Sumerian

Sumerian was spoken in Sumer in southern Mesopotamia (part of modern
Iraq) from perhaps the 4th millennium BC until about 2,000 BC, when
it was replaced by Akkadian as a spoken language, though continued
to be used in writing for religious, artistic and scholarly purposes
until about the 1st century AD. Sumerian is not related to any
other known language so is classified as a language isolate.

Sumerian cuneiform

Sumerian cuneiform is the earliest known writing system. Its
origins can be traced back to about 8,000 BC and it developed
from the pictographs and other symbols used to represent trade
goods and livestock on clay tablets. Originally the Sumerians
made small tokens out of clay to represent the items. The tokens
were kept together in sealed clay envelopes, and in order to show
what was inside the envelopes, they press the tokens into the clay
in the outside.

Examples of Sumerian clay tokens
Examples of the clay tokens

Over time they realised that the tokens were not needed as they
could make the symbols in the clay. They also developed a numeral
system to represent mutiple instances of the same symbol rather than
just inscribing them all. The symbols became stylised over time
and eventually evolved into a complete writing system. The earliest
texts come from the cities of Uruk and Jamdat Nasr and date back to
3,300BC.

The name ‘cuneiform’ means ‘wedge-shaped’ and comes from the
Latin cuneus (wedge). It is based on the appearance of
the strokes, which were made by pressing reed a stylus into clay.
These type of symbol emerged in 3,000 BC.

By about 2,800 BC some of the Sumerian glyphs were being used
to represent sounds using the rebus principle. For example, the
symbol for arrow, pronounced ‘ti’, was used to represent the
word for life (til). There were also many glyphs which were
pronounced the same but represented different words. Later a system
of determinatives, which gave you a hint at the category a
word belonged to, and of phonetic components, which indiciated
how to pronounce a word, developed, and helped disambiguate
the meanings of glyphs.

Here are some examples of how glyphs changed over time:

Some examples of Sumerian glyphs

Source: http://pandora.cii.wwu.edu/vajda/ling201/writingsystems/sumeriancuneiform.htm

Notable features

Type of writing system: semanto-phonetic – the symbols consist of
phonograms, representing spoken syllables, determinatives, which indicate
the category a word belonged to and logograms, which represent words.
Direction of writing: variable – early texts were written vertically
from top to bottom, but by about 3,000 BC the direction had changed
to left to right in horizontal rows. At the same time the signs were
rotated 90° anticlockwise and started to be made up mainly of wedges.
Number of symbols: between about 1,000 in older texts to 400
in later texts.
Many of the symbols had multiple pronunciations.
Used to write: Sumerian

Sumerian syllabic glyps

Sumerian script

Sample text

Sample of Sumerian writing

Sumerian cuneiform tablet, listing herders and cows in the goddess Inana’s fields, 21st–20th century B.C
Source: http://forces.si.edu/soils/04_00_38.html