Van Kranendonk, Martin J.1
1Australian Centre for Astrobiology, University of New South Wales, Kensington, NSW, Australia
Ancient life thrived on an early Earth that was a very different planet to the one we inhabit today, with green seas filled with dissolved iron, an orange sky rich in CO2 and other greenhouse gasses, and small black volcanic protocontinents. Yet by 3.5 billion years ago – “only” 500 million years after the end of the heavy meteorite bombardment and in the oldest rocks that preserve widespread primary features at low metamorphic grade – life had diversified into a variety of niches and employed a variety of metabolisms.
This evidence is preserved in the 3.5 billion-year-old (Ga), shallow water North Pole Chert Member of the Dresser Formation (Pilbara Craton, Western Australia), where putative biosignatures in the form of macroscopic fossil stromatolites, fractionated stable isotopes, and organic matter occurrences are widespread. These have been described from environments that include the shoreline of a shallow water caldera lake, subterranean hydrothermal veins, evaporative barite crystals, and terrestrial hot spring sinter deposits. The discovery of trapped organic matter remnants in columnar Dresser stromatolites that have a similar appearance to extracellular polymeric substances (EPS) of microbial biofilms provide compelling evidence of life. The Dresser stromatolites are unusual in being dominantly composed of nanoporous pyrite, with subordinate sphalerite and dolomite. This assemblage most likely formed via anoxygenic photosynthesis and sulfate reduction, and perhaps also microbes that cycled elemental sulfur and/or sulfide. Putative biosignatures in hot spring deposits on land at this time indicate a microbial community that may have utilised a range of different metabolisms.
The discovery of life on land in the Pilbara 3.5 Ga ago, and in the Barberton Greenstone Belt by 3.2 Ga, changes the way we think about the evolution of life over the course of Earth history and supports recent studies that suggest life may have originated in hot springs on land. Indeed, the Dresser Formation provides a deep-time analogue for better understanding an origin of life on land model.
Additionally, the Dresser Formation provides an important guide in the search for ancient life on Mars. Specifically, the discovery of ancient life signatures preserved in siliceous hot spring deposits from the Dresser Formation, combined with the increasing evidence that life may have originated in hot springs, suggests that these deposits may represent the best chance for success in the search for life on Mars, not only because of the likelihood that hot springs would have been inhabited if life ever got started on Mars, but also because of the proven excellent preservation potential of these rocks over billions of years. Stratiform deposits of nodular opaline silica with digitate protrusions that were observed by the Spirit Rover in the Columbia Hills of Gusev Crater, Mars, are of probable hot spring origin and represent a tantalising astrobiological target. A sample return mission to collect these, and other nearby materials, is being developed.
Professor Van Kranendonk is the Director of the NASA-affiliated Australian Centre for Astrobiology at UNSW. His team investigates the formation mechanisms of ancient crust and the earliest signs of life on Earth, and uses this to guide the search for life on Mars, and to understand the origin of life.