🦴 Was Lucy truly our direct ancestor?

In the arid landscapes of the Afar region in Ethiopia, a series of bones dating back 3.4 million years could shake up what we know about our origins.

Recent discoveries, the result of over a decade of meticulous excavations, invite us to rethink the place occupied by the iconic Lucy and her species, Australopithecus afarensis. Far from being the sole representative of the human lineage at that time, she clearly shared her territory with other hominins who had distinct lifestyles.


This reassessment is based on the definitive attribution of an enigmatic fossil, the "Burtele foot," to the species Australopithecus deyiremeda. This advance, published in Nature, sheds light on a previously unknown diversity in the Pliocene. It suggests that several evolutionary paths were explored simultaneously, blurring the direct lines once drawn between ancestors. Bipedalism itself, a foundational characteristic, is revealed in different forms, each adapted to different environments and behaviors.

An unexpected cohabitation revealed by anatomy

The Woranso-Mille region is a paleontological site of great importance, as it offers tangible proof that two closely related hominin species coexisted in the same space and at the same time. The Burtele foot, with its opposable big toe and long, curved phalanges, has a fundamentally different anatomy from Lucy's. While the foot of A. afarensis is adapted for efficient terrestrial walking with an aligned big toe, that of A. deyiremeda retained a grasping ability inherited from a partially arboreal lifestyle.

This morphological difference indicates distinct locomotor strategies. A. deyiremeda likely practiced mixed locomotion, walking on the ground but also climbing trees with an ease that Lucy had lost. During bipedal walking, it probably propelled itself more from its second toe, a different mechanism from that of modern humans. This "experimentation" shows that the transition to exclusive bipedalism was a long and non-linear process.


The analysis of the teeth and jaws associated with the foot supports this distinction. The canines of A. deyiremeda are smaller and its mandible shows more primitive traits than that of A. afarensis. The discovery of a juvenile jaw allowed, through micro-tomography, the study of dental development. The observed growth pattern, similar to that of great apes and other australopithecines, confirms its status as a distinct species with its own biology.

Ecological niches to avoid competition

The prolonged coexistence of these two species in the same ecosystem implies that they were not in direct competition for the same resources. Isotopic analyses of dental enamel provide a clear answer. The diet of Australopithecus deyiremeda was mainly composed of C3-type plants, from forest trees and shrubs, such as fruits and leaves. In contrast, Lucy and her kin consumed a more varied mix including grasses and sedges (C4 plants), typical of more open environments.

This dietary specialization reflects a partitioning of the habitat. A. deyiremeda moved preferentially in denser wooded environments, exploiting the canopy for food and likely for movement. A. afarensis, on the other hand, exploited a wider range of environments, including shrub savanna areas. This ecological differentiation allowed the two lineages to share the territory without eliminating each other, illustrating a fundamental principle of evolutionary biology.

These discoveries call into question the status of direct ancestor long granted to Lucy's species. The presence of A. deyiremeda, with its more primitive traits closer to older species like A. anamensis, complicates the family tree. It becomes plausible that several branches of australopithecines thrived in parallel, and that our lineage descended from one of these branches, not necessarily the one represented by Lucy. The evolutionary landscape now appears more like a bush.