Was cuneiform a true alphabet, i.e., with correspondence between marks and sounds, as opposed to hieroglyphics or pictograms?
Cuneiform was logographic, not unlike Chinese scripts, but developed before Chinese script and independently from it. For the most part, each character represents a phoneme, comprised of a very stylized pictogram of an underlying and easily recognizable item. So, for example, if we were developing it, we might wind up with Y to represent the word “bird”, because the arms look like wings, and the stem looks like the bird’s body. So “Y” would be pronounced “Bird”.
The flexibility of this system led it to be adapted for many other languages, including Semitic languages and Indo-European languages. Cuneiform was first cracked using Assyrian texts; Assyrian is an extinct Semitic language, with similarities to existing Semitic languages. The failure to find correspondences between the little pictures in each character and the Assyrian words written with the system led to the correct conclusion that the writing system had been adapted to Assyrian.
Having the ability to pronounce the writing even though it was in an unknown language, and finding bilinguals here and there led to the cracking of the Sumerian originals. Sumerian has no known relatives living or extinct, and is considered a language isolate. There are some wild claims to the contrary.
The Sumerians started to use cuneiform in a well-developed form around 5000 years ago; the origin of it is often attributed to much earlier systems of accounting, and tracking property and inventories. The Sumerians were strangers in their own land, having entered from the sea (by their own account), and using the non-Sumerian names for the major rivers, and a number of their greatest cities. The names of the great rivers aren’t taken from any known language, and apparently represent a glimpse into a vanished language and people who lived in a preliterate society.
http://en.wikipedia.org/wiki/Cuneiform_script