TIME: Decoding the Ancient Script of the Indus Valley

The ancient cities of the Indus Valley belonged to the greatest civilization the world may never know. Since the 1920s, dozens of archaeological expeditions have unearthed traces of a 4,500-year-old urban culture that covered some 300,000 square miles in modern-day Pakistan and northwestern India. Digs at major sites such as Mohenjo-daro and Harappa revealed a sophisticated society whose towns had advanced sanitation, bathhouses and gridlike city-planning. Evidence of trade with Egypt and Sumer in Mesopotamia, as well as the presence of mining interests as far as Central Asia, suggests that the fertile Indus River basin could have been home to an empire larger and older than its more famous contemporaries in the Middle East.

But the Indus Valley civilization poses an intractable problem, one that legions of archaeologists and scientists have puzzled over from the first excavations to a new study published last month. Its writing, etched in signs on tiny, intricate seals and tablets, remains undeciphered, shrouding the ancient culture in mystery. A code-busting artifact with bilingual text, like the Rosetta stone, has yet to be found. By some counts, more than 100 decipherments of the civilization's often anthropomorphic runes and signs — known in the field as the Harappan script — have been attempted over the decades, none with great success. Some archaeologists spied parallels with the cuneiform of Mesopotamia. Others speculated an unlikely link between Harappan signs and the similarly inscrutable "bird-men" glyphs found thousands of miles away in the Pacific Ocean on Easter Island.

In 2004, perhaps out of befuddlement and frustration, a group of scholars declared that the script marked only rudimentary pictograms and that the Indus Valley people were functionally illiterate. That hypothesis, which caused a minor uproar in the world of Indus Valley researchers, was recently rejected by a team of mathematicians and computer scientists assembled from institutions in the U.S. and India. That team's study, published initially in April in Science and more extensively in August in the Proceedings of the National Academy of the Sciences, employed computer modeling to prove that the Harappan script communicated language, and has reinvigorated attempts to crack what is one of the lingering puzzles of ancient history.

The group examined hundreds of Harappan texts and tested their structure against other known languages using a computer program. Every language, the scientists suggest, possesses what is known as "conditional entropy": the degree of randomness in a given sequence. In English, for example, the letter t can be found preceding a large variety of other letters, but instances of tx and tz are far more infrequent than th and ta. "A written language comes about through this mix of built-in rules and flexible variables," says Mayank Vahia, an astrophysicist at the Tata Institute for Fundamental Research in Mumbai who worked on the study. Quantifying this principle through computer probability tests, the scientists determined that the Harappan script had a similar measure of conditional entropy to other writing systems, including English, Sanskrit and Sumerian. If it mathematically looked and acted like writing, they concluded, then surely it is writing.

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