25,000 Inhabitants, 2,500 Years Ago

How do archaeologists estimate the size of ancient populations?

Ruins of an abbey

Archaeologists in eastern India have found remains pointing to the existence of a highly developed urban settlement, the BBC reported on Monday. On the basis of recently completed excavations, the research team believes the city had approximately 25,000 inhabitants in the fifth century B.C. How do archaeologists estimate ancient populations?

Fieldwork and guesswork. If archaeologists are lucky, they might uncover written documents—on papyrus or stone tablets—that shed light on the makeup of a long-lost civilization. A Babylonian letter-writer might have noted the proximity of his nearest neighbors. A traveler’s log found at an adjacent site might mention the size of the town in question. Archaeologists also try to extrapolate population figures from a known quantity, like the size of the burial ground. Based on the amount of space allotted to one set of skeletal remains, archaeologists can approximate how many people were buried in a cemetery.

Document analysis and burial-ground excavations are not foolproof methods. Besides being difficult to find, well-preserved letters and travelers’ logs contain subjective observations, which can’t be relied upon in isolation. Since burial practices differ among cultures, there’s no one-to-one relationship between the size of a cemetery and the local population. It might have been traditional to bury adults in a cemetery but not children, or to cremate the elderly and those who died prematurely from illness. Outside information is necessary, but not always available.

Estimations based on residential density are also common practice—the more homes you find, the more people lived there. Soil analysis, pottery shards, old foundation walls, and hearth remains help researchers differentiate buildings from gardens or farmland. Architectural features distinguish residential from civic structures (the latter are larger, and more ornate) and make it possible for archaeologists to establish a density estimate across a sample area—25 homes per hectare, for example. Then they guess how many people lived in each household, on average. If no records are available to illuminate domestic arrangements, researchers study modern village populations in the same area to arrive at a rough estimate—perhaps four people per domicile. Then the archaeologists multiply the number of individuals per household by the households per hectare, and again by the total settlement area.

No matter the methodology, population estimates always involve broad assumptions. Even evidence drawn from an ancient census can be unreliable. Some omitted women, slaves, and children, and we only have access to incomplete records of others. As a result, figures issued by different research teams often vary wildly. Estimations for ancient Alexandria, for example, range from less than 100,000 to almost 1 million.

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Explainer thanks Diana Ng and Christopher Witmore of the Joukowsky Institute for Archaeology and the Ancient World.