A mysterious fossil foot discovered in Ethiopia more than a decade ago has now been linked to its most likely owner, and the revelation adds an important twist to the story of how our ancient relatives lived.
The foot probably belonged to Australopithecus deyiremeda, a 3.4-million-year-old human ancestor that shared the East African region with the famous “Lucy” species. But unlike Lucy’s kind, which was evolving toward more human-like walking, this species kept its tree-climbing abilities, new research reveals.
Scientists from Arizona State University’s Institute of Human Origins spent years searching for the missing pieces of this evolutionary puzzle. When they discovered the peculiar foot bones in 2012 at the Woranso-Mille site in Ethiopia, they knew they had something special. The foot had long, curved toes built for grasping branches and a flexible structure that looked very different from the rigid, arched feet of Lucy’s species, Australopithecus afarensis.
But there was a problem. Without teeth or jaw bones from the same individual, researchers couldn’t definitively identify which species the foot belonged to. The puzzle deepened because the foot came from the same geological layers that had produced fossils of A. deyiremeda, a species named just three years earlier, but the connection couldn’t be proven.
Now, newly discovered fossils have provided strong evidence for the connection. A juvenile jawbone with nearly intact baby and adult teeth, along with several isolated teeth from different individuals, all show the distinctive dental features of A. deyiremeda. These specimens came from the exact same ancient riverbed deposits as the foot, giving scientists solid reasons to link the two.
Curved Toes and Grasping Feet Suggest Tree-Climbing Lifestyle
The findings, published in Nature, show the story of human evolution was never as simple as textbooks made it seem. While Lucy’s species was developing feet adapted for efficient walking on the ground, A. deyiremeda took a different path, keeping features that made it comfortable in the trees as well as on the ground.
The foot bones tell the story. Long toe bones curve from front to back like those of modern apes. The second metatarsal bone (one of the long bones in the middle of the foot) shows unusual twisting and curvature. Most telling of all, the big toe bone is gracile and curved in ways that would have allowed A. deyiremeda to grasp tree branches.
“The foot displays morphological features that suggest a greater ability for grasping than modern humans and most fossil hominins,” the researchers write. The foot structure shows clear adaptations for climbing that simply aren’t present in A. afarensis fossils from the same time period.
The partial foot also lacks the rigid arch structure that makes human feet efficient for walking. Instead, it retains flexibility in the midfoot region, useful for wrapping around branches but less ideal for the kind of obligate bipedality that characterized Lucy’s kind.
Ancient Teeth Solve Decade-Old Mystery
Connecting the foot to A. deyiremeda required detective work spanning more than a decade. The breakthrough came with specimen BRT-VP-2/135, a juvenile jaw discovered about 300 meters from where the foot had been found. The jaw preserves both baby teeth and adult teeth still forming inside the bone, offering a wealth of diagnostic information.
Using micro-CT scanning technology, researchers could examine teeth that weren’t visible from the outside, still tucked away in their developmental crypts within the jawbone. The scans revealed features distinctive to A. deyiremeda that don’t appear in A. afarensis.
The canine teeth lack the prominent vertical ridge on the inner surface that characterizes A. afarensis canines. The premolars have a more primitive shape with cusps positioned more centrally and oriented at different angles than those of Lucy’s species. The upper canines and first molars are smaller than any other australopith species measured, and the molars are much narrower from side to side.
The jaw itself was missing the lateral hollow (an indentation on the outer surface) that’s diagnostic of A. afarensis jaws. Every detail pointed to A. deyiremeda.
Additional isolated teeth from the same geological horizon showed the same distinctive features. With multiple specimens all displaying the diagnostic traits of A. deyiremeda and coming from the exact location and time period as the mysterious foot, researchers had strong evidence to make the connection.
Chemical Analysis Shows Forest Diet Different from Lucy’s
Chemical analysis of tooth enamel provided a window into what A. deyiremeda ate millions of years ago. The research team drilled tiny samples of enamel powder from eight fossil teeth and analyzed their carbon isotopes. Plants use different photosynthetic pathways that leave distinct chemical signatures, and these signatures get locked into tooth enamel as it forms.
The results showed A. deyiremeda ate primarily what scientists call C3 foods, which come from forests and woodlands. These include fruits, leaves, seeds, and underground storage organs like roots and tubers. The isotope values (ranging from -12.4 to -8.8 per mil) matched those of earlier human ancestors like Ardipithecus ramidus and early Australopithecus anamensis, both of which lived in wooded environments.
By contrast, A. afarensis living at nearby sites during the same time period had much more variable isotope values. Lucy’s species ate from both forest plants and grasses from more open habitats, showing dietary flexibility that A. deyiremeda apparently lacked or didn’t need.
The dietary evidence fits well with the foot anatomy. A species spending significant time in trees would naturally have better access to forest foods. The combination of climbing-adapted feet and forest-food diet creates a coherent picture of A. deyiremeda‘s lifestyle.
Early Humans Took Multiple Paths to Walking Upright
For decades, the story of Lucy’s species dominated discussions about how our ancestors transitioned from living in trees to walking upright on the ground. But A. deyiremeda shows there wasn’t just one path.
Both species walked on two legs at least some of the time. But while A. afarensis was evolving rigid feet with arches that made walking more efficient, A. deyiremeda kept feet that could do double duty, handling both ground-based walking and tree climbing.
Study authors point out that large grinding teeth evolved before obligate human-like bipedality. A. deyiremeda had big molars adapted for processing tough plant foods, but it hadn’t given up its climbing adaptations. The package of features didn’t evolve as a unit, the way simple evolutionary narratives might suggest.
This mosaic evolution, where different body parts evolve at different rates and in different directions, appears to have been common among mid-Pliocene human ancestors. Multiple species coexisted, each experimenting with different combinations of primitive and derived features.
The juvenile jaw that helped solve the mystery belonged to an individual who died around 4.5 years old, based on tooth development patterns. The child still had baby teeth in place alongside emerging permanent teeth. The developmental pattern looked more like a chimpanzee’s than a modern human child’s, with incisors delayed in their formation relative to the first molars.
The research team worked at the Woranso-Mille site in Ethiopia’s Afar region, where erosion exposes layers of ancient sediments. The fossils came from sandstone deposits above a volcanic ash layer dated to 3.469 million years ago. Twenty-five hominin specimens have been recovered from the Burtele localities so far.
The evidence joins a growing body of research showing that human evolution wasn’t a simple progression from tree-dwelling apes to ground-walking humans. Instead, the mid-Pliocene saw multiple species coexisting in East Africa, each experimenting with different strategies for moving, eating, and surviving. A. deyiremeda kept its climbing abilities while also walking on two legs, while A. afarensis went further down the path toward obligate bipedality.
Source : https://studyfinds.org/mystery-foot-tree-climbing-human-ancestor-australopithecus/