Tibetan is one of the oldest Sino-Tibetan language to be recorded. The earliest Tibetan inscriptions date from 7th to 8th century CE in what is called the dbu can (which translates into "with a head") script, which also appeared on manuscripts from around 11th CE and remained until the modern day in the form of printed Tibetan text. Another script tradition, the dbu med ("without a head", "acephalous"), appearing first around the 12th century CE. The main difference between the two script traditions is that the dbu can script has the top line (hence the name "with a head") and less cursive than the dbu med script.
The origin of the Tibetan script is rather obscure. Tibetan Buddhist tradition states that it was created by Minister Thon mi Sambhota in northeastern India by order of the Tibetan king Srong btsan sgam po. On the other hand, the Bon po religious tradition maintains that the script came from Iranian or Central Asian origins. However, no matter how it came into Tibetan, the script's structure clearly suggests that its ultimate ancestor is the Brahmi script of India: (a) each sign is actually a syllable consisting of a consonant plus the vowel /a/; (b) the ordering of the characters are same as in Brahmi; and (c) the way the vowels /i/, /u/, /e/, and /o/ are represented by marks above and below the signs.
The following is a list of all the signs in the dbu can script. The first line is the Tibetan sign, and the second line is the traditional transliteration of the sign that reflected how the sign was pronounced in the 7th century CE (see below). Also, notice the dot to the upper right of every sign. When it is present, the sign is meant to be syllabic, i.e. Ca, but without it, it becomes only the consonant.
Retroflex (apico-palatal) consonants (/t/ and /d/) occur only in loanwords from Sanskrit. In these cases, the regular signs for /t/ and /d/, respectively, are flipped horizontally to represent these sounds.
Syllables with vowels other than /a/ is denoted by a set of vowel marks above or below the consonant sign. Hence the default vowel of /a/ is replaced with the vowel of the mark. Initial vowels are denoted by writing the /a/ signs with the desired vowel mark.
The writing system of Tibetan is incredibly conservative, to the point that a written word would sound nothing like the spoken word. Phonological changes since the 7th century CE include simplification of consonant clusters and development of tones, but the writing system has remained formalized and unchanged since the 7th century CE. For example, the word written as brgyad (which means "eight") is now pronounced as /gye/. However, there is really no way to ameliorate this situation since there are thousands of mutually unintelligible Tibetan dialects, all of which use the same writing system.