Human skeletal changes due to bipedalism

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Ape skeletons. A display at the Museum of Zoology, University of Cambridge. From left to right: Bornean orangutan, western gorilla, chimpanzee, human. Great ape skeletons in the Museum of Zoology, University of Cambridge.jpg
Ape skeletons. A display at the Museum of Zoology, University of Cambridge. From left to right: Bornean orangutan, western gorilla, chimpanzee, human.

The evolution of human bipedalism, which began in primates approximately four million years ago, [1] or as early as seven million years ago with Sahelanthropus , [2] [3] or approximately twelve million years ago with Danuvius guggenmosi , has led to morphological alterations to the human skeleton including changes to the arrangement, shape, and size of the bones of the foot, hip, knee, leg, and the vertebral column. These changes allowed for the upright gait to be overall more energy efficient in comparison to quadrupeds. The evolutionary factors that produced these changes have been the subject of several theories that correspond with environmental changes on a global scale. [4]

Contents

Energy efficiency

Human walking is about 75% less costly than both quadrupedal and bipedal walking in chimpanzees. Some hypotheses have supported that bipedalism increased the energetic efficiency of travel and that this was an important factor in the origin of bipedal locomotion. Humans save more energy than quadrupeds when walking but not when running. Human running is 75% less efficient than walking. A 1980 study reported that walking in living hominin bipeds is noticeably more efficient than walking in living hominin quadrupeds, but the costs of quadrupedal and bipedal travel are the same. [5]

Human feet evolved enlarged heels. [6] The human foot evolved as a platform to support the entire weight of the body, rather than acting as a grasping structure (like hands), as it did in early hominids. Humans therefore have smaller toes than their bipedal ancestors. This includes a non-opposable hallux, which is relocated in line with the other toes. [7] The push off would also require all the toes to be slightly bent up. [8]

Humans have a foot arch rather than being flat footed. [7] When non-human hominids walk upright, weight is transmitted from the heel, along the outside of the foot, and then through the middle toes while a human foot transmits weight from the heel, along the outside of the foot, across the ball of the foot and finally through the big toe. This transference of weight contributes to energy conservation during locomotion. [1] [9] The muscles that work along with the hallux has evolved to provide efficient push off. The long arch has also evolved to provide efficient push-off. The stiffening of the arch would be required of an upward gait, all considered that modern bipedalism does not include grasping of tree branches, which also explains the hallux evolving to line up with the rest of the toes. [8]

Knee

Human knee joints are enlarged for the same reason as the hip – to better support an increased amount of body weight. [7] The degree of knee extension (the angle between the thigh and shank in a walking cycle) has decreased. The changing pattern of the knee joint angle of humans shows a small extension peak, called the "double knee action," in the midstance phase. Double knee action decreases energy lost by vertical movement of the center of gravity. [1] Humans walk with their knees kept straight and the thighs bent inward so that the knees are almost directly under the body, rather than out to the side, as is the case in ancestral hominids. This type of gait also aids balance. [7]

Limbs

An increase in leg length since the evolution of bipedalism changed how leg muscles functioned in upright gait. In humans, the push for walking comes from the leg muscles acting at the ankle. A longer leg allows the use of the natural swing of the limb so that, when walking, humans do not need to use muscle to swing the other leg forward for the next step. [7] As a consequence, since the human forelimbs are not needed for locomotion, they are instead optimized for carrying, holding, and manipulating objects with great precision. [10] This results in decreased strength in the forelimbs relative to body size for humans compared to apes. [11]

Having long hind limbs and short forelimbs allows humans to walk upright, while orangutans and gibbons had the adaptation of longer arms to swing on branches. [12] Apes can stand on their hindlimbs, but they cannot do so for long periods of time without getting tired. This is because their femurs are not adapted for bipedalism. Apes have vertical femurs, while humans have femurs that are slightly angled medially from the hip to the knee, thus making human knees closer together and under the body's center of gravity. This adaptation lets humans lock their knees and stand up straight for long periods of time without much effort from muscles. [13] The gluteus maximus became a major role in walking and is one of the largest muscles in humans. This muscle is much smaller in chimps, which shows that it has an important role in bipedalism. When humans run, our upright posture tends to flex forward as each foot strikes the ground creating momentum forward. The gluteus muscle helps to prevent the upper trunk of the body from "pitching forward" or falling over. [14]

Hip and pelvis

Modern human hip joints are larger than in quadrupedal ancestral species to better support the greater amount of body weight passing through them. [7] They also have a shorter, broader shape. This alteration in shape brought the vertebral column closer to the hip joint, providing a stable base for support of the trunk while walking upright. [15] Because bipedal walking requires humans to balance on a relatively unstable ball and socket joint, the placement of the vertebral column closer to the hip joint allows humans to invest less muscular effort in balancing. [7]

