Did this ancient hominin ancestor walk upright over five million years ago?

Nature · Published 3 October 2026 Swipe more like this →

In short: Ardipithecus kadabba, a hominin species living between 5.8 and 5.2 million years ago, occupies a critical place in our evolutionary history. While known primarily from teeth and jaw fragments, a single toe bone suggests these early ancestors may have already begun the transition to bipedalism, long before the rise of Australopithecus.

Discovered in the middle Awash River valley of Ethiopia, Ardipithecus kadabba represents one of the earliest known hominin species. Initially classified as a subspecies of Ardipithecus ramidus, it was elevated to a distinct species in 2004 by researchers Yohannes Haile-Selassie, Gen Suwa, and Tim D. White. This reclassification was driven by the discovery of teeth exhibiting primitive morphology and wear patterns that set them apart from their successors. The name itself, derived from the Afar language, translates to 'basal family ancestor,' reflecting its status as a likely chronospecies in the lineage leading toward later hominins.

The physical evidence for Ar. kadabba is sparse, consisting of nineteen specimens including limb fragments, jaws, and isolated teeth. A significant point of interest is a toe bone recovered from the same age range, which displays a humanlike shape indicative of upright walking. However, because this bone was found in sediments geographically and stratigraphically separate from other remains, its definitive link to the species remains a subject of scientific debate. Some researchers, such as Bernard Wood and Nicholas Lonergan, have questioned the species' placement on the human line, citing its relatively large, ape-like canines as a point of contention.

Despite these debates, Ar. kadabba provides a vital window into the Miocene epoch. Unlike modern apes, it lacks the interlocking honing complex—a feature where canines sharpen against each other—which is considered a key evolutionary shift. By studying these fragments, paleoanthropologists attempt to map the lineage of early humans, potentially linking Ar. kadabba to Ar. ramidus and subsequent species like Australopithecus. Whether these creatures were direct ancestors or merely close relatives remains one of the most compelling questions in the study of human origins.

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