Historians of science in India record that the foundations of scientific thought were laid down as early as the Harappan civilization, also known as the Indus civilization, which lasted from roughly 3100 to 1900 BCE. Facts supplied by archaeologists support this thesis. Covering a little less than half a million square miles, the Harappan civilization was a complex of city-centered communities of agriculturists and craftspeople. The Harappans were a disciplined people, and this discipline was visible in all walks of life. For example, they have been credited with the knowledge of the science of yoga. TheHarappans invented an alphabetic system of writing that simplified a partly pictographic script, using simple cursive signs with basic phonetic values. Harappans became pioneers in studying tides, waves, and currents and put their knowledge to practical use to build tidal docks. There is circumstantial evidence for maritime trade. Extensive trade provided the stimulus for the development of an elaborate system of weights and measures. Archaeologists have also found evidence of a rudimentary astronomical system.