Download Human skeletal changes due to bipedalism

Human skeletal changes due to
bipedalism
Source:
Wikipedia: https://en.wikipedia.org/wiki/Human_skeletal_changes_du
e_to_bipedalism
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 factors that
produced these changes have been the subject of several theories.
Human walking is about 75% less costly than both quadrupedal and
bipedal walking in chimpanzees. Some hypotheses have been
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
human walking. A study helped to prove that walking of living hominin
bipeds is noticeably more efficient than walking of living hominin
quadrupeds, but the costs of quadruped and bipedal travel are the
same.
Foot
Human feet have evolved enlarged heels in order to bear the amount
of weight that has also been increased. The human foot evolved 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 nonopposable hallux, which is relocated in line with the other toes.
Moreover, humans have a foot arch rather than flat feet. When nonhuman 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.
Hip
Modern human hip joints 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. 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, both of these changes combined allows for an increase in
area for the gluteus muscles to attach which helps stabilize the torso
while standing on one leg. The sacrum has also become more broad
which increases the diameter of the birth canal so that it is wider and
makes birthing easier for females. In order to provide more surface for
ligaments to attach to help support the abdominal viscera during erect
posture, the Ischia spines became more prominent and shifted towards
the middle of the body.
Knee
Human knee 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 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.
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. This results in
decreased strength in the forelimbs relative to body size for humans
compared to apes. 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. The
gluteus maximum 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.
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. Furthermore, 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. Increasing
brain size has also been significant in human evolution. It began to
increase around 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 you would expect for our size. The human brain is in fact three to
four times larger than its closest relative - the chimpanzee.
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. Together the lumbar and thoracic curves
bring the body's center of gravity directly over the feet. Also, the
degree of body erection (the angle of body incline to a vertical line in a
walking cycle) is significantly smaller to conserve energy.
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,because the joints support more weight.
Arthritis has been a problem since hominids became bipedal: scientists
have discovered its traces in the vertebrae of prehistoric huntergatherers. Physical constraints have made it difficult to modify the
joints for further stability while maintaining efficiency of locomotion.
References
Kondō, Shirō (1985). Primate morphophysiology, locomotor analyses,
and human bipedalism. Tokyo: University of Tokyo Press. ISBN 4-13066093-4.[page needed]
Rodman, Peter S.; McHenry, Henry M. (1980). "Bioenergetics and the
origin of hominid bipedalism". American Journal of Physical
Anthropology 52: 103–106.doi:10.1002/ajpa.1330520113.
Harcourt-Smith, W.E.H.; Aiello, L.C. (2004). "Fossils, feet and the
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Aiello,Leslie and Christopher Dean (1990). An Introduction to Human
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Latimer B, Lovejoy CO (March 1989). "The calcaneus of
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bipedality". American Journal of Physical Anthropology 78 (3): 369–86.
doi:10.1002/ajpa.1330780306. PMID 2929741.
Wang W, Crompton RH, Carey TS, et al. (December 2004).
"Comparison of inverse-dynamics musculo-skeletal models of AL 288-1
Australopithecus afarensis and KNM-WT 15000 Homo ergaster to
modern humans, with implications for the evolution of bipedalism".
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Lovejoy CO (November 1988). "Evolution of human walking". Scientific
American 259 (5): 118–25.
Bibcode:1988SciAm.259e.118L.doi:10.1038/scientificamerican1188118. PMID 3212438.
Wittman, Anna Blackburn; Wall, L. Lewis (2007). "The evolutionary
origins of obstructed labor: bipedalism, encephalization, and the
human obstetric dilemma".Obstetrical & Gynecological Survey 62:
739–748. doi:10.1097/01.ogx.0000286584.04310.5c.
Saladin, Kenneth S. (2003). 3rd, ed. Anatomy & Physiology: The Unity
of Form and Function. McGraw-Hill. pp. 286–287. ISBN 0-07-1107371.[page needed]
Ruff Christopher (October 2003). "Ontogenetic adaptation to
bipedalism: age changes in femoral to humeral length and strength
proportions in humans, with a comparison to baboons". Human
Evolution 45 (4): 317–349. doi:10.1016/j.jhevol.2003.08.006.
Thorpe SK, Holder RL, Crompton RH (June 2007). "Origin of human
bipedalism as an adaptation for locomotion on flexible branches".
Science 316 (5829): 1328–31.doi:10.1126/science.1140799. PMID
17540902.
Saladin, Kenneth S. "Chapter 8." Anatomy & Physiology: the Unity of
Form and Function. 5th ed. Dubuque: McGraw-Hill, 2010. 281. Print.
Wittman, Anna Blackburn; Wall, L. Lewis (2007). "The evolutionary
origins of obstructed labor: bipedalism, encephalization, and the
human obstetric dilemma".Obstetrical & Gynecological Survey 62:
739–748. doi:10.1097/01.ogx.0000286584.04310.5c.
Jacob C. Koella; Stearns, Stephen K. (2008). Evolution in Health and
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Further reading
Crompton RH, Sellers WI, Thorpe SK. (2010). "Arboreality,
terrestriality and bipedalism.". Philos Trans R Soc Lond B Biol Sci. 365
(1556): 3301–14.doi:10.1098/rstb.2010.0035. PMC 2981953. PMID
20855304.
Grabowski MW, Polk JD, Roseman CC. (2011). "Divergent patterns of
integration and reduced constraint in the human hip and the origins of
bipedalism.". Evolution65 (5): 1336–56. doi:10.1111/j.15585646.2011.01226.x. PMID 21521191.