Austrian ethnologist Christoph von Fürer-Haimendorf theorized that untouchability originated as class stratification in urban areas of the Indus Valley Civilisation. According to this theory, the poorer workers involved in 'unclean' occupations such as sweeping or leather work were historically segregated and banished outside the city limits. Over time, personal cleanliness came to be identified with "purity", and the concept of untouchability eventually spread to rural areas as well. After the decline of the Indus Valley towns, these untouchables probably spread to other parts of India. Scholars such as Suvira Jaiswal reject this theory, arguing that it lacks evidence, and does not explain why the concept of untouchability is more pronounced in rural areas.