Alfred Kröner is Professor emeritus at Mainz University, Germany, and visiting Professor at the Beijing SHRIMP Centre, Institute of Geology, Chinese Academy of Geological Sciences. He is a field geologist with interest in tectonics and geochronology and has mainly worked on aspects of crustal evolution in Africa, specifically the Kaapvaal craton of southern Africa. He has published more than 400 scientific papers and was editor or was on the editorial board of several international earth science journals.  Axel Hofmann is Associate Professor at the University of Johannesburg. He combines field-based studies of volcano-sedimentary successions with trace element and stable isotope geochemistry for the understanding of Archaean surface processes, early life and sediment-hosted mineral deposits. The Kaapvaal craton with its unique record of more than 3-billion-years old strata of low metamorphic grade is the focus of his research.