Complex sentence: The focus was initially on the well-known Indo-European languages, many of which had long written histories, and on the other hand on the Uralic languages, another European language family for which less early written material exists.
Split into simpler sentences: The focus was initially on the well-known Indo-European languages, many of which had long written histories. They also studied the Uralic languages, another European language family for which less early written material exists.