.
“We found that it has some characteristics of higher fungi and some characteristics of lower fungi, but is definitely not a primitive fungus,” Loron tells Haaretz. “It’s already in the crown fungi line, so of course they had to have existed before.”
Science has yet to find fossilized specimens of the One True Ancestral Fungus, but the magnificent if microscopic giraldae is the oldest found so far — and by a long shot.
Until the discovery of abundant fungi in the billion-year-old Grassy Bay Formation mud shale, the earliest previously known fungus hailed from Wisconsin and dated to the mid-Palaeozoic era, a mere 450 million years ago. That clearly wasn’t the earliest, Loron explains.
To be fair, there had been candidate fossil fungi from Precambrian times, making them older than the Wisconsin find, but their fungoid identity was not categorical. Even some Ediacarans — a group of very weird and mostly sessile multicellular animals living about 600 million years ago — were thought to have lichen-like characteristics, but nothing jumped out at the researchers screaming “fungus.”
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“A lot of work was done, but it was all based on morphology,” such as the possession of filaments, Loron says. “That is very important, of course, but is not enough on its own to be unambiguously saying that this or that fossil is a fungus.”
Form in the world of microscopic life is very plastic. “Nothing looks more like a microorganism than another microorganism,” Loron points out. So in the world of microbiology, shape and size alone mean little, in isolation.
So to study the latest finds, the team employed multiple techniques. Morphology was just one, and this creature had the telltale branching filaments of true fungi.
They also used ultrastructure analysis and analyzed chemical composition of the microfossils, and using these multiple approaches they concluded that among the many and myriad creatures dwelling in the muddy bottom of the river in what is today the Canadian arctic, these creatures were, unambiguously, fungoid.
The origin of life remains one of those great mysteries. If we accept that what appear to be fossil bacterial mats found in rare outcrops of truly ancient rock in Canada and Australia are indeed fossil bacterial mats, then life dates back over 4 billion years, starting when the planet itself was still very young.
But even if those really are fossil bacterial mats, we don’t know “who” made them: bacteria or archaea, or whether these two primitive prokaryotes (lacking a cellular nucleus) arose from an even more primitive common ancestor.
Maybe one didn’t predate the other, since we cannot state that all life began with one cell that sprang to life at one spot. Maybe organic chemicals and serendipity came together in multiple spots, forming different forms of life in the primordial warm seas.
And we don’t know when fungi, which are eukaryotes with defined nuclei, arose.
What seems quite sure is that fungi were a lot older than the 450-million-year-old specimens called Tortotubus found in Wisconsin, let alone the 400-million-year-old fungus found in Rhynie, Scotland (which is also home to fossil lichen from the same time).
In fact, Loron and others first reported on the multicellular microfossils in Canada in January and suspected they’d found a fungus, so looked deeper, he explains.
Their discovery is consistent with molecular clock estimates putting the emergence of the fungus at about a billion years ago, the mid-Proterozoic era, says Loron.
It is also consistent with the blossoming of the eukaryotic domain during the Proterozoic era, which was a time of climatic flux and change on the planet, characterized by volcanism and sundry upheaval. So in a sense, finding a billion-year-old fungus that seems to have lived in nutrient-rich river sludge is not a surprise, per se; it’s just really lucky and difficult.
Finding the ancestor of this billion-year-old fungus will be even harder. In any case, the discovery bolsters the hypothesis that fungi were key to life leaving the seas — for instance, by symbiotically aiding the colonization of land by plants. The Tortotubus species found in Wisconsin lived on land and was even said to be the earliest form of terrestrial life ever discovered. Again, that probably just means we haven’t found the earlier ones yet.