Bipedalism

Bipedalism is a type of terrestrial animal movement involving ambulation by use of two legs. Humans, apes, some lizards and insects, and birds are the only living animals that can move bipedally, although the skill is believed to have begun with reptiles that preceded the dinosaurs. Humans are the only hominids, the family of great apes that includes chimpanzees and gorillas, that can walk on two legs consistently. Early humans evolved the muscles and skeletal system required to walk upright between three and six million years ago. The human species, Homo sapiens, is distinct from other great apes in its proficiency in bipedalism.

Background

For many years, paleontologists thought bipedalism originated with dinosaurs as a skill used for chasing prey. They believed this because the more primitive group of large land reptiles that existed directly before the dinosaurs were quadrupeds, animals that walked on four legs. In the early 2000s, however, paleontologists discovered that another type of reptile actually evolved the ability to run on two legs about sixty million years before the rise of the dinosaurs. This was a species of bolosaurus called Eudibamus cursoris.

This small reptile had short front limbs, long back limbs, and a long tail. Scientists cited this bone structure as evidence for Eudibamus cursoris being a biped. They claimed that the animal was an herbivore and likely used its ability to run on two legs to escape from predators. This made Eudibamus cursoris a highly adaptable species and is probably the reason the reptile survived long enough to spread across China, Russia, central Europe, and North America.

Birds, thought to be the descendants of dinosaurs, later developed bipedalism on their own. When on the ground, birds travel using bipedal walking, running, and hopping. Some types of lizards occasionally ambulate bipedally. The Callisaurus genus of lizards, for instance, can move on two legs for multiple steps, while the Basiliscus genus can run short distances on two legs across the surfaces of water.

Meanwhile, humans developed the skills that led to bipedalism over millions of years. The hominids of tens of millions of years ago were still apelike in their biology and behavior. They had long fingers and toes, long arms, and short legs, all of which evolved as a result of the hominids spending most of their time in trees.

Some scientists estimate that early humans began walking upright about seven million years ago. As evidence of this, they identify the fossils of Sahelanthropus tchadensis, believed by some to be an ancient ancestor to modern humans. The placement of the foramen magnum, the hole in the bottom of the skull through which the spinal cord passes, in this species is located almost directly on the bottom of the skull instead of further toward the back. This indicated to scientists that Sahelanthropus tchadensis walked upright, on two legs, rather than on all four legs, which would have positioned the animal's spinal cord and head differently. Furthermore, subsequent discovery of a femur and two ulnae indicate bipedality as well as continued arboreal activity.

Fossils from the ancient hominid Orrorin tugenensis, dated from about six million years ago, display more definite evidence of bipedalism. The thigh bones of Orrorin tugenensis were angled toward the knees, which placed the animal's feet just below the body's center of gravity. This would have made Orrorin tugenensis more stable and balanced when walking upright.

Exactly why early hominids stood up and began walking on two legs at all has been more mysterious to scientists. Nineteenth-century English naturalist Charles Darwin suggested that hominids needed to become bipeds to free their hands for tool use. Others later asserted that bipedalism emerged among hominids as an adaptation to changing environments, as forests shrank and flat grasslands emerged.

Impact

Early hominids' adapting of bipedal locomotion ultimately led to the emergence of modern humans, Homo sapiens. For this to occur, the human body had to evolve over millions of years to accommodate a radically different posture. About six million years ago, human ancestors' thigh bones began angling toward the feet, forming a stronger connection to the hip that would support the additional weight of an upright body.

About 4.1 million years ago, early humans evolved strong, wide knees capable of supporting their weight. One hundred thousand years later, human ancestors were walking upright consistently, although they were still biologically disposed to climbing trees. By 2.5 million years ago, the early human spine had curved to allow it to absorb shock from walking. The natural curve of the lower part of the human spine separates modern humans from apes.

The hip bones of the early human species Homo erectus had broadened by 1.95 million years ago. These bones closely resembled those of modern humans, meaning Homo erectus had stopped climbing trees entirely in favor of walking on two legs. The next one hundred thousand years saw Homo erectus develop longer leg bones. These, combined with enlarged hips and thighs, helped Homo erectus walk long distances throughout eastern Africa, where grasslands continued to expand.

Homo sapiens, the modern human species, appeared in Africa about two hundred thousand years ago. By this time, their skeletons and muscles had been fully adapted to bipedalism. Homo sapiens is distinguished from other hominids by its ability to walk on two legs consistently. Chimpanzees can walk upright for limited periods, but they and the other apes are still quadrupeds overall. This is because their skeletons are not adapted to bipedal walking. Chimpanzees' spinal cords connect to their skulls from the back rather than from underneath, as in humans. Chimpanzees' hip and thigh bones are also not long enough to support bipedal ambulation.

Bipedalism has had advantages and disadvantages for Homo sapiens. Early bipedal humans freed their hands to pick fruit from tree branches and carry food, tools, and children. The ability to use two hands remains important to modern humans. At the same time, humans have developed various medical problems due to placing their full body weight on only two legs. These common problems include lower back pain, slipped disks, arthritis of the hips and knees, and collapsed foot arches.

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