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How do paleoanthropologists date hominin fossils?

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Illustration showing side-by-side reconstructions of five hominins in the human lineage: Australopithecus afarensis, Homo habilis, Homo erectus, Homo neanderthalensis, and Homo sapiens, with each figure labeled and depicted in a walking pose to highlight evolutionary changes in posture and body structure. A scale bar at the top right indicates relative height. — human evolution An artist's depiction of five species of the human lineage.

Paleoanthropologists determine the age of hominin fossils by employing a combination of relative and absolute dating techniques, often requiring interdisciplinary knowledge of the physical sciences.1

Relative Dating

Relative dating establishes the chronological order of fossils based on their position in geological layers. The principle of superposition dictates that in undisturbed sedimentary sequences, older layers are found at the bottom and younger layers at the top. Archaeologists and paleoanthropologists use stratigraphy to observe these successions. Additionally, chemical analysis, such as measuring the fluorine content in bones, helps verify if remains found in the same level are truly contemporary or if later intrusions have occurred.23

Absolute Dating

Absolute dating provides specific numerical ages, primarily through radiometric methods that measure the decay of radioactive isotopes:34

Potassium–argon dating: This is a critical method for dating early hominin sites, particularly those in East Africa, as it can date volcanic rocks millions of years old.35
Radioactive carbon dating: Used for organic remains, this method measures the decay of carbon-14. It is effective for specimens up to approximately 50,000 years old.34

These techniques allow researchers to construct a reliable chronology for human evolution.36