Xapian is a modern class library, but has evolved out of more than 30 years of commercial and academic experience.
Xapian is partly derived from the Open Muscat engine, developed by BrightStation PLC and released under the GPL. Open Muscat was built to be a replacement for the proprietary Muscat 3.6 information retrieval system, which was written almost entirely in BCPL, and becoming hard to extend in the ways they wanted.
Muscat was originally written by Dr. Martin Porter at Cambridge University. In 1984, Martin and John Snyder founded Cambridge CD Publishing to commercially exploit the technology; the company was soon renamed Muscat Ltd when the focus shifted from CDs to the web. Muscat Ltd was bought by Maid PLC, who renamed themselves first to Dialog Corporation, and then to BrightStation PLC (when they sold the Dialog brand and content to Thomson).
In early 2001, BrightStation's management renamed Open Muscat to Omsee, and shortly afterwards they sadly announced they were taking development closed-source.
A number of developers (both former BrightStation employees and others from the fledgling Open Muscat community) took the last GPLed version and have been continuing development under the GPL. This project was initially known as OmSeek, but BrightStation complained this was too close to their (untrademarked and unpublicised) name Omsee. It was simpler to change it than to argue, and the name Xapian was chosen.
The last numbered BrightStation release was Open Muscat 0.4.1, although there was extensive development in CVS after that. The first official Xapian release was 0.5.0.
Back in 2000, Open Muscat (using the muscat36 backend) was the retrieval engine which powered BrightStation's Webtop search engine (archive.org snapshot), which offered a sub-second search over around 500 million web pages (around 1.5 terabytes of database files). The whole search engine, including the web crawlers and index building, ran on 30 Intel boxes. Webtop also incorporated technology from Muscat Ltd's EuroFerret search engine.