Change in the shape of the hip may have led to the decrease in the degree of hip extension, an energy efficient adaptation. [1] [14] The ilium changed from a long and narrow shape to a short and broad one and the walls of the pelvis modernized to face laterally. These combined changes provide increased area for the gluteus muscles to attach; this helps to stabilize the torso while standing on one leg. The sacrum has also become more broad, increasing the diameter of the birth canal and making birthing easier. To increase surface for ligament attachment to help support the abdominal viscera during erect posture, the ischial spines became more prominent and shifted towards the middle of the body. [16]

Vertebral column

The vertebral column of humans takes a forward bend in the lumbar (lower) region and a backward bend in the thoracic (upper) region. Without the lumbar curve, the vertebral column would always lean forward, a position that requires much more muscular effort for bipedal animals. With a forward bend, humans use less muscular effort to stand and walk upright. [15] Together the lumbar and thoracic curves bring the body's center of gravity directly over the feet. [7] Specifically, the S-shaped curve in the spine brings the center of gravity closer to the hips by bringing the torso back. Balance of the whole vertebral column over the hip joints is a major contribution for efficient bipedalism. [17] The degree of body erection (the angle of body incline to a vertical line in a walking cycle) is significantly smaller [1] to conserve energy.

The Angle of Sacral Incidence was a concept developed by G. Duval-Beaupère and his team at the University of René Descartes. It combines both the pelvic tilt and sacral slope to determine approximately how much lordosis is required for the upright gait to eliminate strain and fatigue on the torso. Lordosis, which the inward curvature of the spine, is normal for an upright gait as long as it is not too excessive or minimal. If the inward curvature of the spine is not enough, the center of balance would be offset causing the body to essentially tip forward, which is why some apes that have the ability to be bipedal require large amounts of energy to stand up. In addition to sacral angles, the sacrum has also evolved to be more flexible in comparison to the stiff sacrum that apes possess. [17]

Skull

The human skull is balanced on the vertebral column. The foramen magnum is located inferiorly under the skull, which puts much of the weight of the head behind the spine. The flat human face helps to maintain balance on the occipital condyles. Because of this, the erect position of the head is possible without the prominent supraorbital ridges and the strong muscular attachments found in, for example, apes. As a result, in humans the muscles of the forehead (the occipitofrontalis) are only used for facial expressions. [10]

Increasing brain size has also been significant in human evolution. It began to increase approximately 2.4 million years ago, but modern levels of brain size were not attained until after 500,000 years ago. Zoological analyses have shown that the size of human brains is significantly larger than what anatomists would expect for their size. The human brain is three to four times larger than its closest relative, which is the chimpanzee. [16]

Significance

Even with much modification, some features of the human skeleton remain poorly adapted to bipedalism, leading to negative implications prevalent in humans today. The lower back and knee joints are plagued by osteological malfunction, lower back pain being a leading cause of lost working days, [18] because the joints support more weight. Arthritis has been an obstacle since hominids became bipedal: scientists have discovered its traces in the vertebrae of prehistoric hunter-gatherers. [18] Physical constraints have made it difficult to modify the joints for further stability while maintaining efficiency of locomotion. [7]

There have been multiple theories as to why bipedalism was favored, thus leading to skeletal changes that aided the upward gait. The savannah hypothesis describes how the transition from arboreal habits to a savannah lifestyle favored an upright, bipedal gait. This would also change the diet of hominins, more specifically a shift from primarily plant-based to a higher protein, meat-based diet. This would eventually increase the size of the brain, changing the skeletal structure of the skull. [19] Transitions from the forests to the savannah meant that sunlight and heat would require major changes in lifestyle. Being a biped on an open field is also an advantage because of heat dispersal. Walking upright reduces the amount of direct sun exposure and radiation in comparison to being a quadruped which have more body surface on top for the sun to hit. [20] Increased capabilities of postural/locomotor neural control is hypothesis suggesting that the transition from quadrupedal to habitual upright bipedal locomotion was caused by qualitative changes in the nervous system that allowed controlling the more demanding type of posture/locomotion. Only after the more demanding posture was enabled by changes in the nervous system, could advantages of bipedal over quadrupedal locomotion be utilized, including better scanning of the environment, carrying food and infants, simultaneous upper extremity movements and observation of the environment, limitless manipulation of objects with upper extremities, and less space for rotating around the Z-axis. [21]

See also

Related Research Articles

<span class="mw-page-title-main">Bipedalism</span> Terrestrial locomotion using two limbs

Bipedalism is a form of terrestrial locomotion where a tetrapod moves by means of its two rear limbs or legs. An animal or machine that usually moves in a bipedal manner is known as a biped, meaning 'two feet'. Types of bipedal movement include walking or running and hopping.

