INVITED: THE IMPLICATIONS OF ARCHAEAN LIFE IN THE SEARCH FOR EXTRATERRESTRIAL LIFE

The data pertaining to life in the Archaean Epoch are few and disputed. Nevertheless, they present a consistent picture of the widespread existence of microorganisms identical to modern prokaryotes by 3.5 Ga. The micropalaeontological, biogeochemical and isotopic data show that chemolithotrophic microorganisms colonised the surfaces of volcanic sedimentary grains and the glassy surfaces of basalts and that consortia of chemolithrophs and anoxygenic photosynthesisers formed microbial biofilms and mats on the surfaces of sediments deposited in the photic zone. Microorganisms were distributed in a variety of environments, including shallow water basins, pore waters within sediments, in evaporitic, littoral environments, and around hydrothermal vents/springs. Pervasive, hydrothermal silicification of the Early-Mid Archaean microorganisms and rocks has preserved them well. These early signatures of life are not associated with large-scale carbonate deposits; indeed the latter did not exist as such on the early Earth. Despite the widespread distribution of the microorganisms and their already relatively evolved level of evolution, the traces of Early-Mid Archaean life are subtle and require sophisticated methods of detection. Although life must have appeared and evolved much earlier than 3.5 Ga, there is a lack of well-preserved rocks from the first billion years of Earth's history (the heavy metamorphism of the3.8 Ga rocks from Isua make interpretation of potential biogenic signatures contentious). The earliest fossil record is therefore missing on Earth but could potentially be found in Noachian sediments from the Southern Highlands of Mars. If life did arise on Mars, it would probably not have reached the level of oxygenic photosynthesis before the loss of its surface volatiles at ~3.5 Ga and hence, loss of its surface habitability. This is because, on Earth, the earliest putative evidence of oxygenic photosynthesis is 2.9 Ga. Given the subtlety of the early signatures of life on Earth, in situ detection on Mars might be difficult: sample return missions of previously well-characterised rocks is therefore recommended.