Humans have always wondered where they came from. Fossils contain clues, and scientists search for them carefully. Each discovery reshapes what we know about evolution.
A new study from Washington University in St. Louis has reignited a century-old debate. Researchers revisited Ardipithecus ramidusor “Ardi,” the 4.4 million-year-old ancestor who could bridge the gap between apes and humans.
Ardi lived long before “Lucy” and provides a window into how early ancestors balanced tree life and land life. Previous studies claimed that Ardi moved differently from modern African apes.
A new look at Ardipithecus ramidus
Scientists took a close look at the ankle bone (astragalus) of Ardipithecus ramidusas well as the talus and calcaneus. Compared to those of apes and early humans, the results are remarkable.
A key angle – the “talus angle,” essentially the tilt of the ankle joint – was 14.5°, the highest of any fossil hominid studied and squarely in the range of non-human primates, like gorillas and bonobos.
They also noted a deep groove for the tendon of the flexor hallucis longus (a flexor of the big toe), as well as a lined articular surface – together forming a mosaic: monkey-like features for climbing combined with adjustments that would help “push off” during bipedal walking.
These values suggest powerful ankles for climbing and strong support for walking upright.
Crucial stage of evolution
The authors even point out another Ardipithecus ankle from Ethiopia that might have had a smaller angle, hinting at normal variation that natural selection might later shape toward better two-legged walking.
This balance of movement marks a crucial stage of evolution. Ardi was not entirely adapted to the ground but was not tied to trees either. Its bones bear witness to a lifestyle in transition.
“One of the surprises of this discovery was that Ardi walked upright, while still retaining many ape-like features, including a grasping foot,” said Thomas (Cody) Prang, assistant professor of biological anthropology.
The team’s work corrects previous interpretations that separated Ardi too much from the apes. Instead, it shows that Ardi’s anatomy kept one foot in the trees and another on dry land.
Evolution by similar paths
The study also compared Ardi’s anatomy to that of New World monkeys like those in the Atelidae family. The two groups independently evolved similar climbing traits, a case of convergent evolution.
This means that evolution has found similar solutions to similar challenges, even in distantly related species.
Ardi’s ankle didn’t evolve by chance. It adapted to handle both climbing and limited vertical walking, paving the way for later hominids.
Prang and his co-authors concluded that Ardi’s anatomy allows movements similar to those of African apes – a mix of vertical climbing and walking on all fours with heels touching the ground.
“Based on their analysis, they concluded that living African apes – like chimpanzees and gorillas – are like dead ends or dead ends of evolution, rather than stages of human emergence,” Prang said.
This discovery challenges the long-held view that humans evolved from a general arboreal ancestor. Instead, it suggests that early humans came from apes already skilled at climbing and moving on the ground.
“No one disputes the importance of (Ardi’s) discovery, of course, but many people in the field would say that the initial interpretation was probably wrong. And so, this paper is a correction of that initial idea that distanced Ardi from chimpanzees and gorillas,” Prang noted.
Questions left unanswered
The team used detailed 3D measurements and statistical tests to sort ankle shapes and estimate the evolution of different traits.
They recognize some overlap – different animals may share similar ankle shapes – so no single measurement “proves” behavior.
But the overall ankle pattern, along with other skeletal clues, strongly indicates vertical climbing associated with early forms of upright walking.
The study also notes that there may have been normal variation within Ardipithecus populations that natural selection could later transform into more efficient bipedalism.
Ardipithecus and human evolution
These findings challenge the idea that our last common ancestor with chimpanzees had a more generalized, purely arboreal body plan.
Instead, the evidence on the ankle suggests that early hominids – including Ardipithecus – retained a climbing style similar to that of African apes while beginning to experiment with upright walking.
The authors say it is difficult to reject the “African ape ancestor” model based on what we see in Ardipithecus the hands, the feet, the waist of the body and now the ankle.
In short: Early hominids probably moved through trees like apes and also took the first steps toward bipedalism.
The results do not suggest that humans evolved directly from chimpanzees. Instead, the two species share a common ancestor that moved like modern chimpanzees.
Lessons from Ardipithecus ramidus
This idea changes the way scientists think about bipedalism. It was not a sudden leap from the trees, but a gradual process based on strength, balance and adaptation.
“This finding is both controversial and consistent with what people originally thought,” Prang concluded.
Ardi’s story proves that evolution is rarely simple. It twists, overlaps and sometimes revisits old forms.
By reconstructing bones like his, scientists can take a step closer to answering the age-old question: Where did we come from?
The study is published in the journal Communication biology.
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