Interpreting is ancient. Maybe as ancient as languages or mankind. Interestingly enough there are references to interpreters in many different historical sources. Like the representation of an interpreter in General Horemhebs grave, Unprofessional translation has an interesting post on ancient Egypt and interpreters there.
Cicero in ancient Rome spoke highly of his interpreter and the services the interpreter did for him. In the Ottoman empire interpreters were called dragoman and their role was not just interpreting but also acting as guides, go-betweens and door-openers to the Ottoman empire. The Ottoman empire also had sworn court interpreters, as can be seen from old court records from the Ottoman empire. Update December 6, 2010: Another interesting post on dragomans and the history of interpreting by Unprofessional Translations
There were also sworn court interpreters in Spain in the 16th Century. And interpreters were also used by the conquistadors to communicate with the indigenous people in the Americas. Although the training those interpreters recieved were perhaps not to be envied. Natives were brought back to Spain where they worked as slaves and learnt the language. If they were judged good enought they were brought back to their origins to act as interpreters.
But interpreting hit the headlines with the Nüremberg Trials. Although interpreting was used at the international organisations before the Second World War, this was the first time that large scale simultaneous interpreting was used. Technology now allowed interpreters to listen to the original in head phones and interpret into a microphone that broadcasted the interpreting to listeners. Hardly any of the interpreters who interpreted at the Nüremberg Trials had any interpreting training. But most of the interpreters there then went on to a career in interpreting. These interpreters were the founding fathers and mothers of the profession. They were active in the professionalization of interpreters, they helped training new interpreters and they lay the foundations of aiic, the international association for conference interpreters.
Community interpreters are a different case. Community interpreting has not started its professionalization until the past 10 or 15 years. Community interpreters were typically friends and and family of the person needing community interpreting. However, thanks to researchers and very active community interpreters, and in particular thanks to the Critical link conference, community interpreting is slowly gaining professional standards in the same way as its big sister conference interpreting.