<span class="mw-page-title-main">Walking</span> Gait of locomotion among legged animals

Walking is one of the main gaits of terrestrial locomotion among legged animals. Walking is typically slower than running and other gaits. Walking is defined by an "inverted pendulum" gait in which the body vaults over the stiff limb or limbs with each step. This applies regardless of the usable number of limbs—even arthropods, with six, eight, or more limbs, walk. In humans, walking has health benefits including improved mental health and reduced risk of cardiovascular disease and death.

<span class="mw-page-title-main">Foramen magnum</span> Opening in the occipital bone of the skull

The foramen magnum is a large, oval-shaped opening in the occipital bone of the skull. It is one of the several oval or circular openings (foramina) in the base of the skull. The spinal cord, an extension of the medulla oblongata, passes through the foramen magnum as it exits the cranial cavity. Apart from the transmission of the medulla oblongata and its membranes, the foramen magnum transmits the vertebral arteries, the anterior and posterior spinal arteries, the tectorial membranes and alar ligaments. It also transmits the accessory nerve into the skull.

<i>Australopithecus afarensis</i> Extinct hominid from the Pliocene of East Africa

Australopithecus afarensis is an extinct species of australopithecine which lived from about 3.9–2.9 million years ago (mya) in the Pliocene of East Africa. The first fossils were discovered in the 1930s, but major fossil finds would not take place until the 1970s. From 1972 to 1977, the International Afar Research Expedition—led by anthropologists Maurice Taieb, Donald Johanson and Yves Coppens—unearthed several hundreds of hominin specimens in Hadar, Ethiopia, the most significant being the exceedingly well-preserved skeleton AL 288-1 ("Lucy") and the site AL 333. Beginning in 1974, Mary Leakey led an expedition into Laetoli, Tanzania, and notably recovered fossil trackways. In 1978, the species was first described, but this was followed by arguments for splitting the wealth of specimens into different species given the wide range of variation which had been attributed to sexual dimorphism. A. afarensis probably descended from A. anamensis and is hypothesised to have given rise to Homo, though the latter is debated.

<i>Oreopithecus</i> Extinct genus of hominid from the Miocene

Oreopithecus is an extinct genus of hominoid primate from the Miocene epoch whose fossils have been found in today's Tuscany and Sardinia in Italy. It existed nine to seven million years ago in the Tusco-Sardinian area when this region was an isolated island in a chain of islands stretching from central Europe to northern Africa in what was becoming the Mediterranean Sea.

<span class="mw-page-title-main">Brachiation</span> Form of arboreal locomotion involving swinging by the arm

Brachiation, or arm swinging, is a form of arboreal locomotion in which primates swing from tree limb to tree limb using only their arms. During brachiation, the body is alternately supported under each forelimb. This form of locomotion is the primary means of locomotion for the small gibbons and siamangs of southeast Asia. Gibbons in particular use brachiation for as much as 80% of their locomotor activities. Some New World monkeys, such as spider monkeys and muriquis, were initially classified as semibrachiators and move through the trees with a combination of leaping and brachiation. Some New World species also practice suspensory behaviors by using their prehensile tail, which acts as a fifth grasping hand. Evidence has shown that the extinct ape Proconsul from the Miocene of East Africa developed an early form of suspensory behaviour, and was therefore referred to as a probrachiator.

<span class="mw-page-title-main">Laetoli</span> National Historic Site of Tanzania

Laetoli is a pre-historic site located in Enduleni ward of Ngorongoro District in Arusha Region, Tanzania. The site is dated to the Plio-Pleistocene and famous for its Hominina footprints, preserved in volcanic ash. The site of the Laetoli footprints is located 45 km south of Olduvai gorge. The location and tracks were discovered by archaeologist Mary Leakey and her team in 1976, and were excavated by 1978. Based on analysis of the footfall impressions "The Laetoli Footprints" provided convincing evidence for the theory of bipedalism in Pliocene Hominina and received significant recognition by scientists and the public. Since 1998, paleontological expeditions have continued under the leadership of Amandus Kwekason of the National Museum of Tanzania and Terry Harrison of New York University, leading to the recovery of more than a dozen new Hominina finds, as well as a comprehensive reconstruction of the paleoecology. The site is a registered National Historic Sites of Tanzania.

<span class="mw-page-title-main">Gait (human)</span> A pattern of limb movements made during locomotion

A gait is a manner of limb movements made during locomotion. Human gaits are the various ways in which humans can move, either naturally or as a result of specialized training. Human gait is defined as bipedal forward propulsion of the center of gravity of the human body, in which there are sinuous movements of different segments of the body with little energy spent. Varied gaits are characterized by differences such as limb movement patterns, overall velocity, forces, kinetic and potential energy cycles, and changes in contact with the ground.

