Fossils of what may be the earliest fungi have been discovered in rocks dating back 2.4 billion years old. The finding of the fossilized filaments, if it can be confirmed, would push back the earliest known date of eukaryotic cells – the group that contains plants, fungi, and crucially animals – by 500 million years. The study describing the fossils has been published in Nature Ecology & Evolution.
The age of the fossils, discovered by accident in South Africa, is what shocked the researchers when they reanalyzed the rocks in which they were found. Initially, they thought that they were around 2.2 billion years old, which in itself would have made the fossil fungi – if they can be verified as such – the oldest eukaryotic cells discovered to date, but this was later pushed back by a further 200 million years.
This is significant because it crosses a point in the Earth’s geological history known as the Great Oxidation Event. Thought to have occurred roughly 2.3 billion years ago, the Great Oxidation Event marks the point in which the levels of oxygen in the Earth’s atmosphere rapidly increased, due to the activity of the first photosynthetic microbes called cyanobacteria.
If the dating of these latest fossils is accurate then, it means that not only did this fungus evolve in the dark depths of the ocean sediment, but they did so in the absence of oxygen, which for most eukaryotic cells is a critical component. The researchers, therefore, suggest that these early fungi may have been living in a symbiotic relationship with some form of microbes, using the chemically stored energy that they produced.
What is also surprising about these potential fossil fungi is where they have been found. It has long been assumed that the organisms evolved on land, as this is where the majority of fungi species are found today. But the rocks in which these latest fossils were discovered formed deep beneath the waves, under an ancient ocean seabed.
This is a region known as the “deep biosphere”, and despite hosting a significant amount of the Earth’s biomass, is poorly understood. “The deep biosphere (where the fossils were found) represents a significant portion of the Earth, but we know very little about its biology and even less about its evolutionary history,” explained Professor Bengtson, who led the research, to BBC News.
Up to now, the first fossil traces of eukaryotes dates to around 1.9 billion years old, and the oldest fungi to 1.2 billion years. This would make these latest fossils a significant discovery in the history of life on Earth, challenging our understanding of where and when eukaryotes first evolved.