Up until modern times, the alphabet was a work-in-progress that went as far back as ancient Egypt. We know this because the earliest evidence of a consonant-based alphabet, in the form of graffiti-style inscriptions, was discovered along the Sinai peninsula.
Not too much is known about these mysterious scripts except they're likely a collection of characters adapted from Egyptian hieroglyphs. It's also unclear whether these early scripts were written by the Canaanites who inhabited the area around 19th century BC or a Semitic population that occupied central Egypt in 15th century BC.
Whatever the case, it wasn't until the emergence of the Phoenician civilization, a collection of city-states strewn along Egypt's Mediterranean coast, that the Proto-Sinaitic script was widely used. Written from right to left and consisting of 22 symbols, this unique system would eventually spread throughout the middle east and across Europe through maritime merchants who carried out commerce with nearby groups of people.
By the 8th century BC, the alphabet had made its way to Greece, where it was altered and adapted to the Greek language. The biggest change was the addition of vowel sounds, which many scholars believed marked the creation of the first true alphabet that allowed for a clear pronunciation of specific Greek words. The Greeks also later made other significant modifications such as writing letters from left to right.
At about the same time toward the east, the Phoenician alphabet would form the early basis for the Aramaic alphabet, which serves as the foundation for Hebrew, Syriac, and Arabic written systems. As a language, Aramaic was spoken throughout the Neo-assyrian empire, Neo-babylonian empire and perhaps most prominently among Jesus Christ and his disciples. Outside of the middle east, remnants of its use have also been found in parts of India and central Asia.
Back in Europe, the Greek alphabet system reached the Romans around the 5th century BC, through exchanges between Greek and Roman tribes that resided along the Italian peninsula. The Latins made some minor changes of their own, dropping four letters and adding others. The practice of modifying the alphabet was commonplace as nations began to adopt it as a writing system. The Anglo-Saxons, for instance, used Roman letters to write old English after the kingdom's conversion to Christianity, and made a series of alterations that later became the foundation for the modern English we use today.
Interestingly enough, the order of the original letters has managed to remain the same even as these variants of the Phoenician alphabet were changed to suit the local language. For example, a dozen stone tablets unearthed in ancient Syrian city of Ugarit, which dated back to the 14th century BC, depicted an alphabet that resembled bits of the Latin alphabet in its standard letter order. New additions to the alphabet often were placed at the end, as was the case with X, Y, and Z.
But while the Phoenician alphabet can be considered the father of just about all written systems in the west, there are some alphabets that bear no relation to it. This includes the Maldivian script, which borrows elements from Arabic but derived many of its letters from numerals. Another one is the Korean alphabet, known as Hangul, which groups various letters together into blocks that resemble Chinese characters to produce a syllable. In Somalia, the Osmanya alphabet was devised for the Somali in the 1920s by Osman Yusuf Kenadid, a local poet, writer, teacher, and politician. Evidence of independent alphabets were also found in medieval Ireland and the old Persian empire.
And in case you're wondering, the alphabet song used to help young children learn their ABCs only came about relatively recently. Originally copyrighted by Boston-based music publisher Charles Bradlee under the title "The ABC: A German Air With Variations for the Flute With an Easy Accompaniment for the Piano Forte," the tune is modeled after Twelve Variations on "Ah vous dirai-je, Maman," a piano composition written by Wolfgang Amadeus Mozart. The same tune has also been used in "Twinkle, Twinkle, Little Star" and "Baa, Baa, Black Sheep."