10.4: The Evolution of Upright Walking

Rethinking Our Origins

Chances are that when you think of human evolution (or Google it) you’ll likely get that iconic image of a chimp walking on all fours and slowly evolving into a human. As we’ve learned in previous lessons, this image is both misleading (evolution does not happen in a linear fashion) and incorrect (chimps are our cousins, not our ancestors), but it stems from earlier ideas about the evolution of bipedalism itself.

For decades, scientists believed that humans evolved from large, knuckle-walking ancestors similar to modern chimpanzees and gorillas. This seemed logical since we share close genetic relationships with these great apes. However, the fossil evidence tells a different story. As paleoanthropologist Jeremy DeSilva notes, "not a single fossil has been found from a large, knuckle-walking ape that lived when humans, chimps, and gorillas split from their common ancestor."²

Instead, the fossils from this crucial time period during the Miocene (23-5.3 million years ago) come from small-bodied apes that hung by their arms and had flexible backs that supported upright posture. This evidence has revolutionized our understanding of how and why humans began walking on two legs, leading many scholars to conclude that early human ancestors never passed through a dedicated knuckle-walking phase.

A recent discovery has provided some of the most compelling evidence for this new understanding of bipedalism's origins. Found in 2016, Danuvius guggenmosi, a small-bodied, 11-million-year-old fossil ape from Germany, shows remarkable upright adaptations in its legs, ankles, and vertebrae. Its ankle joint is remarkably similar to both modern humans and Lucy’s species, and its spine exhibits the characteristic curves that support upright posture.

This discovery, along with studies of modern gibbons, has led DeSilva to propose a revolutionary idea: that the iconic image of “the March of Progress may be backward. We didn't evolve bipedalism from a knuckle-walking ancestor. Instead, knuckle-walkers evolved from apes that were, at least occasionally, bipedal. In other words, bipedalism isn't a new locomotion at all, just an old locomotion in a new setting."³

What Made Bipedalism Possible: The Anatomy

Walking upright required significant changes to the human body. These anatomical adaptations tell the story of our evolutionary journey.

Position of foramen magnum - In bipeds, the foramen magnum is centralized on the bottom of the skull, closer to the teeth so that the head sits on top of the trunk of the body.

S-Shaped Spine - The spine has a characteristic S-shape curvature with vertebrae that increase in size and robusticity as you move down the spine towards the pelvis. This S-shape curvature aids in bipedalism by keeping the head centered above the pelvis to maintain balance, while the larger (lower) vertebrae help support the weight of the body. The curve at the base of the lumbar spine is unique to upright walking and helps to absorb shock while walking or running.

Shortened, broad pelvis - A biped's pelvis has a very different shape from an ape pelvis. Especially distinctive is the short ilium (hip bones). This morphology is an essential element of the stability of the pelvis during standing, walking, and running. The round, bowl shaped structure with robust hip bones is also important for supporting upper body weight.

Repositioned Gluteal Muscles - In chimpanzees, the lesser gluteal muscles attach to the back of the pelvis, which helps them extend their legs backward for climbing. However, this positioning makes them unstable when walking upright and requires much more energy. In humans, these muscles moved to the sides of our bodies. When we take a step, these muscles contract to keep us balanced and upright.

Length of the leg: In bipeds, the leg is generally longer relative to the arm.

Bicondylar Angle - Human femurs (thigh bones) tilt inward at the knee joint, creating what scientists call the bicondylar angle. Interestingly, human babies are born with straight femurs, but as they begin to walk, this signature angle develops. Chimpanzees never develop this angle, and neither do humans who for various reasons never walk.

Specialized Feet - Since the feet are the only part of our skeleton that actually touches the ground while walking, they show some dramatic adaptations. Bipedal feet have large, robust heels that can withstand the impact of heel strikes, rigid ankles, long Achilles tendons, an arch that acts as a spring to counteract the weight of the body pressing down on the feet, and non-grasping big toes that are short and bend upward to help propel us forward. In contrast, ape toes are long and bend downward for grasping branches.

The Environmental Context

Early bipedal ancestors lived in a challenging world and occupied an environmental niche on the edges of Africa's shrinking forests. These forest edges provided both food and safety, but venturing onto the dangerous open ground meant facing large, fierce predators. Success required caution and careful movement on two legs. Importantly, size and speed were not the driving forces behind bipedalism. Early hominins weren't trying to outrun predators or overpower competitors. Instead, bipedalism served other purposes that we're still working to fully understand. Research comparing the energy expenditure of quadrupeds versus bipeds reveals another advantage of upright walking. When tested on treadmills, chimpanzees expend double the energy that humans do for the same amount of locomotion.⁴ This efficiency would have provided significant survival advantages for early hominins who needed to travel long distances while foraging.

The Social Revolution

According to DeSilva, upright walking fundamentally altered how early humans raised their children, ultimately laying the foundation for human society as we know it. With feet adapted for terrestrial walking rather than grasping, early hominin babies from established bipeds like Lucy's species could no longer cling to their mothers while they climbed into trees for safety. Meanwhile, the demands of foraging required covering vast distances to find diverse food sources. This created a dangerous vulnerability that could only be solved through cooperation. Caring for helpless infants while foraging required enlisting help from other group members. This cooperation demanded a high level of trust, reciprocation, and social coordination. Group living became essential for survival, and foraging had to be carefully timed to avoid peak predator activity. These social innovations—cooperation in food gathering and sharing, group child care, and coordinated group activities—are the fundamental qualities that define human society today. Remarkably, they all began because our ancestors walked on two legs rather than four.

An Evolving Picture

The evolution of bipedalism represents one of the most significant transitions in human evolution, yet many questions remain unanswered. Each new fossil discovery fills in pieces of the puzzle while raising new questions about our origins. What we do know is that bipedalism is not simply about anatomy or locomotion—it's about the birth of human society itself. The moment our ancestors stood upright, they began a cascade of changes that led to cooperation, trust, shared child-rearing, and ultimately, the complex social structures that define humanity today. As scientists continue to uncover new fossils and develop new analytical techniques, our understanding of this crucial evolutionary transition continues to evolve. The story of bipedalism reminds us that in science, each answer opens the door to new questions, and our understanding of human origins remains a work in progress.

References

1 Orwell, G. (1946). Animal Farm. New York: Signet Classics.

2 DeSilva, J. (2021). First Steps: How Upright Walking Made Us Human. New York: Harper Collins Publishers.

3 Ibid.

4 Ibid.