Critical testing of Earth's oldest putative fossil assemblage from the ∼3.5 Ga Apex chert, Chinaman Creek, Western Australia

Abstract

Structures resembling cyanobacterial microfossils from the ca. 3465 Ma old Apex chert of the Warrawoona Group in Western Australia have until recently been accepted as providing the oldest morphological evidence for life on Earth, and have been taken to support an early beginning for oxygen-releasing photosynthesis. Eleven species of filamentous prokaryote, principally distinguished by shape and geometry, have been put forward as meeting the criteria required of authentic Archaean microfossils. They were contrasted with other microfossils that were dismissed as either unreliable or irreproducible. The aim of this paper is to provide a detailed account of research recently reported by us on the type and recollected material, involving optical and electron microscopy, digital image analysis and other techniques. All previously figured holotype materials are illustrated here, and the context for all the published materials is re-evaluated. The Apex chert ‘microfossils’ occur near the top of a 1.5-km long chert dyke complex associated with major synsedimentary growth faults. Highly localised, glassy felsic tuffs erupted explosively from this and other fissures during the early stages of volcanism, and were followed by the deposition of essentially hydrothermal black and white BaSO 4 rich cherts that infiltrated the feeder dykes, underplating and dilating adjacent stratiform cherts before the start of the next volcanic cycle. The Apex chert ‘microfossils’ occur within multiple generations of these metalliferous hydrothermal vein cherts some 100 m down the dyke system. Comparable structures occur in associated volcanic vent glass and in hydrothermal cherts at least 1 km deep. We find no supporting evidence for a primary biological origin. We reinterpret the purported microfossil-like structures as pseudofossils that formed from the reorganization of carbonaceous matter, mainly during recrystallization from amorphous to spherulitic silica.