Figure 1. Examination of the carcinogenicity of compounds with the Ames test. Slight growth of the his-minus mutant on the Petri dishes can be seen (background growth). Three revertants grew on the control dish, but many more grew when the tested compound was present, indicating a positive test. (Diagram: Anne Kemmling, Goettingen, Germany.)

Figure 2. Early proterozoic stromatolites formed 2.2 billion years ago in the eastern Andes south of Cochabamba (Bolivia). The dark bands consist of cyanobacteria in carbonate-containing sediment layers. (Photograph: Joachim Reitner, Goettingen, Germany.)