Mushrooms are not known for their suitability for fossilizing, so we have little evidence about their early evolution. However, an extraordinary series of events turned one humble 113- to 120-million-year-old fungus to stone, pushing the record for mycelium back 16 million years.
Back when dinosaurs roamed the Earth, and South America and Africa were still one continent, a mushroom fell into a river in what is now north-eastern Brazil. It floated to a lagoon too salty for the microbes that would normally degrade such a tasty morsel, sank to the bottom, and became covered in sediments. There it mineralized with iron pyrites, commonly known as fool's gold, replacing its tissues. Over time the ex-mushroom turned to goethite, a sort of mineral, which became embedded in a sandstone Lagerstätte, the name given to deposits that preserve fossils of soft tissues that usually decay without being preserved.
"Most mushrooms grow and are gone within a few days," said finder Dr Sam Heads of the University of Illinois. "The fact that this mushroom was preserved at all is just astonishing. When you think about it, the chances of this thing being here – the hurdles it had to overcome to get from where it was growing into the lagoon, be mineralized and preserved for 115 million years – have to be minuscule.”
The previous record holder for oldest mushroom was not something left at the bottom of a sharehouse fridge, but one trapped in Burmese amber. The same method preserved the other nine known fossil mushrooms. Although amber has been an outstanding preservative of many ancient lifeforms, it usually only captures small objects. Heads' discovery, which he named Gondwanagaricites magnificus, was 5 centimeters (2 inches) high, with a 1-centimeter-wide (0.4-inch-wide) cap.
The preservation is sufficient to reveal spore-releasing gills under its cap, a feature shared with some, but far from all, modern mushrooms. Unfortunately, no actual spores can be seen, preventing its placement within one of the major mushroom families.
Besides being frequently delicious, fungi were essential to the development of life on land, forming symbiotic relations that allowed plants to move onto the land. More recently, mushrooms formed the basis of the diet of Neanderthals, and quite likely our own ancestors, in Spain. They could be important to our future as well, whether easing depression or being turned into longer-lasting anodes for lithium-ion batteries, opening the path to cheap storage of renewable energy.
Gondwanagaricites magnificus, which Heads announced in PLOS One, is a long way from the original ancestral mushroom, from which all these descended. That lived at least 500 million years go. This discovery, however, may be the closest we get for quite a long time.