As Greek became the official language of Christianity and the New Testament, the Septuagint's Greek at times overshadowed even the original Hebrew of the Bible, along the way creating more than a little controversy.
As parts of the original Hebrew were lost to time, the Greek translation preserved texts of the Bible that would otherwise have been unrecoverable.
And as the pronunciation of ancient Hebrew was forgotten, the Septuagint's Greek served as a virtual recording to help scholars recreate the ancient sounds that shaped Western civilization.
For these reasons and more, it is impossible to fully understand the Bible without taking the Septuagint into account.
This impressive story is reflected in the very name "Septuagint," which comes from the Greek septuaginta, "seventy." (In a typical move for academia, though, scholars abbreviate the Greek name with the Roman numeral LXX.)
The account in the Letter of Aristeas is unlikely to be accurate, so the name "Septuagint" is misleading. It's also vague, because the original translation into Greek covered only the Five Books of Moses. The rest of the Old Testament was added later, as part of a process that included significant emendation of the original Greek work. What we now call the "Septuagint" is in fact but one version of a Greek tradition that began in ancient Egypt.
Still, it is a rich source of information about the Bible, even if it is imperfect.
Sometimes the translation is sloppy, mixing up near synonyms. For example, the Hebrew words alma ("young woman") and b'tulah ("virgin") were nearly synonymous in antiquity, because most female virgins were young women, and most young women were virgins — similar to "high-schooler" and "teenager" in modern America. A careful translation would still distinguish between the two Hebrew words, but the Septuagint often translates them both as the Greek parthenos ("virgin").
In Genesis 24:16, this gives us the fairly odd, "the virgin ... was a virgin" in Greek, for what should have been, "the young woman was a virgin." But other than the stylistic oddity, at least the Greek there means roughly the same thing as the Hebrew.
In a more significant departure from literal translation, the Septuagint takes the Hebrew phrase, "the young woman is pregnant" in Isaiah 7:14 and turns it into the very different Greek, "the virgin is pregnant." This passage was then quoted, in Greek, in Matthew 1:23 as part of the description of Jesus's birth to the virgin Mary. Mary, according to Matthew 1:23, was a virgin just like the "virgin" in Isaiah 7:14, even though the young woman was only called a virgin in Greek, not in the original Hebrew.
Though this kind of loose translation surprises many modern readers, it was the norm in antiquity, and the author of Matthew surely knew that his comparison only worked in Greek. He didn't care.1
In a similarly non-literal vein, the Septuagint takes the fashion accessories in Isaiah 3:18-24 — which comprise examples of haughty dress — and adds to them some more culturally relevant Egyptian items, to help drive home the point to ancient Egyptian readers. So along with the "signet rings and nose rings" of Isaiah 3:21 in Hebrew, the Greek adds transparent clothing from Sparta. Again, this kind of departure from strict translation into the realm of interpretation surprises many modern readers, but it was common.
In perhaps its biggest surprise, the Septuagint reframes the whole book of Esther with an introduction that does not appear in the Hebrew. The narrative in Esther — closely connected to the Jewish holiday of Purim, celebrated by Jews to this day — is widely regarded as historical fiction, an allegory set in Persia during the reign perhaps of Xerxes the Great. The Hebrew version of the story jumps right into the narrative during King Ahasuerus's (Xerxes's) reign in Persia.
But the Septuagint account begins instead with a dream (ina section of text called "Addition A"). Mordecai, the story's hero, sees two serpents "ready for battle" come forward amid thunder and earthquakes on a day of darkness and doom across the entire earth. Through their cry every nation prepares for war to battle the "righteous nation," which in response calls out to God. Then, "like a great river from a small spring" comes light with the sunrise, and the "lowly are exalted." At the end (in text called "Addition F"), Mordecai understands God's plan from the dream. But it's not in the Hebrew text of the Bible.
In addition to helping preserve the text of the Bible, the Septuagint is an invaluable guide to how Hebrew may have sounded in antiquity. This is important, because the Hebrew writing from the same time period records very few of the vowels. Greek supplies more detail.
For instance, Rebbecca's name in Hebrew is spelled R-B-K-H, which could be pronounced "rubeko," "ravka" (because "V" and "B" are written the same consonant in Hewbrew), etc. Her name in Hebrew is commonly pronounced "rivka," which is one of the possibilities, but certainly not the only one.
Indeed, the current Biblical Hebrew pronunciation tradition dates back only about 1,100 years, to the Masoretes, who lived long after Hebrew had ceased to be spoken natively. They are the ones who give us "rivka" for Rebbecca's name.
Even though the Masoretes recorded what they thought the sounds of Biblical Hebrew were, there's no reason to think they got it right. How could they know for sure how Hebrew had been pronounced nearly a millennium before they lived?
In this case, the Masoretes give us a two-syllable word. But the Septuagint gives us the three-syllable "rebekah," which is why the Jewish matriarch's name in English is Rebbecca. According to the Septuagint, the Masoretes got the vowels wrong on this one — and in many other cases.
Finally, every so often, for reasons we don't know, the Septuagint has a different understanding of the text than the original Hebrew. One bizarre case involves the animal called in Hebrew a r'em, a horned animal of some sort, perhaps from the antelope family, perhaps closer to an ox.
But the Septuagint calls the animal a unicorn! This is how unicorns entered the Bible.
So the Septuagint contributed to the theology of the New Testament, preserved important parts of the Old Testament, and even threw in some curious creatures.