In the Torah, Aaron is the first high priest of the Levitical order of priests. He had four sons: Nadab, Abihu, Eleazar, and Ithamar. The first two died in Leviticus 10, leaving Eleazar as the eldest inheritor of the high priestly line. We see in Joshua that Eleazar's son Phinehas had become the high priest by the time of the conquering of the land. Then we lose any record of what was going on with tabernacle worship until we get to Samuel, where there seems to be a fixed temple structure built up around the tabernacle implements of worship from the end of Exodus. Perhaps the most surprising feature of the priestly situation at the beginning of the book of Samuel is that the high priest Eli was not descended from Eleazar but his younger brother Ithamar. Where are the descendants of Eleazar, then? What happened after Phinehas?
This surprising fact in the Samuel history has led a number of scholars to propose a skeptical reconstruction of what really happened. On this view, the high priestly family has always been descended from Ithamar, and Eli's family in Samuel was the original high priestly family. With David we get the insertion of a priest named Zadok alongside the final remaining Elide priest Abiathar/Ahimilech. I Chronicles 24 tells us that Zadok is the head of the the Eleazar clan of priests, which Ahimelech (perhaps the same man called Abiathar in Samuel, perhaps his son) was head of the Ithamar clan of priests. The revisionist theory takes Zadok to be a complete outsider from the conquered Jebusite city of Jerusalem. David allowed him to continue his priestly duties, casting him as a priest under the order of Melchizedek, the original priest-king of Salem (which became Jerusalem) from Genesis 14. This allowed David to assert his legitimacy to be king in the Jebusite city, and then Chronicles and the other places that list him as a descendant of Eleazar are just reworking the tradition to make him fit the Israelite origin story, casting Zadok as a son of an older brother of the ancestor of the Elide priests. Thus no sign was left of the Jebusite origin of Zadok.