<span class="mw-page-title-main">Knuckle-walking</span> Form of quadrupedal walking using the knuckles

Knuckle-walking is a form of quadrupedal walking in which the forelimbs hold the fingers in a partially flexed posture that allows body weight to press down on the ground through the knuckles. Gorillas and chimpanzees use this style of locomotion, as do anteaters and platypuses.

Orthograde is a term derived from Greek ὀρθός, orthos + Latin gradi that describes a manner of walking which is upright, with the independent motion of limbs. Both New and Old World monkeys are primarily arboreal, and they have a tendency to walk with their limbs swinging in parallel to one another. This differs from the manner of walking demonstrated by the apes.

Henry Malcolm McHenry is a professor of anthropology at the University of California, Davis, specializing in studies of human evolution, the origins of bipedality, and paleoanthropology.

<span class="mw-page-title-main">Terrestrial locomotion</span> Ability of animals to travel on land

Terrestrial locomotion has evolved as animals adapted from aquatic to terrestrial environments. Locomotion on land raises different problems than that in water, with reduced friction being replaced by the increased effects of gravity.

A facultative biped is an animal that is capable of walking or running on two legs (bipedal), as a response to exceptional circumstances (facultative), while normally walking or running on four limbs or more. In contrast, obligate bipedalism is where walking or running on two legs is the primary method of locomotion. Facultative bipedalism has been observed in several families of lizards and multiple species of primates, including sifakas, capuchin monkeys, baboons, gibbons, gorillas, bonobos and chimpanzees. Several dinosaur and other prehistoric archosaur species are facultative bipeds, most notably ornithopods and marginocephalians, with some recorded examples within sauropodomorpha. Different facultatively bipedal species employ different types of bipedalism corresponding to the varying reasons they have for engaging in facultative bipedalism. In primates, bipedalism is often associated with food gathering and transport. In lizards, it has been debated whether bipedal locomotion is an advantage for speed and energy conservation or whether it is governed solely by the mechanics of the acceleration and lizard's center of mass. Facultative bipedalism is often divided into high-speed (lizards) and low-speed (gibbons), but some species cannot be easily categorized into one of these two. Facultative bipedalism has also been observed in cockroaches and some desert rodents.

Lucy (<i>Australopithecus</i>) 3.2-million-year-old fossilized hominid

AL 288-1, commonly known as Lucy or Dinkʼinesh, is a collection of several hundred pieces of fossilized bone comprising 40 percent of the skeleton of a female of the hominin species Australopithecus afarensis. It was discovered in 1974 in Ethiopia, at Hadar, a site in the Awash Valley of the Afar Triangle, by Donald Johanson, a paleoanthropologist of the Cleveland Museum of Natural History.

<i>Ardipithecus ramidus</i> Extinct hominin from Early Pliocene Ethiopia

Ardipithecus ramidus is a species of australopithecine from the Afar region of Early Pliocene Ethiopia 4.4 million years ago (mya). A. ramidus, unlike modern hominids, has adaptations for both walking on two legs (bipedality) and life in the trees (arboreality). However, it would not have been as efficient at bipedality as humans, nor at arboreality as non-human great apes. Its discovery, along with Miocene apes, has reworked academic understanding of the chimpanzee–human last common ancestor from appearing much like modern-day chimpanzees, orangutans and gorillas to being a creature without a modern anatomical cognate.

The endurance running hypothesis is a series of conjectures which presume humans evolved anatomical and physiological adaptations to run long distances and, more strongly, that "running is the only known behavior that would account for the different body plans in Homo as opposed to apes or australopithecines".

Ardi (ARA-VP-6/500) is the designation of the fossilized skeletal remains of an Ardipithecus ramidus, thought to be an early human-like female anthropoid 4.4 million years old. It is the most complete early hominid specimen, with most of the skull, teeth, pelvis, hands and feet, more complete than the previously known Australopithecus afarensis specimen called "Lucy". In all, 125 different pieces of fossilized bone were found.

Muscular evolution in humans is an overview of the muscular adaptations made by humans from their early ancestors to the modern man. Humans are believed to be predisposed to develop muscle density as early humans depended on muscle structures to hunt and survive. Modern man's need for muscle is not as dire, but muscle development is still just as rapid if not faster due to new muscle building techniques and knowledge of the human body.

The savannah hypothesis is a hypothesis that human bipedalism evolved as a direct result of human ancestors' transition from an arboreal lifestyle to one on the savannas. According to the hypothesis, hominins left the woodlands that had previously been their natural habitat millions of years ago and adapted to their new habitat by walking upright.

Danuvius guggenmosi was an extinct species of great ape that lived 11.6 million years ago during the Middle–Late Miocene in southern Germany. It is the sole member of the genus Danuvius. The area at this time was probably a woodland with a seasonal climate. A male specimen was estimated to have weighed about 31 kg (68 lb), and two females 17 and 19 kg. Both genus and species were described in November 2019.

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