Three million years ago on the East African plains, a tense scene might have played out. A group of Australopithecus afarensis—small, upright-walking hominins—gathers around a carcass, quickly slicing off scraps of meat with sharpened stones. Their actions do not go unnoticed. From the tall grass, a Homotherium—a scimitar-toothed cat—lurks, poised to charge. The hominins scatter. But could they actually outrun the predator?
A new study, published in Current Biology1, has modeled how Australopithecus afarensis ran, offering the most detailed analysis yet of their locomotor abilities. The findings suggest that, while they were competent bipeds, their running capabilities lagged far behind those of later human ancestors.
"Our simulations revealed that Lucy wasn’t as efficient or as fast at running as modern humans," the researchers explain.
While humans today can sprint at speeds exceeding 20 mph, the best A. afarensis could likely manage was around 11 mph—barely enough to escape a determined predator.
The Running Experiment: Reconstructing Lucy in 3D
To test Australopithecus afarensis’ running ability, researchers digitally reconstructed the skeleton of Lucy, the most famous representative of the species. Where bones were missing, they used scaled versions of other Australopithecus fossils. They also incorporated comparative data from chimpanzees and modern humans to estimate muscle placement and function.
Using 3D modeling software, they then "fleshed out" the skeleton with muscles, simulating different configurations—from a more human-like anatomy to a more ape-like one. Finally, they tested multiple versions of A. afarensis with and without an Achilles tendon, a key structure that aids in running efficiency in modern humans but is absent in chimpanzees.
The digital models were then placed into a physics-based simulation program called GaitSym, which allowed researchers to simulate running motion, measure energy efficiency, and determine top running speeds.
Slow and Inefficient: The Limits of Early Hominin Locomotion
The results painted a clear picture: while A. afarensis was capable of running, it was neither fast nor efficient. The fastest simulated hominin reached just 11 mph—comparable to a slow jog in modern humans. Even untrained human sprinters can hit speeds of around 17.6 mph.
"The metabolic cost of running in Australopithecus afarensis was up to three times higher than in modern humans," the researchers report.
This means that every step took considerably more energy, making sustained running over long distances impractical. The presence or absence of an Achilles tendon also made a significant difference: when removed from the simulation, running efficiency dropped dramatically, reinforcing its importance in later hominin evolution.
Could Australopithecus afarensis Hunt or Escape?
These findings challenge the idea that A. afarensis could have engaged in endurance hunting, a strategy where early humans chased prey until it collapsed from exhaustion. This technique, seen in some modern hunter-gatherer groups, relies on a combination of efficient cooling (via sweating), a long Achilles tendon, and an energy-efficient gait—traits that appear to have developed later in hominin evolution.
If A. afarensis wasn’t built for endurance running, how did they obtain meat? Cut-marked bones dating to 3.4 million years ago suggest they had access to animal flesh, but whether they hunted or scavenged remains unclear. Given their limited running ability, scavenging—perhaps snatching scraps from a predator’s kill—seems the more likely scenario.
As for escaping predators, the data suggests they wouldn’t have been able to outrun a fast-moving threat. Instead, they may have relied on group coordination, vigilance, and possibly taking refuge in trees.
"If faced with a predator like a Homotherium, running away probably wasn’t an option," the researchers note.
A Step Toward Human Endurance
The study highlights a crucial transition in human evolution. While Australopithecus afarensis had already shifted to full-time bipedalism, they had not yet acquired the long-distance running abilities that characterize later members of the genus Homo.
This shift likely occurred around 2 million years ago with Homo erectus, whose longer legs, shorter arms, and well-developed Achilles tendon suggest a body built for efficient locomotion. The ability to run farther and faster would have given Homo erectus a major advantage in acquiring food and avoiding predators.
Rewriting the Story of Early Human Locomotion
For decades, Lucy has been a symbol of the evolutionary leap toward bipedalism. But this new research reminds us that walking on two legs didn’t immediately translate to efficient running. Instead, human endurance running was likely a gradual adaptation, appearing only with later hominins.
So, if an Australopithecus afarensis had been caught in the middle of an ancient savanna chase, the odds were not in their favor. Their best bet? Spot danger early, and climb fast.
Related Research
For those interested in further reading, here are additional studies on early human locomotion:
Bramble, D. M., & Lieberman, D. E. (2004). "Endurance running and the evolution of Homo." Nature, 432(7015), 345–352. DOI: 10.1038/nature03052
Pontzer, H., Raichlen, D. A., & Wood, B. M. (2014). "Hunter-gatherers as models in public health." Obesity Reviews, 15(Supplement 1), 12–23. DOI: 10.1111/obr.12174
Sockol, M. D., Raichlen, D. A., & Pontzer, H. (2007). "Chimpanzee locomotor energetics and the origin of human bipedalism." Proceedings of the National Academy of Sciences, 104(30), 12265–12269. DOI: 10.1073/pnas.0703267104
These studies, along with the latest findings on Australopithecus afarensis, continue to refine our understanding of how and when humans became the long-distance runners they are today.
Bates, K. T., McCormack, S., Donald, E., Coatham, S., Brassey, C. A., Charles, J., O’Mahoney, T., van Bijlert, P. A., & Sellers, W. I. (2025). Running performance in Australopithecus afarensis. Current Biology: CB, 35(1), 224-230.e4. https://doi.org/10.1016/j.cub.2024.11.025
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