The search for extraterrestrial life and habitable planets is at the heart of many space missions. A vast range of biosignatures have been proposed for this purpose, but such signatures are predicated on a concatenation of circumstances related to the geological evolution of the Earth and the co-evolution of life. However, an extraterrestrial observer could probably have detected any biosignature for Earth only in the last 800 Myr, even though Earth has probably been inhabited since the Hadean (4.56-4.0 Ga). The nature of life for the greater part of a planet's history would thus be undetermined. Here we analyse 3.45-Gyr-old volcanic sediments from the Kitty's Gap Chert (Pilbara, Australia), a Mars-analogue location, to characterize the nature of the microfossils contained therein. We conclude that the most common forms of extraterrestrial life on a rocky planet will be similar to terrestrial chemotrophic organisms, specifically chemolithotrophs, which are notoriously difficult to detect and identify. This study highlights the difficulties in determining the biogenicity and syngenicity of signatures in a sample and shows the importance of terrestrial analogue examples for demonstrating the methodological challenges.