Earwax contains a complex community of microorganisms that is individual to the ear that produced it. This can be used to identify an individual from a trace sample of their earwax, much in the same way as a fingerprint.
Introducing the earwax of another individual to your own ears creates a completely new bacterial signature, so criminals will routinely share cotton wool buds to prevent their earwax from being used to identify them. It is for this reason that earwax profiling has never been used as extensively as fingerprinting by the police service.