A new study led by a researcher at Penn State University suggests that the way humans walk—by landing on the heel first—allowed early humans to walk longer distances more efficiently. However, this walking style comes with a trade-off: it places greater impact forces on bones and joints. The findings, published in the Proceedings of the National Academy of Sciences, suggest that this evolutionary adaptation helped early humans develop hunting and gathering strategies and spread across the globe. The study challenges previous assumptions that humans share the heel-strike walking style with other great apes, such as chimpanzees, and instead reveals significant differences in how humans and chimpanzees walk. To investigate, researchers used high-speed, high-resolution video cameras and a flat runway lined with metal plates that measure impact forces. They studied how three male chimpanzees walked on all fours and on two legs at Stony Brook University. Additional data from other studies involving 15 chimpanzees were also analyzed. Meanwhile, 12 human participants walked on a similar runway at the University at Buffalo, both using their natural heel-first gait and an altered "midfoot strike" that mimicked how chimpanzees typically walk. The researchers also used a portable respirometry system to measure the metabolic energy used by the human participants during different walking styles. The study found that chimpanzees vary their walking style, using a mix of heel and midfoot strikes. When walking on two legs, they used fewer heel strikes than when moving on all fours, suggesting a preference for a "softer" landing. In contrast, all human participants used heel strikes, with a very consistent angle of foot contact relative to the ground. The range of foot-strike angles in chimpanzees was 2.4 to 8.6 times greater than in humans. Researchers also found that heel strikes generated significantly higher impact forces than midfoot strikes in both species. In chimpanzees walking on two legs, heel strikes caused up to 138% more force than midfoot strikes, while in humans, the difference was up to 162%. Despite the higher impact forces, heel-first walking saved human participants an estimated 26% to 41% in metabolic energy compared to walking on the ball of the foot. This efficiency likely helped early humans travel long distances more easily, supporting survival strategies like hunting and gathering. However, the high impact forces required later anatomical adaptations, such as stronger bones and joints, to allow safe, consistent walking. The researchers suggest that while modern environments with hard, paved surfaces may affect how our bodies handle these forces, walking remains a healthy exercise. Our bodies evolved to cope with the impact, and we shouldn’t avoid it.