3D models of Australopithecus afarensis hint at the muscular adaptations that made modern humans better runners
Ancient human relatives ran on two legs, like modern humans, but at a much slower pace, suggest 3D computer simulations of Australopithecus afarensis – a small hominin that lived more than three million years ago.
The analysis offers a detailed snapshot of the hominin’s running speed and the muscular adaptations that enabled modern humans to run long distances, says Herman Pontzer, an evolutionary anthropologist at Duke University in Durham, North Carolina. The findings were published this week in Current Biology.
A. afarensis walked upright on two legs, making its fossils a favourite for researchers looking to unpick how bipedalism evolved in the human lineage. But few studies have explored the hominin’s running ability because it requires more than studying fossilized footprints and bones, says study co-author Karl Bates, an evolutionary biomechanics researcher at the University of Liverpool, UK.
### Lucy’s Speed Limit
Bates and his colleagues created a 3D digital model of the ‘Lucy’ skeleton – a near-complete 3.2-million-year-old A. afarensis specimen discovered in Ethiopia half a century ago. They used the muscular features of modern apes and the surface area of Lucy’s bones to estimate the ancient hominin’s muscle mass. The researchers then used a simulator to make their Lucy model ‘run’ and compared its performance with that of a digital model of a modern human.
The simulations showed that Lucy could run on two legs, despite lacking the lengthened Achilles tendon and shortened muscle fibres that benefit endurance running in modern humans. But speed wasn’t Lucy’s strength: she could reach a maximum of only around five metres per second, even after the researchers remodelled her with human muscles. By contrast, the human model ran at roughly 8 metres per second.
### Energy Expenditure
Next, the researchers assessed whether certain muscles have a role in energy expenditure during running. When they added human-like ankle muscles to the Lucy model, the energy cost was comparable to that of other animals of a similar size. But running became more taxing for Lucy when the team replaced the human ankle muscles with ape ones. This suggests that adaptations in the Achilles tendon and surrounding muscles enable modern humans to run for extended periods.
Bates and his colleagues are now planning to investigate whether fatigue and bone strain also influenced Lucy’s running.
### Conclusion
While early human ancestors like Australopithecus afarensis were capable of jogging on two legs, they were not built for long-distance running like modern humans. Understanding the muscular adaptations that enabled humans to become efficient long-distance runners sheds light on the evolution of bipedalism and endurance in the human lineage. Further research into ancient hominins’ running abilities may provide more insights into the physical capabilities of our early ancestors.
Reproduced with permission from the original article published on December 19, 2024.