Script evolution

links for world alphabets
STEP 1: Humans sketch drawings.
Cave drawings are not yet a consciously organized script-system.
STEP 2: Homo sapiens sapiens writes symbols
Sketches of objects iconogrammes. These become ideogrammes.
Script appeared approximately during the same time (before 6 thousand years) in:
Mesopotamia (sumerians et al.)
the Indus Valley (Harappa civilization)
Aegypt (check Günter Dryer's findings)
It seems that the human species was ready to make this step, so, it happened tautochronously without any contact among those people.
STEP 3: Phonetic scripts
phonetic concept: symbols stand for sounds of speech rather than an actual object.
STEP 3a: Syllabic scripts
So, we first have syllabogrammes and consonantgrammes. Elaborate writing systems have been used by soumerians, babylonians, assyrians, hettites, since 4000 a.e.v.. The phoenicians created a consonant-system-script, that was destined to be our great grandmother script!
STEP 3b: Precise phonetic alphabetic scripts
Vowels would be the next step. Here is where the greeks come in, in our story. Their script is the grandmother of today's roman alphabet.
Today's scripts
Both systems are in use today.
PHONETIC SCRIPTS: Most contemporary languages are written in phonetic scripts. Vowels are marked either with letters (as in european scripts) or with other diacritics (as in arabic). English spelling is not a precise phonetic representation of sound. German uses a very precise phonetic script. The only absolutely phonetic script is the International Phonetic Alphabet.
NON PHONETIC SCRIPTS: as in chinese script. In this case, speakers of different languages and dialects can read texts without any translation needed. But the ideogrammatic system has thousands of symbols and is very difficult to learn.