Tefillin are arguably not biblical requirements
Rabbis have taught that the command to wear Tefillin is contained in four passages Exodus 13:9, 13:16, and Deuteronomy 6:8-9, and 11:18, and these passages were placed in the Tefillin boxes. The passages do not mention Tefillin but say, quoting Exodus 13:9, “And it shall be for a sign unto thee upon thy hand, and a memorial between thine eyes.”
While the rabbis interpreted “upon thy hand” as a biblical command to don Tefillin on one’s arm (not hand), and “between thy eyes” as a biblical mandate to place Tefillin on one’s forehead (not between the eyes), the great Bible commentator Rashbam (born around 1085) understood the verse metaphorically. “According to its plain meaning, it (the Torah teaching) should be remembered always as if it had been written upon your hand, similar to ‘Set me as a seal upon thy heart and a seal upon thine arm’ (Song of Songs 8:6). ‘Between your eyes’, it should be like a piece of jewelry or gold chain that people put on the forehead for decoration” (Rashbam on Exodus 13:9).
I would suggest as small change from what Rashbam wrote. Yes, what we see here are two metaphors, but what the Torah is saying is “sign upon thy arm,” think of the Torah teaching whenever you use your arm, in whatever work you do. And “a memorial between thine eyes” means think about the Torah teaching whenever you see something and whenever you think of something.
Rashbam realized that the four verses that speak about the hand and between the eyes should not be taken literally. They are similar to the passage that Rashbam quoted and to “Hear, my son, the instruction of thy father, and forsake not the teaching of thy mother; for they shall be a chaplet of grace unto thy head and chains about thy neck” (Proverbs 1:8-9), and to “My son, keep the commandments of thy father, and forsake not the teaching of thy mother; bind them continually upon thy heart, tie them about thy neck” (Proverbs 6:20-21). The “Tefillin” passage are encouraging the Israelites to remember the teaching of the Torah always, even when you do any work (upon thy hand) or think about something (between thy eyes).
When did the new interpretation begin?
Tzvi Adams alerted me to the interesting scholarly book “Tangled up in Text: Tefillin and the Ancient World.” In his book, Yehudah Cohn, notes that the earliest Tefillin that have been found are from Qumran, the site of the Essenes near the Dead Sea in Israel, which was destroyed around the time of the destruction of the second temple in 70 CE. Cohn also notes that these Tefillin are “remarkable for its diversity, and evidences significant differences with later rabbinic dicta,” showing that there was no set rule what the Tefillin at that time should contain. He also notes that the ancient documents of the time – the Letter of Aristeas and Philo – do not interpret the four so-called “Tefillin” passages to require donning Tefillin.
Cohn suggests that the Tefillin were first used sometime after Jews came in contact with Greeks who wore amulets. Amulets were widely used by superstitious Greeks to protect them from medical problems and demons, and to cure diseases. The Greek “widespread use of protective amulets would have led Jews toward an integration of Greek practice with their own paramount source of authority, namely the Torah…. One can readily imagine that Jews would have been as eager as gentiles to avail themselves of such useful objects.” But instead of pagan texts or those mentioning idols, the Jews would use verses from the Torah, such as the promise in Deuteronomy 11:21 of “long life.”
It is no surprise, therefore, that II Maccabees 12:34-40 speaks about Jewish soldiers killed during a battle because each wore a pagan and not a Jewish amulet.
In short, the verses that the rabbis use to prove a biblical mandate to wear Tefillin do not mention Tefillin, they speak about placing something on the hand and between the eyes while the rabbis required setting the Tefillin on the arm and forehead; the verses should be understood as metaphors; the earliest find of Tefillin are from near the start of the common era but may have existed earlier; Jews came into contact with Greeks after 322 BCE; the Greeks wore amulets; it is likely that the Jews also wanted the protection afforded by amulets but felt it proper to place Torah verses in the amulets; we know that Jews used amulets for centuries even during the period of the Maccabees and many Jews use them even today; thus it is possible that the practice of wearing Tefillin began as protective amulets which the rabbis elevated and spiritualized later by reinterpreting the use of the boxes in a non-superstitious manner and assigning biblical verses to support their innovation.
To be continued: Is the mezuzah mandated by the Torah? And if the practice is old – either from biblical time or the Greek era, why was there a disagreement about the contents of the Tefillin boxes in the twelfth century? And what is the meaning of “it” in “it shall be a sign,” what Torah teaching is the Torah emphasizing?
 The translations of all biblical passages are from The Holy Scriptures, JPS, 1960.