Recent discoveries of fossilized bones have enabled researchers to connect a mysterious 3.4-million-year-old hominin foot, initially unearthed in 2009, to a distinct species separate from the renowned Lucy fossil. This crucial association bolsters the evidence suggesting that two distinct early human ancestor species coexisted in the identical geographic area during that era.
Discovery of the Enigmatic Burtele Foot
Back in 2009, a team of scientists under the leadership of paleoanthropologist Yohannes Haile-Selassie from Arizona State University excavated eight bones comprising the foot of an early human relative from sediments dating back 3.4 million years in Ethiopia’s Afar Rift region. Dubbed the Burtele Foot, this fossil was retrieved from the Woranso-Mille paleontological locality and was officially documented in a scientific publication released in 2012.
“Upon discovering the foot in 2009 and publicizing it in 2012, we immediately recognized its differences from the species of Lucy, known as Australopithecus afarensis, which is extensively documented from that period,” explained Haile-Selassie, who serves as director of the Institute of Human Origins and a professor within Arizona State University’s School of Human Evolution and Social Change.
“That said, in paleoanthropology, it’s uncommon to designate a new species solely based on postcranial remains—those parts of the skeleton situated below the neck. Our hope was to locate cranial remains, jaws, or teeth in direct association with the foot. Typically, such elements like skulls, jawbones, and dentition form the basis for identifying and classifying species.”
Linking the Burtele Foot to Australopithecus deyiremeda
At the time the Burtele Foot was first scientifically described, several teeth had already been found in the nearby vicinity. Nonetheless, there was uncertainty regarding whether these teeth originated from the precise sedimentary layer as the foot itself. In 2015, the research group proclaimed the identification of a novel species from the area, named Australopithecus deyiremeda, yet they refrained from attributing the Burtele Foot to it at that stage, despite some fossils being unearthed in very close proximity, as noted by Haile-Selassie.
Throughout the subsequent ten years, multiple excavation campaigns and further fossil recoveries have provided the scientists with a more comprehensive dataset. According to Haile-Selassie, they now possess sufficient evidence to definitively associate the Burtele Foot with A. deyiremeda.
Coexistence of Two Hominin Lineages in Shared Habitats
Assigning the Burtele Foot to a particular species represents just one facet of a broader narrative. The significance of the Woranso-Mille site lies in its provision of unequivocal proof that two closely related hominin species inhabited the same territory concurrently.
The Burtele Foot, now linked to A. deyiremeda, exhibits more primitive characteristics compared to the feet belonging to Lucy’s species, A. afarensis. In contrast to Lucy, this foot retained an opposable big toe, a feature advantageous for arboreal climbing activities. Nevertheless, on terrestrial surfaces, A. deyiremeda was bipedal, propelling itself forward mainly via the second toe instead of the big toe—a departure from the modern human gait pattern.
“The abducted big toe observed in Ardipithecus ramidus came as a major shock, given that at approximately 4.4 million years ago, an early hominin still possessed this climbing-adapted trait, which was entirely unanticipated,” Haile-Selassie remarked.
“Then, just one million years afterward, at 3.4 million years ago, the Burtele Foot emerges, which is even more astonishing. This occurs during an epoch dominated by species such as A. afarensis, whose individuals displayed complete bipedalism with an aligned big toe. This indicates that bipedal locomotion among these primordial human forebears manifested in diverse styles. Discoveries like the Burtele Foot illustrate that early bipeds employed multiple terrestrial walking strategies, rather than a singular method that only evolved later on.”
Isotopic Analysis Reveals Distinct Dietary Preferences
To gain deeper insights into the dietary habits of A. deyiremeda, Naomi Levin, a professor at the University of Michigan, employed isotopic analysis on eight out of the 25 teeth unearthed from the Burtele locality. The process involves meticulously cleaning the tooth exterior before extracting solely the enamel layer for examination.
“I utilize a dental drill equipped with a minuscule bit, smaller than 1 millimeter—similar to tools employed by dentists,” Levin described. “Using this, I extract tiny quantities of enamel powder, which I then collect in a plastic container and transport to our laboratory at the University of Michigan for detailed isotopic evaluation.”
