New York: Researchers have prepared the first spatially explicit data set of the location and size of urban settlements globally over the past 6,000 years -- offering fresh clarity as to how the growth of cities transforms humans into an "urban species". The researchers from the Yale School of Forestry and Environmental Studies created maps through digitising, transcribing, and geocoding a deep trove of historical, archaeological, and census-based urban population data previously available only in tabular form. The study, published recently in the journal Scientific Data, has made it possible to access information on urban centres from 3700 B.C. to A.D. 2000. "To better understand urbanisation today it is helpful to know what urbanisation looked like through history," said lead author Meredith Reba. "By understanding how cities have grown and changed over time, throughout history, it might tell us something useful about how they are changing today," she added. The findings have broad applications. The dataset offers an important first step toward understanding the geographic distribution of urban populations throughout history and across the world. Currently the only spatially explicit data available at a global scale is the United Nations World Urbanisation Prospects, which provides population values, latitudes, and longitudes for places with populations of 300,000 or more. However, it goes back only to 1950. The new study allows researchers to map and visualise city level population changes through time. For example, Istanbul in Turkey underwent a major period of population decline between AD 1057 and AD 1453. During this time the population dropped from approximately 300,000 to 45,000 due to a series of events including a city sacking by the Crusaders and a bout with the plague. According to the researchers, the ability to pinpoint the size and location of human populations over time will help researchers understand the evolving characteristics of the human species -- particularly human interactions with the environment.