Human skeletal changes due to bipedalism
Encyclopedia
The evolution of human bipedalism approximately four million years ago has led to morphological
alterations to the human skeleton
including changes to the arrangement and size of the bones of the foot, hip size and shape, knee
size, leg
length, and the shape and orientation of the vertebral column
. The evolutionary factor
s that produced these changes have been the subject of several theories.
evolved to act as a platform to support the entire weight
of the body, rather than acting as a grasping structure, 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. Moreover, humans have a foot arch
rather than flat feet. 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.
joint
s are larger than in quadrupedal ancestral species
to better support the greater amount of body weight passing through them, as well as having 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. Also, 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
. Change in the shape of the hip may have led to the decrease in the degree of hip extension
, an energy efficient adaptation.
joints are enlarged for the same reason as the hip – to better support an increased amount of body weight. 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. Humans walk with their knees kept straight and the thigh
s 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.
Having long hindlimbs and short forelimbs allows humans to walk upright, while orangutans and gibbons had the adaptation of longer arms to swing on branches. Apes can stand on their hindlimbs, but they cannot do so for long periods of time without getting tired. This is because they haven’t adapted their femur for bipedalism. Apes have vertical femurs, while humans have femurs that are slightly angled medially from the hip to the knee. This adaptation allows our knees to be closer together and under the body’s center of gravity. This permits humans to lock their knees and stand up straight for long periods of time without much effort from the muscles.
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. Furthermore, the flat human face helps to maintain balance on the occipital condyle
s. Because of this, the erect position of the head is possible without the prominent supraorbital ridge
s 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.
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
alterations to the human skeleton
Human skeleton
The human skeleton consists of both fused and individual bones supported and supplemented by ligaments, tendons, muscles and cartilage. It serves as a scaffold which supports organs, anchors muscles, and protects organs such as the brain, lungs and heart....
including changes to the arrangement and size of the bones of the foot, hip size and shape, knee
Knee
The knee joint joins the thigh with the leg and consists of two articulations: one between the fibula and tibia, and one between the femur and patella. It is the largest joint in the human body and is very complicated. The knee is a mobile trocho-ginglymus , which permits flexion and extension as...
size, leg
Human leg
The human leg is the entire lower extremity or limb of the human body, including the foot, thigh and even the hip or gluteal region; however, the precise definition in human anatomy refers only to the section of the lower limb extending from the knee to the ankle.Legs are used for standing,...
length, and the shape and orientation of the vertebral column
Vertebral column
In human anatomy, the vertebral column is a column usually consisting of 24 articulating vertebrae, and 9 fused vertebrae in the sacrum and the coccyx. It is situated in the dorsal aspect of the torso, separated by intervertebral discs...
. The evolutionary factor
Evolutionary factor
Evolutionary factors in biology are all factors that modify the gene pool in the evolutionary process.The most concise definition comes from population genetics:...
s that produced these changes have been the subject of several theories.
Foot
The human footFoot
The foot is an anatomical structure found in many vertebrates. It is the terminal portion of a limb which bears weight and allows locomotion. In many animals with feet, the foot is a separate organ at the terminal part of the leg made up of one or more segments or bones, generally including claws...
evolved to act as a platform to support the entire weight
Weight
In science and engineering, the weight of an object is the force on the object due to gravity. Its magnitude , often denoted by an italic letter W, is the product of the mass m of the object and the magnitude of the local gravitational acceleration g; thus:...
of the body, rather than acting as a grasping structure, as it did in early hominids. Humans therefore have smaller toes than their bipedal ancestors. This includes a non-opposable hallux
Hallux
In tetrapods, the hallux is the innermost toe of the foot. Despite its name it may not be the longest toe on the foot of some individuals...
, which is relocated in line with the other toes. Moreover, humans have a foot arch
Arches of the foot
-External links:* * *...
rather than flat feet. When non-human hominids walk upright, weight is transmitted from the heel
Heel
In human anatomy, the heel is the prominence at the posterior end of the foot. It is based on the projection of one bone, the calcaneus or heel bone, behind the articulation of the bones of the lower leg.- Human anatomy :...
