Hydrothermal alteration and microfossil artefacts of the 3,465-million-year-old Apex chert

The 3,465-million year-old Apex chert of the Pilbara Craton in Western Australia has been controversially reported to contain some of the oldest fossil evidence for life: tiny carbonaceous filaments interpreted as cyanobacteria(1). Yet, on the basis of the presence of barite and native metals, it has been suggested that the chert formed from hydrothermal fluids at temperatures greater than 250 degrees C (refs 2, 3), an unlikely environment for cyanobacteria. Here we use scanning electron microscopy to assess the mineralogy and depositional setting of the chert veins surrounding the nodes that contain the microfossils. In addition to rare native metals, we find an assemblage of iron oxides and clay minerals, which we interpret to have formed during repeated pulses of hydrothermal alteration at low to medium temperatures. We also find micrometre-sized silica structures that resemble microbial exopolymers(4), and textures formed by the partial dissolution of tubular minerals that look similar to fossilized microbial mats(5). Other branched microstructures suggest post-depositional colonization of microcracks and fissures by microbes(5). These observations are not directly applicable to the carbonaceous filaments purported to be the earliest fossil evidence of life, as we did not recover any in our sample. However, because of the observed hydrothermal and groundwater alteration, we conclude that the Apex chert is unlikely to have preserved any early forms of life.