For centuries now, historians and biblical scholars alike have come to the conclusion that Mary and Joseph were related. A close study of both geneological trees in Matthew and Luke show that they shared the same grandfather, called Matthat by Luke which is a form of the name Matthan used by Matthew. In other words, Mary and Joseph were first cousins. This would explain why Luke 2:4-5 shows Joseph taking Mary to register for the Roman census in Bethlehem, a city of David. [NOTE: the alternative reading of Luke 2:4 in the recently discovered 4th century mss., called a Palimpsest, saying they enrolled together in Bethlehem because they were both of the house of David.]

We also know from the historical records governing the laws of the Roman census that each tribe was called to a different town. This small but significant historical tidbit is carefully noted by the historian Luke when he says that everyone returned to their own ancestral towns to register for the census (Luke 2.3).

Also, it’s clear from the account by Luke that both the angel and Mary know she’s a descendant of David.

Luke 1:31 “Listen: You will become pregnant and give birth to a son, and you will name him Jesus. 32 He will be great, and will be called the Son of the Most High, and the Lord God will give him the throne of his father David. 33 He will reign over the house of Jacob forever, and his kingdom will never end.” 

Mary is not in the least surprised by the words of the angel, i.e., she doesn’t say How can this be since I’m not a descendant of David. But instead rightly asks:

Luke 1:34 “How, if I have not had sexual relations with a man?” 35 The angel replied, “The Holy Spirit will come upon you, and the power of the Most High will overshadow you. For this reason the child to be born will be holy; he will be called the Son of God.”

This crucial exchange shows that Jesus’ royal, biological lineage comes from Mary and not Joseph.

All this is further verified by the historian Luke’s later remark in Luke 3:23 that Jesus “was the son (as was supposed) of Joseph”! Also note that in Luke 1.42 Elizabeth rejoiced and exclaimed to Mary: “God has blessed you more than any other woman! And blessed is the fruit of your womb [koilia]!” Elizabeth was actually repeating OT prophetic words about David that now apply to Mary! In the LXX of Ps 132.11 God makes a promise to David: “I will place the fruit of your womb [same Greek word used in Lk 1.42, koilia] upon your throne.” Now, while this may sound odd to our Western ears, in the Bible the word womb is sometimes used of the father as well. Job 19:17 refers to his children as “the sons of my womb.” Most translators change biblical words perhaps in order to remove confusion from modern day readers. And so, other words like body or loins are used for men instead of womb.

So, while both Matthew and Luke may be a record of the line of Joseph only, both Gospels show God fathering Jesus out of Mary [Mat 1.16; Luke 1:31-35].

Lastly, this is all in fulfillment of Gen 3.15 where God had originally said to the serpent that there would be enmity between him and the seed of the woman. Both the Hebrew and Greek words translated as seed obviously carry a metaphorical meaning eventually fulfilled in Mary. That is, the Messiah would come from the biological line of David through the woman and not the man!