2.4-billion-year-old fossils that exhibit similar structural characteristics to some living fungi are reported in a paper published online this week in Nature Ecology & Evolution. The filamentous shape of the fossils strongly suggests they are either extremely early fungi, or a previously unknown filamentous organism also from the eukaryotic branch of life (which includes all animals, plants and fungi, but not bacteria or archaea). The discovery is important for understanding the development of early life as the fossils are either the earliest fungi by one to two billion years, or the earliest eukaryote fossils by five hundred million years.
Conservative estimates suggest fungi originated about 400 million years ago, although some recent studies have reported possible fossil evidence for fungi that dates from 1.4 billion years ago. It has been suggested that the eukaryotic branch of life evolved about 2.7 billion years ago, but the earliest currently known eukaryotic fossil (Grypania spiralis) is only 1.9 billion years old.
In this study, Stefan Bengtson and colleagues describe fossils from a core drilled deep into the Ongeluk Formation in South Africa, and use micro- and spectroscopic techniques to confirm the biological origin of the filaments. They find that the fossils are the remains of tiny organisms (0.002-0.012 millimetres in diameter) that were thinner than a human hair and lived in cavities within the volcanic rock. As fungi are thought to have originated on land, the authors note it is significant that the Ongeluk Formation was below the sea at the time the organisms were alive. Fossils indicative of such early fungal origins also raise the question of whether other major eukaryote branches existed at this time.