Orthograde posture
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Orthograde is a term derived from
Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...
ὀρθός, ''orthos'' ("right", "true", "straight") + Latin ''gradi'' (to walk) that describes a manner of
walking Walking (also known as ambulation) 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 o ...
which is upright, with the independent motion of limbs. Both
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and
Old World monkeys Old World monkey is the common English name for a family of primates known taxonomically as the Cercopithecidae (). Twenty-four genera and 138 species are recognized, making it the largest primate family. Old World monkey genera include baboons ...
are primarily
arboreal Arboreal locomotion is the Animal locomotion, locomotion of animals in trees. In habitats in which trees are present, animals have evolved to move in them. Some animals may scale trees only occasionally, but others are exclusively arboreal. Th ...
, 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
ape Apes (collectively Hominoidea ) are a clade of Old World simians native to sub-Saharan Africa and Southeast Asia (though they were more widespread in Africa, most of Asia, and as well as Europe in prehistory), which together with its sister g ...
s.
Chimpanzee The chimpanzee (''Pan troglodytes''), also known as simply the chimp, is a species of great ape native to the forest and savannah of tropical Africa. It has four confirmed subspecies and a fifth proposed subspecies. When its close relative th ...
s,
gorilla Gorillas are herbivorous, predominantly ground-dwelling great apes that inhabit the tropical forests of equatorial Africa. The genus ''Gorilla'' is divided into two species: the eastern gorilla and the western gorilla, and either four or fi ...
s,
orangutan Orangutans are great apes native to the rainforests of Indonesia and Malaysia. They are now found only in parts of Borneo and Sumatra, but during the Pleistocene they ranged throughout Southeast Asia and South China. Classified in the genus ...
s and
human Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...
s, when walking, walk upright, and their limbs swing in opposition to one another for balance (unlike monkeys, apes lack a
tail The tail is the section at the rear end of certain kinds of animals’ bodies; in general, the term refers to a distinct, flexible appendage to the torso. It is the part of the body that corresponds roughly to the sacrum and coccyx in mammals, r ...
to use for balance). Disadvantages related to upright walking do exist for primates, since their primary mode of locomotion is
quadrupedalism Quadrupedalism is a form of locomotion where four limbs are used to bear weight and move around. An animal or machine that usually maintains a four-legged posture and moves using all four limbs is said to be a quadruped (from Latin ''quattuor' ...
. This upright locomotion is called "orthograde posture". Orthograde posture in humans was made possible through millions of years of evolution. In order to walk upright with maximum efficiency, the skull, spine, pelvis, lower limbs, and feet all underwent evolutionary changes.


Origin

The definition of orthograde posture can easily be derived from its roots “ortho-” meaning “upright” and “-grade” meaning “ascent.” This was true for the early hominidae, whose transition to upright walking took place approximately six to seven million years ago evident in ''Orrorin tugenensis''. These hominin were some of the first bipeds who propagated forward one leg at a time, step by step.


Evolutionary significance

The first definitive evidence of habitual orthograde posture in human evolutionary lineage begins with ''Ardipithecus ramidus'', dating between 5.2 and 5.8 million years ago. The skeletal remains of this hominid exhibit a mosaic of morphological characteristics that would have been both adapted to an arboreal environment and walking upright terrestrially. The earliest evidence of a hominid exhibiting skeletal morphology capable of achieving orthograde posture dates to 9.5 million years ago, with the discovery of a Miocene ape, Dryopithecus in Can Llobateres, Spain. Several million years after ''
Orrorin tugenensis ''Orrorin tugenensis'' is a postulated early species of Homininae, estimated at and discovered in 2000. It is not confirmed how ''Orrorin'' is related to modern humans. Its discovery was used to argue against the hypothesis that australopitheci ...
'', australopithecines such as ''Au. africanus'' and ''Au. afarensis'' also practiced habitual bipedalism. These tree-dwellers were arboreal and inhabited the wooded areas of forest canopies. Some
hominin The Hominini form a taxonomic tribe of the subfamily Homininae ("hominines"). Hominini includes the extant genera ''Homo'' (humans) and '' Pan'' (chimpanzees and bonobos) and in standard usage excludes the genus ''Gorilla'' (gorillas). The t ...
in that time period still used knuckle walking, a practice common in other apes. However habitual bipedalism in australopiths meant though they nested among the branches in trees at night, they moved with orthograde posture such that their hands could also be used for gathering, feeding, weight transfer, or balance during the day. From fossil evidence and hypotheses state that upright posture was a quintessential reaction to changes in environment and competition. Due to the more wooded barren savannahs of northern Africa, ''O. tugenensis'' and australopiths began to change, which is evident in morphological data accumulated from the remains of the different species. These major morphological changes differentiate them from pronograde hominin seen in the skull, vertebral column, pelvis, and femur fossils.


