The limits of palaeontological knowledge: finding the gold among the dross

Palaeontological interpretation rests on two interwoven sets of comparisons with the modern world. Palaeobiological interpretation relies on the placement of fossils within a phylogenetic and functional framework based primarily on the comparative biology of living organisms. Analogy to currently observable chemical, physical and taphonomic processes enables palaeoenvironmental inferences to be drawn from geological data. In older rocks, comparisons with the modern Earth can become tenuous, limiting palaeontological interpretation. The problem reaches its apogee in Archaean successions, yet pursuit of multiple lines of evidence establishes that complex microbial communities, fuelled by autotrophy and, likely, photoautotrophy, existed 3500 million years ago. Although Archaean palaeontology has to date focused on silicified coastal sediments, improved understanding of Earth's earliest biosphere may depend on the development of alternative environmental and taphonomic analogies. Spring precipitates and hydrothermal metal deposits are promising candidates. Terrestrial organisms may be of limited value in interpreting such fossils as may be found on Mars, although some points of comparison could prove general. Given limited opportunities for exploration, proper choice of environmental analogy is critical. Spring precipitates constitute excellent deposits for addressing questions of biology on another planet.