The first time Grigory Kessel held the ancient manuscript, its animal-hide pages more than 1,000 years old, it seemed oddly familiar.

A Syriac scholar at Philipps University in Marburg, Germany, Dr. Kessel was sitting in the library of the manuscript’s owner, a wealthy collector of rare scientific material in Baltimore. At that moment, Dr. Kessel realized that just three weeks earlier, in a library at Harvard University, he had seen a single orphaned page that was too similar to these pages to be coincidence.

The manuscript he held contained a hidden translation of an ancient, influential medical text by Galen of Pergamon, a Greco-Roman physician and philosopher who died in 200 A.D. It was missing pages and Dr. Kessel was suddenly convinced one of them was in Boston.

Dr. Kessel’s realization in February 2013 marked the beginning of a global hunt for the other lost leaves, a search that culminated in May with the digitization of the final rediscovered page in Paris.