, 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
Ball (anatomy)
The ball of the foot is where the toes join with the rest of the foot.Source:...
and finally through the big toe. This transference of weight contributes to energy conservation
Conservation of energy
The nineteenth century law of conservation of energy is a law of physics. It states that the total amount of energy in an isolated system remains constant over time. The total energy is said to be conserved over time...
during locomotion.
Hip
Modern human hipHip (anatomy)
In vertebrate anatomy, hip refer to either an anatomical region or a joint.The hip region is located lateral to the gluteal region , inferior to the iliac crest, and overlying the greater trochanter of the femur, or "thigh bone"...
joint
Joint
A joint is the location at which two or more bones make contact. They are constructed to allow movement and provide mechanical support, and are classified structurally and functionally.-Classification:...
s are larger than in quadrupedal ancestral species
Common descent
In evolutionary biology, a group of organisms share common descent if they have a common ancestor. There is strong quantitative support for the theory that all living organisms on Earth are descended from a common ancestor....
to better support the greater amount of body weight passing through them, as well as having a shorter, broader shape. This alteration in shape brought the vertebral column
Vertebral column
In human anatomy, the vertebral column is a column usually consisting of 24 articulating vertebrae, and 9 fused vertebrae in the sacrum and the coccyx. It is situated in the dorsal aspect of the torso, separated by intervertebral discs...
closer to the hip joint, providing a stable base for support of the trunk while walking upright. Also, because bipedal walking requires humans to balance on a relatively unstable ball and socket joint
Ball and socket joint
A ball and socket joint is a joint in which the distal bone is capable of motion around an indefinite number of axes, which have one common center...
, the placement of the vertebral column closer to the hip joint allows humans to invest less muscular effort in balancing
Balance (ability)
In biomechanics, balance is an ability to maintain the center of gravity of a body within the base of support with minimal postural sway. When exercising the ability to balance, one is said to be balancing....
. Change in the shape of the hip may have led to the decrease in the degree of hip extension
Extension (kinesiology)
In kinesiology, extension is a movement of a joint that results in increased angle between two bones or body surfaces at a joint. Extension usually results in straightening of the bones or body surfaces involved. For example, extension is produced by extending the flexed elbow. Straightening of...
, an energy efficient adaptation.
Knee
Human kneeKnee
The knee joint joins the thigh with the leg and consists of two articulations: one between the fibula and tibia, and one between the femur and patella. It is the largest joint in the human body and is very complicated. The knee is a mobile trocho-ginglymus , which permits flexion and extension as...
joints are enlarged for the same reason as the hip – to better support an increased amount of body weight. 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. Humans walk with their knees kept straight and the thigh
Thigh
In humans the thigh is the area between the pelvis and the knee. Anatomically, it is part of the lower limb.The single bone in the thigh is called the femur...
s 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.
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. 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.Having long hindlimbs and short forelimbs allows humans to walk upright, while orangutans and gibbons had the adaptation of longer arms to swing on branches. Apes can stand on their hindlimbs, but they cannot do so for long periods of time without getting tired. This is because they haven’t adapted their femur for bipedalism. Apes have vertical femurs, while humans have femurs that are slightly angled medially from the hip to the knee. This adaptation allows our knees to be closer together and under the body’s center of gravity. This permits humans to lock their knees and stand up straight for long periods of time without much effort from the muscles.
Skull
The human skullHuman skull
The human skull is a bony structure, skeleton, that is in the human head and which supports the structures of the face and forms a cavity for the brain.In humans, the adult skull is normally made up of 22 bones...
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. Furthermore, the flat human face helps to maintain balance on the occipital condyle
Occipital condyle
The occipital condyles are undersurface facets of the occipital bone in vertebrates, which function in articulation with the superior facets of the atlas vertebra....
s. Because of this, the erect position of the head is possible without the prominent supraorbital ridge
Supraorbital ridge
The supraorbital ridge, or brow ridge, refer to a bony ridge located above the eye sockets of all primates. In Homo sapiens sapiens the eyebrows are located on their lower margin.Other terms in use are:* supraorbital arch...
s and the strong muscular attachments found in, for example, apes. As a result, in humans the muscles of the forehead (the occipitofrontalis
Occipitofrontalis muscle
The occipitofrontalis or epicranius is a muscle which covers parts of the skull. It consists of two parts or bellies: The occipital belly, near the occipital bone, and the frontal belly, near the frontal bone....
) are only used for facial expressions.