Original: The project was led by Udi Manber of Google, it was announced on December 13, 2007 and was opened in beta to the public on July 23, 2008 with a few hundred articles mostly in the health and medical field.
Simplified: The project was  announced on December 13, 2007 and opened in beta to the public on July 23, 2008. When it was opened, there were a few hundred articles mostly about health and medicine.