Found naturally in the stones of cherries, plums and peaches, the cores of apples and the leaves of the laurel plant, cyanide evolved as a plant protection mechanism of grazing animals (interestingly, a number of bacteria, fungi and algae are also found to produce the chemical). Ingestion of moderate amounts of these natural substances cause headaches accompanied by mild heart palpitations, more than enough to steer animals two-legged or four clear. However, the Middle Eastern people of ancient times made the discovery that the distillation by evaporation of laurel leaves produced lethal concentrations of this innocent plant product. from
http://www.bbc.co.uk/dna/h2g2/A4113983