There are wildly different starting dates given for Syriac, a script descended from Aramaic and used, over time, to write several different languages. I believe this has to do with Syriac script evolving slowly into a distinct script, Syriac spoken language evolving slowly into a distinct spoken language, and lack of precision in distinguishing between the spoken language and the script. (There are way more spoken language geeks than writing systems geeks, so that’s perhaps not surprising.)
It is clear that by 6 AD, Syriac script was a distinct script. While several of the previous writing systems had a cursive form, in which letters were sometimes joined (e.g. hieratic Egyptian), as well as a discontinuous form, Syriac is unusual of the languages up to this point in only having a joined form.
Syriac script, like Hebrew script, has different forms of letters depending upon where in the word the letter is located. Unlike Hebrew, which only has differences between ending and non-ending, Syriac also distinguishes between beginning and middle. (Not all letters have three visually distinct glyph forms, but many do.)
A bitter schism in the Syrian community rent the Syrian language community in two in 431 AD. Because the two groups basically stopped talking to each other, their spoken and written languages diverged. Eventually, the child scripts developed their own unique identity in both glyph shape and in the representation of vowels. The earlier script, called Estrangela, did not have vowels. The Eastern script (also called Nestorite) developed arrangements of dots around the consonants to denote vowels in around 350 AD; the Western script (also called Serta) starting using tiny little Greek letters above or below the consonants to denote vowels around 700 AD. There is some thought that the Eastern dots influenced the Hebrew script’s vowel pointing.
As was common back when writing materials were expensive, there were lots of abbreviations. An interesting feature developed by Syriac was the use of a horizontal line (with dots at the beginning, center, and end of the line) over the characters that were the abbreviation, to help disambiguate (similar to our ending abbreviations with a dot). This might have been adopted by Greek for their “nomina sacra” abbreviations of sacred names.
They also would use a horizontal line over letters to indicate that they were being used as numbers. (It was very common in many languages to use letters for numbers, e.g. Roman numerals.)
In addition, Syriac has a diacritic to change a consonant from an hard, unaspirated form to a soft, aspirated form. There is also an optional double-dot to denote plurals, and a single dot to denote a feminine form.
Syriac was the language of Christianity in Central Asia and eastward for several centuries, reaching even to Kerala in India, where there are still some native speakers.