1. Darwin, C. R. The Origin of Species (Murray, London, 1859).
2. Schopf, J. W. (ed.) Earth's Earliest Biosphere: its Origin and Evolution (Princeton Univ. Press, Princeton, 1983).
3. Knoll, A. H. Scient Am. 265, 65–73 (1991).
4. Harland, W. B. et al. A Geological Time Scale 1989 (Cambridge Univ. Press, Cambridge, 1990).
5. Krogh, T. E., Strong, D. F., O'Brien, S. J. & Papezik, V. S. Can. J. Earth Sci. 25, 442–453 (1988).