After reading the 2010 paper When Does Human Life Begin? Conception and Ensoulment which I exclusively used for this answer, it appears that they didn't use Ps 51:5 (which is popular among pro-life activists today) because they wanted to avoid the Traducianism view and opted for the Creationism view (see Complete Answer below).
Augustine and Aquinas then simply adopted Aristotle's view: 40 days for males, 80 days for females. It may not be as "primitive" as you think, if we understand why they chose the Creationism view, which has to do with separating LIFE (which plants and animals have) and SOUL (which in Christianity must be individually created by God because of its spiritual essence). Modern theologian Louis Berkhof also held the Creationism view but used other Bible verses (see Complete Answer below).
Another 2014 paper Traducianism? Creationism? What Has An Ancient Debate To Do With The Modern Debate over Abortion? has a more detailed discussion why Augustine and Aquinas chose Creationism.
The paper discusses 3 views of ensoulment and various religions's view on ensoulment, including a history of Christian position from the early church father until modern times. My answer is exclusively based on this paper.
Bible verses by pro-life activists
The paper starts by reviewing a few Bible verses often used by Christian pro-life activists, but which in the paper author's opinion have ambiguous meaning so not used in the history of Christianity until modern times:
- Ps 139:13: "you [God] created my inmost being; you knit me together in my mother's womb" (ambiguous because the intent is not scientific, but a metaphor that God knows David intimately)
- Jer 1:5: "Before I formed you in the womb I knew you, before you were born I set you apart." (ambiguous because it merely communicates God's sovereign plan of Jeremiah, similar to Eph 1:4 that all of God's elect were chosen "before the creation of the world.")
... Souls are preexistent entities who await bodies to enter. ... Historically, very few within Christian circles have held or taught this view, though the Church of Jesus Christ of Latter Day Saints adopted it in the 19th century and certain New Age groups ...
"soul" is present in both the sperm and the egg when they unite. The combination forms a new "soul" automatically and immediately. Traducianism has been held by ... Tertullian (c.160-c.225) ... [who] wrote that "we allow that life begins with conception, because we contend that the soul also begins from conception; life taking its commencement at the same moment and place that the soul does." Clement of Alexandria presented a much more detailed description:
The embryo is a living thing; for that the soul entering into the
womb after it has been by cleansing prepared for conception, and
introduced by one of the angels who preside over generation,
and who knows the time for conception, moves the woman to intercourse;
and that, on the seed being deposited, the spirit, which
is in the seed, is, so to speak, appropriated, and is thus assumed
into conjunction in the process of formation.
This view is also held by Gregory of Nyssa (335-c.394) and Maximus the Confessor (c.580-662). The latter is based on the argument that Jesus was fully human at conception, thus possessing a spiritual soul from that instant. Because Jesus was fully human, every human being receives soul similarly (immediately) at conception.
Although this view best explains the transmission of original sin (since God is only indirectly party to the transmission of sin) this makes the parents to be the true creators of life and God only a (dis)interested observer, implying a deistic God except in the case of Adam and Eve.
... the "soul" is created and introduced into a fetus by God at a point of his choosing, either at the time of a fetus's first breath, as was the case with Adam in Gen 2:7, or when God in his soverignty knows that a fetus is not going to be spontaneously (meaning "naturally") or intentionally aborted.
The [20th century t]heologian Louis Berkhof [is] a staunch proponent [who] sees a marked distinction in the Bible between the body, which is taken from earth, and the soul, which is given by God. [He cited Gen 2:7, Eccl 12:7, and Heb 12:9.] ... Physical substance comes from physical origins, and spiritual essence from a spiritual source .... Creationism is the most biblically-based view, claiming that "it is more consistent with the prevailing representations of Scripture than Traducianism."
Berkhof is just one of the more recent representatives of a stream
of thought that is rooted both in ancient Hebrew beliefs and in Aristotelian
philosophy, a stream that is shared today by rabbinic Judaism
and by much of Islam. Aristotle equated “life” and “soul,” but described
different kinds of the latter: vegetative, sensitive, locomotive, and intellectual. “In general,” Aristotle believed, “soul is imparted to
the body in stages as each part is formed, and the specific soul is not
actually present until the form is complete.” This “completion of
form” takes place on the fortieth day after conception for males, and
on the eightieth day for females. Augustine of Hippo (354–430) was a
proponent of this view, and Thomas Aquinas (1205–1274) adopted
Aristotle’s schema practically in its entirety. Aquinas held that
the body was formed gradually through the power transmitted by
the male seed but the spiritual soul was directly created by God
when the body was ready to receive it. Thus the embryo was believed
to live at first the life of a plant, then the life of a simple
animal, and only after all its organs, including the brain, had
been formed, was it given, by the direct and creative act of God,
an immortal spiritual soul.
The Creationist views of Augustine and Aquinas were the norm
in the Christian West from the early fifth century to the late nineteenth
The rest of the Christian section of the paper discusses the view of various Popes, John Calvin, Martin Luther, etc.