Morphological characteristics

In order for animals to have the ability to walk upright, there are certain anatomical requirements. For mammals that exhibit orthograde posture, the scapula is more dorsally placed than in animals with a pronograde posture. The scapular index, the measure of width to length of the scapula, is decreased in animals exhibiting orthograde posture. This means that the scapula is broader than it is long. The
rib cage The rib cage, as an enclosure that comprises the ribs, vertebral column and sternum in the thorax of most vertebrates, protects vital organs such as the heart, lungs and great vessels. The sternum, together known as the thoracic cage, is a semi- ...
is more flattened and the acromion process on the scapula is much larger. This is because there is more of a need for the deltoid muscle in orthograde posture, due to the availability of resource manipulation by the freeing up of hands.


Morphological changes

In 1924, the discovery of remains of the
Taung Child The Taung Child (or Taung Baby) is the fossilised skull of a young ''Australopithecus africanus''. It was discovered in 1924 by quarrymen working for the Northern Lime Company in Taung, South Africa. Raymond Dart described it as a new species ...
in South Africa provided further evidence of bipedalism and orthograde posture. The skull belonged to a three-year-old child, later identified as ''Australopithecus africanus.'' The skull was an indicator of orthograde posture because of the location and orientation of the foramen magnum. The foramen magnum is the space in the skull that acts as the bridge to the central nervous system from the spinal cord to the brain. For animals with "pronograde posture, the foramen magnum is dorsally oriented, whereas in humans it is anteriorly located and forwardly inclined. In the Taung Child despite lacking the forward inclination seen in humans, the foramen magnum is also anteriorly oriented. Similarly in ''Australopithecus afarensis'', the site of the space in the skull is even more human-like, inferiorly located such that the spinal cord would run perpendicularly to the ground. Relating this orientation to the
encephalization Encephalization quotient (EQ), encephalization level (EL), or just encephalization is a relative brain size measure that is defined as the ratio between observed to predicted brain mass for an animal of a given size, based on nonlinear regressi ...
of hominin of the time, the position foramen magnum assisted in balance and supported upright posture. More evidence in hominidae that enabled orthograde posture is present in the vertical column or lumbar vertebra of ''Australopithecus afarensis''. The human lumbar column consists of five vertebrae that connect the twelve
thoracic vertebrae In vertebrates, thoracic vertebrae compose the middle segment of the vertebral column, between the cervical vertebrae and the lumbar vertebrae. In humans, there are twelve thoracic vertebra (anatomy), vertebrae and they are intermediate in size b ...
to the sacrum and pelvis. Primates with pronograde posture such as gorillas have four lumbar vertebrae that connect to twelve thoracic vertebrae. The difference in vertebra number results in a greater range of movement for humans with less thoracic vertebrae than gorillas with more lumbar vertebrae. ''Au. afarensis'' has six total lumbar vertebrae with also twelve thoracic vertebra Another key characteristic that enforced upright posture in hominin was the shape of the lumbar vertebra. The “s” shape of the lumbar vertebra is called spinal
lordosis Lordosis is historically defined as an ''abnormal'' inward curvature of the lumbar spine. However, the terms ''lordosis'' and ''lordotic'' are also used to refer to the normal inward curvature of the lumbar and cervical regions of the human spi ...
, which produces the unique convex curvature seen in upright bipeds. The vertebral column of australopith fossils also share the curved morphology of modern humans. Lordosis in the lower lumbar spine centers the mass of the body on the lower joints such as the pelvis and femur such that the body is self-stabilizing and can remain upright. The earliest habitual bipeds of the hominins were ''Orrorin tungenenisis''. Evidence draws from three femur fragments, including the left shaft and head, and the head of the right femur. Linking the legs to the pelvis and lumbar vertebra, the femur quintessentially supports body weight as it is transferred from the pelvis to the knee and lower limbs. The femoral neck specifically, which connects the head of the femur to its primary shaft absorbs the force of impact when an upright biped assumes movement. In ''Orrorin tugenensis'', the orientation of the head condyles of the broadened femur is wider and thicker in comparison to that of chimpanzees and other great apes.


See also

*
Bipedalism Bipedalism is a form of terrestrial locomotion where an organism 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' (from Latin ''bis'' 'double' a ...
*
Pronograde Quadrupedalism is a form of locomotion where four limbs are used to bear weight and move around. An animal or machine that usually maintains a four-legged posture and moves using all four limbs is said to be a quadruped (from Latin ''quattuor ...


References


Further reading

* * * * {{refend Walking