Human skeletal changes due to bipedalism
The evolution of
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]
Foot
Human feet evolved enlarged heels.
Humans have a foot
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
Change in the shape of the hip may have led to the decrease in the degree of hip
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
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.
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,
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
References
- ^ ISBN 978-4-13-066093-8.[page needed]
- S2CID 234630242.
- ^ Staff (August 14, 2016). "What Does It Mean To Be Human? – Walking Upright". Smithsonian Institution. Retrieved August 14, 2016.
- S2CID 761213.
- PMID 6768300.
- PMID 15198703.
- ^ ISBN 978-0-12-045591-1.[page needed]
- ^ PMID 30655349.
- PMID 2929741.
- ^ ISBN 978-0-07-110737-2.
- PMID 14585245.
- S2CID 85992565.
- ISBN 978-0-07-727620-1.
- ^ PMID 3212438.
- ^ PMID 15566947.
- ^ S2CID 9543264.
- ^ S2CID 3645143.
- ^ ISBN 978-0-19-920746-6.[page needed]
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- .
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Further reading
- Grabowski MW, Polk JD, Roseman CC (May 2011). "Divergent patterns of integration and reduced constraint in the human hip and the origins of bipedalism". Evolution; International Journal of Organic Evolution. 65 (5): 1336–1356. PMID 21521191.
- Crompton RH, Sellers WI, Thorpe SK (October 2010). "Arboreality, terrestriality and bipedalism". Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences. 365 (1556): 3301–3314. PMID 20855304.
External links
- Human Timeline (Interactive) – Smithsonian, National Museum of Natural History (August 2016).
million years ago ) |