The results proved startling.
Whereas Lucy’s species, A. afarensis, maintained a varied diet incorporating both C3 plants (derived from trees and bushes) and C4 vegetation (such as tropical grasses and sedges), A. deyiremeda showed a stronger dependence on C3 resources.
“The clarity and consistency of the carbon isotope signatures, mirroring those from earlier hominins like A. ramidus and Au. anamensis, caught me off guard,” Levin shared. “I anticipated greater difficulty in distinguishing the diets of A. deyiremeda and A. afarensis, but the data distinctly demonstrate that A. deyiremeda did not exploit the same breadth of food sources as A. afarensis, marking the latter as the earliest hominin known to incorporate C4 grass-related foods.”
Precise Dating and Paleoenvironmental Reconstruction
A vital component of the study entailed accurately determining the chronological placement of the fossils and recreating the environmental contexts these hominins occupied. Correlating fossil-bearing strata across spatial and temporal dimensions is essential for discerning the timelines and ecological niches of each species.
“Our extensive fieldwork at Woranso-Mille has meticulously mapped the relationships among various fossil layers, which is fundamental to grasping the temporal and ecological frameworks of these species,” stated Beverly Saylor, a professor of earth, environmental, and planetary sciences at Case Western Reserve University. Saylor spearheaded the geological investigations that confirmed the stratigraphic connection between the foot and A. deyiremeda.
Insights from a Juvenile Jaw into Ontogeny
In addition to the 25 teeth from Burtele, Haile-Selassie’s team recovered a juvenile jawbone that, through dental morphology, was unmistakably attributable to A. deyiremeda. Gary Schwartz, a research scientist at the Institute of Human Origins and professor in the School of Human Evolution and Social Change, observed that this specimen housed a full complement of deciduous teeth, alongside numerous permanent teeth in advanced stages of formation within the mandible.
Employing computed tomography (CT) scans, the team visualized the entirety of the developing dentition. Given the tight correlation between dental eruption and somatic growth, these observations permitted an age estimation of approximately 4.5 years at death for the individual.
“In this young hominin, we identified pronounced disparities in maturation rates between anterior incisors and posterior molars, akin to patterns in extant apes and fellow early australopiths including Lucy’s kind,” Schwartz noted.
“The most striking revelation is that, notwithstanding the profound diversity among early australopith species—in body size, feeding ecology, movement patterns, and skeletal morphology—their developmental trajectories appear strikingly uniform.”
Ecological Niches Enabling Sympatric Hominins
Integrating data on locomotion, nutrition, and habitat yields fresh perspectives on the mechanisms permitting multiple hominin taxa to occupy overlapping ranges without competitive exclusion. Variations in gait, climbing proficiency, and foraging strategies likely facilitated niche partitioning across the shared terrain.
“Our investigations into ancient ecosystems from epochs millions of years distant transcend mere intellectual curiosity or ancestral genealogy,” Haile-Selassie emphasized. “They inform our comprehension of contemporary dynamics and prospective trajectories alike.”
“Without a firm grasp of history, we cannot adequately interpret the present or anticipate the future. Phenomena akin to today’s climatic shifts recurred frequently during the eras of Lucy and A. deyiremeda. Lessons from those periods hold potential to alleviate severe repercussions of modern climate change.”
Research Publication and Collaborative Efforts
The study, titled “New finds shed light on diet and locomotion in Australopithecus deyiremeda,” is published in the prestigious journal Nature. The collaborative effort involved experts from Arizona State University, Washington University in St. Louis, Case Western Reserve University, Berkeley Geochronology Center, Universitat de Barcelona, University of Tampa, and University of Michigan. The author roster comprises Yohannes Haile-Selassie, Gary T. Schwartz, Thomas C. Prang, Beverly Z. Saylor, Alan Deino, Luis Gibert, Anna Ragni, and Naomi E. Levin.
Support for this project was provided by the National Science Foundation and the W.M. Keck Foundation. Excavations and analyses in Ethiopia were facilitated by the Ethiopian Heritage Authority.
