Abstract
Africa is set to drive future global urbanization, yet the evolution of its urban systems remains underexplored. Here we show this evolution during 1950–2020 using three urban system rules based on a unified urban definition across the African continent. Zipf’s law quantified increasing population concentration in large cities, leading to a shift from dispersed to centralized city-size distributions. As Gibrat’s law predicted, urban population growth was independent of its initial size since the 1990s. Relative growth among different-sized cities changed urban hierarchy, thus the integration of Zipf’s and Gibrat’s laws uncovered imbalances in urban system structures. The third rule we focus on is the scaling law, where unexpected super-linear scaling relationships between built-up areas and population size indicated lower land-use efficiency in large cities, particularly in Eastern and Western Africa. However, the overall decline in built-up scaling exponents over time suggested improving economies of scale as urban systems matured. This study evidenced simple rules behind complex urban systems that help us understand broader urban complexities and urbanization dynamics.
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Data availability
The population and built-up areas of African urban agglomerations are publicly available from Africapolis (https://africapolis.org/en). The total urban population and urbanization level of Africa are from the World Population Prospects 2022 (https://population.un.org/wpp/). Macro socio-economic statistics at the country level, such as GDP per capita, are from the World Bank (https://data.worldbank.org/). The geospatial data of administrative boundaries for mapping are from the Global Administrative Areas (GADM; https://gadm.org/).
Code availability
Statistical analyses are performed using R programming (version 1.4.1717) and maps are drawn using ArcGIS (version 10.6). The R code is available on GitHub at https://github.com/xugang2016/african-urban-system.
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Acknowledgements
This research was supported by the National Natural Science Foundation of China (42101460 to G.X., 42201373 to B.C. and 42371423 to L.J.), the International Science and Technology Cooperation Project of Hubei Province, China (2023EHA001 to G.X.), the Fundamental Research Funds for the Central Universities (2042024kf0004 to G.X.), the Natural Science Foundation of Hubei Province, China (JCZRYB202400503 to G.X.), the Research Grants Council of Hong Kong Early Career Scheme (HKU27600222 to B.C.) and General Research Fund (HKU17601423 to B.C. and HKU17307024 to P.G.), and the Croucher Foundation (CAS22902/CAS22HU01 to P.G.). The funders had no role in study design, data collection and analysis, decision to publish or preparation of the manuscript. We greatly appreciate the Africapolis project for providing the valuable dataset to understand urbanization in Africa.
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G.X., L.J. and P.G. designed the research framework. B.C., M.S. and Z.X. improved the research design. G.X. and M.Z. developed the algorithm, performed data analyses and wrote the draft. B.C., P.B.C., X.L., N.S.S. and X.Z. analyzed and interpreted the results. All authors contributed to editing the paper.
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Nature Cities thanks Rafael Prieto-Curiel, Ivan Turok and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Xu, G., Zhu, M., Chen, B. et al. Underlying rules of evolutionary urban systems in Africa. Nat Cities 2, 327–335 (2025). https://doi.org/10.1038/s44284-025-00208-y
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DOI: https://doi.org/10.1038/s44284-025-00208-y
