The Indus Valley Civilization was the first major urban culture of South Asia. It reached its peak from 2600 BC to 1900 BC roughly, a period called by some archaeologists "Mature Harappan" as distinguished from the earlier Neolithic "Early Harappan" regional cultures. Spatially, it is huge, comprising of about 1000 settlements of varying sizes, and geographically includes almost all of modern Pakistan, parts of India as far east as Delhi and as far south as Bombay, and parts of Afghanistan.
The main corpus of writing dated from the Indus Civilization is in the form of some two thousand inscribed seals in good, legible conditions. (In case you don't know what seals are, they are used to make impressions on malleable material like clay.)
Although these seals and samples of Indus writing have been floating around the scholastic world for close to 70 years, little progress has been made on deciphering this elegant script. However, we should not blame scholars for their lack of progress, for there are some major impediments to decipherment:
1. Very short and brief texts. The average number of symbols on the seals is 5, and the longest is only 26.
2. The language underneath is unknown.
3. Lack of bilingual texts.
For instance, consider Champollion, who deciphered Egyptian hieroglyphs with all of these 3 important clues: there were very long Egyptian texts; he knew Coptic, a descendant of Egyptian; and the Rosetta Stone, a bilingual text between Greek and two written forms of Egyptian.
But the script isn't as bad as undecipherable. For one, even though scholars don't have long texts and bilingual texts, they can still theorize about the language underneath the writing system. There are several competing theories about the language that the Indus script represent:
The major problem with this model is the fact that horses played a very important role in all Indo-European cultures, being a people constantly on the move. "There is no escape from the fact that the horse played a central role in the Vedic and Iranian cultures..." (Parpola, 1986) Sidenote: "Vedic" means from the time of the Vedas, the earliest text in India, and the Vedic culture is from around 1500 to 500 BC. However, no depiction of horses on seals nor any remains of horses have been found so far before 2000 BC. They only appear after 2000 BC. Very likely there were no Aryan speakers present before 2000 BC in the Indus Valley.
The Dravidian model isn't just an unapplicable theory...But first we have to know what kind of writing system is the Indus script.
A count of the number of signs reveal a lot about the type of system being used. Alphabetic systems rarely have more than 40 symbols. Syllabic systems like Linear B or Cherokee typically have 40 to 100 or so symbols. The third ranges from logophonetic to logographic, running upwards of hundreds of signs (like 500 signs in Hieroglyphic Luwian, and 5000 symbols in modern Chinese).
It appears that the maximum number of Indus script symbols is 400, although there are 200 basic signs (ie signs that are not combined from others). This means that the Indus script is probably logophonetic, in that it has both signs used for their meanings, and signs used for their phonetic values.
Many signs start off as pictorial representation of a physical object, often misleadingly called pictograms. They really are should be called logograms because they represent words in the language. However, it's next to impossible to write out a word with abstract meaning pictorially. What all early writers figured out was to use a logogram not for the object or idea it was originally supposed to stand for, but for all words sounding similar to the original word for that object or idea. For example, in English to write "leave" we can use a picture of a "leaf". This is called rebus writing, and is a tremendously common pattern in all early writing systems. We could also then use the same "leaf" symbol to stand for the sound in "relief", adding another symbol in front of the "leaf" symbol in order to indicate the "re" sound. So the logogram gained a phonetic value as well.
Testing the theory
How can we take the theoretical framework so far and apply it to archaeological data?
Numerals seem to represented by vertical lines (represented by number of lines in the glyph), but they only go up to 7. Analysis reveal 4 more signs that appear in the same context as these numerals, and so they likely represent numbers higher than 7.
The fact that no vertical-line numeral sign denotes 8 very likely means the Harappan language is based 8. (For example, the Arabic numerals that we use has symbols from 0 to 9, and to write "ten" we have to combined the symbols 1 and 0, which identify our number system as based ten.)
Base 8 languages are rare in the world, but it does appear that early Dravidian is base 8, but later changed to base 10 (possibly under Indo-European influence). When translated, the count from 1 to 7 is familiar to us: "one", "two", "three", "four", "five", "six", "seven". However, above seven, the number's etymologies become non-numerical: 8 is "number", 9 is "many minus one", and 10 is "many". (Fairservis 1983)
But can we actually read (not interpret) any symbol on the seals? We should start with "pictograms", as this one:
Many scholars (Knorozov, Parpola, Mahadevan, etc) see this sign as a fish. Fish in reconstructed Proto-Dravidian is *mîn. Coincidentally, *mîn is also the word for star. On many pots from Mohenjo Daro, an Indus site, there are drawings of fish and stars together, and so affirming this linguistic association.
Going further, often the numeral six appears before the fish. Either it means 6 fish, or 6 stars. Old Tamil (a Dravidian language still spoken today) texts from just around the 1st century AD recorded the name of the Pleiades, a star cluster visible during autumn and winter just above Orion, as "Six-Stars", or aru-mîn.
.Throughout the world, titles with celestial connotations are very common, and the clause Six Stars forming part or whole of a Harappan title is not unreasonable. (Parpola, 1986)
Sometimes symbols are added to the basic sign to make new signs. Of these, the one that looks like a circumflex accent placed on top of the fish is quite interesting. It is theorized to mean "roof", and in Proto-Dravidian it is *vêy/mêy. This is phonetically similar to Proto-Dravidian word for "black", *may. Together with fish, it spells out mai-m-mîn, or "black star", which in Old Tamil means the planet Saturn. In Sanskrit texts, Saturn is associate the color black. The god of death, Yama, is the presiding of this planet, and is usually depicted as riding on a dark buffalo.
But the "fish" reading isn't accepted by all scholars. William Fairservis saw it as a combination of a loom twist and a human sign, and form a honorific title pertaining to rulership (Fairservis, 1983). I, however, am more inclined to accept the fish identification.