What is the oldest family name in the world?

The oldest family name in the world that is still in use today is believed by many historians to be the name Katz or variants like Cohen, with claims dating back thousands of years, but there are several contenders depending on definition. The surname Katz is an acronym for Kohen Tzedek meaning righteous priest in Hebrew, and Kohanim are descendants of Aaron, brother of Moses, with lineage claimed to go back over 3,000 years to around 1300 BCE, making Cohen and its variants like Katz, Kohn, and Kahn among the oldest continuous family identifiers. Another contender is the name Zhang in China, which is one of the oldest and most common Chinese surnames with recorded use over 4,000 years dating back to ancient China, though as a family name its continuous use is well documented. In Europe, the name O'Cleirigh or O'Clery in Ireland is considered one of the oldest surnames with recorded use from the 10th century. The surname Bryant, which you asked about earlier, is much more recent, originating in medieval England and Ireland around the 12th century. Overall, there is no single undisputed oldest family name because naming conventions evolved differently across cultures, but Hebrew priestly names like Cohen and Katz and Chinese names like Zhang are among the oldest continuously used family names with documented history spanning millennia.