Ammonia, which is found in urine, is also a cleaning agent that can kill bacteria. The ancient Egyptians would whiten their teeth with a mixture of pulverized pumice and wine vinegar.
Strangely enough, centuries later, teeth whitening methods got worse! In the 1800s they used to whiten teeth by filing them down and then washing them in nitric acid. This also destroyed the protective enamel of the teeth (not a good idea!).
A history of teeth whitening on eHow.com. http://www.ehow.com/about_4687154_history-teeth-whitening.html