Probably the most foundational item of the hypothetical Priestly (“P”) source in Wellhausenesque source criticism of the Hebrew Bible, effectively the traditional scholarly standard, is that P is responsible for the genealogies in the OT, and various genealogy-like elements (lists, passages like Genesis 1 displaying a particularly repetitive structure, etc, considered so on the analogy of “things vaguely genealogy-like”). The source of this rather non-intuitive connection of genealogies in particular with priests is found in one of the stories of the return in Ezra 2, where certain of the returning priests were not able to be found listed in the (seemingly official) genealogies, and so were excluded by the governor (2.63) from eating the priestly portion of the sacrifices. The intervention of the governor, rather than the high priest, may also be taken to indicate that the genealogical information was under the governor’s control, not the priests’. Yet there is further indication that the genealogies are not particularly “priestly.” Firstly, in this particular instance in Ezra 2, we find the priests interested in their own genealogies, not particularly everyone else’s, with a particular focus on who is eligible to receive portions of the sacrifices. The importance of this ritual aspect of the interest in the families of the returned priests doesn’t, however, apply to all the various genealogies found in the Pentateuch and Former Prophets, where such ritual matters are completely unmentioned in connection with the genealogies. It’s a bit of special pleading to stretch the Ezra 2 evidence so far. Secondly, there is a trend to be found in the genealogies as they are found in the Pentateuch and Former Prophets, and this tendency or direction is an important one. It indicates that David, and thereafter the Son of David, is not only rightful king of Israel as seen through this genealogy, but he is essentially the firstborn son of God, and the firstborn or the entire human race.
Notice the “drilling down” of the genealogies and related stories, preserving the line of Adam’s firstborn in Genesis 5 and 11, leading to Abraham, then of course the unusual birth of Isaac in Genesis 21, then the birth of Jacob and Esau, and Esau selling his birthright to Jacob in Genesis 25 and Jacob tricking Isaac to receive the blessing in Genesis 27, and then the birth of all of Jacob’s sons, the eponymous heads of the tribes of Israel. Things then get tricky! Since Jacob has two wives and two concubines, and has had 12 sons and one daughter through them, which one is depicted as his firstborn? We find that this position of favored son moves around quite a bit. First, Reuben is Jacob’s true firstborn son (as explicitly stated in Genesis 35.23, 46.8, and 49.3), born to his wife Leah, followed by Simeon, Levi, and Judah, before any other children are born to any of the other ladies (Gen 29.31-35). Reuben, however, does the unthinkable and sleeps with his father’s concubine (Gen 35.22). This apparently bumps the favored son status in two different directions, however. Firstly, Simeon and Levi lose out because of their violence in killing all the incapacitated men of Shechem (Gen 34; cf Gen 49.5-7), and this leaves the role of head of the sons with Judah, as seen in Genesis 49.9-10. Secondly, some part of the birthright blessing is indicated as having moved from the sons born to Jacob’s first wife Leah, to those born of his second, more loved wife Rachel (cf the blessing of Joseph’s sons in Gen 48, and the explicit statement in this regard in 1 Chronicles 5.1). So, we see a split blessing, of rulership in Judah as the firstborn, and the “birthright” (of “firstborn’s blessing” likely referring to the “double portion” to be inherited by the firstborn, as in Deuteronomy 21.17) in Ephraim and Manasseh. It’s unclear whether the “double portion” for Ephraim and Manasseh is considered to be the two separate tribes themselves (Ephraim and Manasseh, rather than a single tribe of Joseph), or the large, Jordan-straddling territory of Manasseh in combination with the fertility of that land and that of Ephraim, or perhaps all of the above. And, of course, David was depicted as descended in a direct line from Judah through Perez (who possesses an unusual birth tale of his own in Genesis 38), and so on, down through Boaz and Ruth, to Obed, then Jesse, David’s father (see Ruth 4 and 1 Chronicles 2). While this part of the genealogy is fully preserved only in those two places, it would likely have been well-publicized in the days of David.
I suggest that part of that publicizing is precisely the Pentateuch and Former Prophets (Joshua through Samuel, with 1 Kings up through, likely, only chapter 10 initially) with the inclusion of Ruth. The original framework of the structure of Genesis, the genealogies upon which the stories are hung like charms on a bracelet, in connection particularly with the book of Ruth, lead in the direction of legitimizing David’s rule as not only as ruler of Israel through his birthright, but as ruler of the entire world as in the line of the firstborn son of Adam, and also thereby, the line of the firstborn son of God (see especially the high view of the newborn Son of David in Isaiah 7, and Psalms 2 and 110). This group of books, I suggest, was first put together in the days of Solomon to provide this legitimation, and to enhance the standing of the Son of David in his particular role religiously as Son of God, as well. In contrast to the Wellhausenian approach of removing the genealogies as a particular source, this suggestion views them as an important clue as to the original intent of the work in which they appear. The various gymnastics that are described in the transfer of the birthright blessings are typically not understood as leading in any particular direction, their being usually understood separate sources. However, they do tell an important story in the books as they stand, one of legitimation just as necessary to every new dynast as in any other ancient nation, where we find iterations of lists of prior kings to which the newcomer is always somehow related (see the Assyrian King Lists, especially relevant, which preserve the fiction [?] of only a single dynasty of rulers throughout Assyrian history). For this reason, the genealogies must be seen not as intrusive sections of a fictive Priestly source, but rather as a part of the original author’s work of providing a history that legitimizes David and his dynasty in the eyes of other Israelites as not only the heir to the Patriarchs, but indeed also as Son of God.