Bipedalism defines a method of locomotion by which organisms maneuver in their environment on two feet, and includes actions such as running, hopping, and walking. Organisms that habitually walk on two feet are called habitual bipeds and inhabit terrestrial environments. Organisms that occasionally support their weight on two hind legs, such as when fighting, foraging, copulating, or eating, are said to exhibit limited bipedalism. Organisms whose only method of locomotion on land involves two feet are called exclusive bipeds.
Other terms used to label types of bipedal movement include facultative and obligate bipedalism; however, the distinction between organisms that do and don’t use bipedalism isn’t so clear. Bipedal behaviors are found on a spectrum upon which animals can be on the facultative end, the obligate end, or somewhere in between.
Skeletal Changes for Bipedalism in Humans
In humans, the foramen magnum—the hole in the skull through which the spinal cord leaves the head—is positioned more directly under the skull compared to quadrupeds, allowing bipeds to hold their heads erect when walking upright.
The chest of a human is flatter (dorsal to ventral) than that of a quadruped. This keeps most of the weight of the chest near the spine and above one’s center of gravity, effectively helping to increase balance and stop us from falling forward.
The spine of humans has a characteristic S-shaped curvature. The concave curve of the S positions the chest directly above where the spine and pelvis meet, and puts the weight of the chest, again, at the center of gravity. Aside from offering better balance, the S-shaped spine is also good for absorbing the mechanical shock that comes from walking.
The pelvis of humans is wide and short. This squat shape provides increased stability to hold up an erect torso and to transfer much of the mechanical stress of weight bearing to the two lower limbs.
Humans have legs longer than their arms, while quadrupeds have arms longer than their legs. The femur of humans is longer, straighter, and thinner than that of their quadruped counterparts. The longer bones allow for a bigger stride. The straighter shape ensures that the weight is evenly distributed down the length of the bone. In addition, the thinness of a biped femur makes for a lighter structure.
The valgus angle (the angle at which the femur descends from the pelvis) in a biped is bigger than the angle in a quadruped. In a quadruped, the angle is zero; the femur descends with no slant, providing a wider stance. In a human, the femur angles inwards to bring the knees together, providing support at the center of gravity when walking upright.
The feet of humans are specialized for walking only. Human feet are arched and have lost the ability to grasp objects. The arch acts like a spring that absorbs shock, and allows the weight of the body to be transferred from the heel to the ball of the foot as we stride. The toes are positioned to provide a thrusting motion against the ground to push the body forward.
Bipedalism Origin Theories
There are several highly contested theories surrounding the evolution of bipedalism in hominines. Some have already been debunked, and others are still viable contenders. Most of these hypotheses, however, emphasize some sort of environmentally-based pressure that favored the bipedal gait. Others focus on how bipedalism facilitates food acquisition, predator avoidance, and reproductive success.
The Savanna-based Theory
Widely debunked by modern-day scientists, this theory posits that bipedalism first arose when hominines migrated out of shaded forests and into the dry heat of the grasslands because of receding forests during a time of climate change. With tall grasses that obscured vision, and no trees to shelter our early ancestors from the sun, this theory suggests that bipedalism was advantageous in that it allowed early hominines to see farther into the distance by providing the height needed to peer over the grass. It was also believed that the bipedal posture acted as a thermoregulatory mechanism that decreased the surface area of skin exposed to an overhead sun (thermoregulatory model). Other advantages that pushed the evolution of bipedalism in the savanna, according to this theory, were that the upright posture made it easier to acquire food in trees that would have otherwise been out of reach, and walking on two legs was more energy efficient. We know now, however, that hominines had already acquired the ability to walk upright while still living the aeries of the forests, before moving out into the savannah. Hence, the challenges of the savannah and the advantages of bipedalism within it are irrelevant to the rise of bipedalism in human evolution.
Aquatic Ape Hypothesis
More popular among the general public than with scientists themselves, this hypothesis suggests our early ancestors took to a more aquatic lifestyle after being outcompeted and forced from the forest trees. This band of apes, then, depending heavily on aquatic food sources, acquired bipedalism to allow them to wade into deeper waters for improved food acquisition.
Supporting this hypothesis, the wading model is based on the fact that great apes and other large primates will wade into waters looking for food and begin walking upright when they’ve waded in waist-deep to keep their heads above water. While recent studies have found some supportive evidence for this hypothesis, there is still not enough to allow us to conclusively accept or reject it. Still, most scientists do not consider this hypothesis seriously, especially because most primates will avoid going into the water unless absolutely necessary. The waters are inhabited by creatures deadly to apes and humans, such as crocodiles and hippopotami.
The Postural Feeding Hypothesis
One of the more likely scenarios of the evolution of bipedalism is the postural feeding hypothesis. This hypothesis is based on the observation that chimpanzees, our closest living relatives, use bipedalism only while eating. Being able to stand on two hind feet allows chimps on the ground to stand and reach for low-hanging fruit, as well as allowing chimps in the trees to stand and reach for a higher branch. This hypothesis suggests that these occasional bipedal actions eventually became habitual actions because of how advantageous they were in acquiring food.
Further evidence supporting the idea that bipedalism arose from the need to maneuver better in the treetops comes from the observation of orangutans using their hands for stability when the branches they were moving through were unsteady. In other words, orangutans were relying on their bipedal actions to move through the trees, only using their hands for extra support when needed. This adaptation would have been very useful while the forests and their trees were thinning.
Early Bipedalism in Homininae Model
An interesting idea surrounding the evolution of bipedalism is that it was a trait in all early hominids that was either lost or retained in varying lineages. The idea came about when a 4.4-million year old fossil of A. ramidus was found. The fossil structures suggested that the organism was bipedal, and this gave birth to the idea that chimpanzees and gorillas both started out with a bipedal gait, but that each evolved more specialized means of locomotion for their different environments. Some chimps, this model posits, lost the ability to walk upright when they settled into arboreal habitats. Some of these arboreal chimps later acquired a particular type of locomotion called knuckle-walking, as seen in gorillas. Other chimps acquired a different type of locomotion that made it easier to walk and run on the ground, as seen in humans.
The Threat Model
A recent suggestion posits that bipedalism was used by early hominines to make themselves look bigger and more threatening to prevent predators from seeing them as easy prey. Many non-bipeds will do this when threatened; however, this idea states that early hominines used bipedalism for as long, and as often, as they could, whether or not they were currently being threatened, until it eventually became habitual.
The (Male) Provisioning Model
This theory on the evolution of bipedalism links bipedalism to the practice of monogamy. It suggests that, in order to improve the chances that an offspring will survive, early hominines entered into a binding paired relationship in which the male would forage for provisions and the female, in return for these provisions, would reserve herself for her partner and care for their offspring. The provisions-providing males supposedly walked upright to free their arms to carry food and resources back to their families. Furthermore, as bipedalism emerged and physiological changes arose to accommodate the new lifestyle, newborn babies would experience increased difficulty in independently hanging onto the mother, requiring the mother to use her arms to carry the child and forcing the use of a bipedal gait.