Fats on the skin break down to form volatile, strong-smelling substances called ketones and aldehydes when they come into contact with iron - whether it comes from the environment or from haemoglobin in blood - says Dietmar Glindemann, a chemist at Virginia Polytechnic Institute and State University in Blacksburg.
Glindemann and his team identified the chemicals after analysing vapours produced when seven volunteers rubbed metal objects on their skin. The strongest-smelling is 1-octen-3-one, the researchers report in Angewandte Chemie International Edition (vol 45, p 7006).
Glindemann then established that the same chemicals are produced by reactions between iron in blood and chemicals in the skin by rubbing his own blood on his skin and analysing the resultant vapour. He suggests that the ability to detect traces of the smelly chemicals allowed our ancestors to sniff out freshly wounded animals.
"Take this tip from nature: The forest would be a very silent place if no birds sang except those who sang best."