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Human Evolution to Bipedalism
Human evolution to bipedalism started with Hominins, our ancestors, standing upright as early as around 7 million years ago.
Thanks to them for turning us into a unique species standing up as straight as no other mammal ever did! Consequently it has increased our energy efficiency as far as walking is concerned by 75% compared to what it was. It also did lot many other things that turned us what we really are today.
This was a great leap forward in the history of evolution that set human skeleton to undergo huge morphological changes. The said changes involved in this process of skeletal reorientation were the following:
- New arrangement and size of foot bones
- Hip size and shape
- Knee size
- Leg length
- Shape and orientation of vertebral column
- Skull orientation
Human Evolution to Bipedalism: Foot
With the help of newly evolved larger heels, the foot increased in size. In fact this enabled it to work like a platform on which the body weight could easily rest. It also turned the toes smaller than before as they were no more used to grasp things like tree trunks.
Foot as a platform didn’t evolve as a flat surface but did so as an arched one. Consequently it distributed the body weight on feet through their heels to the balls of feet behind big toes. The said weight distribution saved a lot of energy while walking.
(But that is only the ideal state of body weight distribution in bipedal homo sapiens. What I have noticed in most of my trainees coming to me for re-posturing is not so. Owing to posturing their body in a wrong way, they support their body weight on the outsides of their feet. Most of them do so towards the outer sides of their heels whereas the rest do towards their little toes. I always check it the very first through checking the soles of their shoes where they have worn the most. My data suggest that most of modern homo sapiens support their body weight on their feet the wrong way.)
Today, the human foot is a complex structure of 26 bones, 33 joints and hundreds of muscles, tendons and ligaments. The main joints of the foot are…
A foot has three parts to it…
- Hindfoot includes ankle bone (talus) and heel bone (calcaneous). Ankle bone connects to tibia and fibula in the lower leg and forms ankle joint. At its other end, it connects to heel bone and forms subtalar joint.
- Midfoot has five bones, cuboid, navicular and three cuneiform bones from its arches, serving as a shock absorber. It’s connected to hind foot and fore foot with muscles and plantar fascia,
- Forefoot comprises five toes and five proximal long bones forming metatarsus. Toe bones are called phalanges and make joints among them, named interphalangeal joints and with metatarsus, named metatarsophalangeal joints.
Human foot has two longitudinal and one transverse arches. They make walking and running more energy-efficient.
Human Evolution to Bipedalism: Knee
Human knee joints have also evolved to support the increased body weight on two of them owing to bipedalism. Also, the degree of knee extension, meaning the angle between the thigh and the shank in the walking cycle, has decreased. In fact this change called double knee action reduces energy lost by vertical movement of the center of gravity. It also enables homo sapiens to keep their knees straight right under the body improving its balance.
The knee joins the thigh and the leg with two joints, tibiofemoral joint connecting femur with tibia and patellofemoral joint connecting femur and patella. In fact this modified hinge joint is the largest joint in the body. It also very efficiently permits flexion and extension along with a limited internal and external rotation.
Human Evolution to Bipedalism: Limbs
Human evolution to bipedalism changed the length of the legs since they were the only ones to function in upright gait. In fact leg muscles are the ones that give a push to the action of walking at its every step through their ankles. Long legs also give an easy swing to the steps without using muscles every time. Since forelimbs don’t play any role in the action of walking, they become free to carry other stuff.
Human evolution to bipedalism also provided homo sapiens with upper limbs for visually guided manipulation and lower limbs for mechanically guided bipedal gait. It also optimized the location of the body’s center of gravity. It further reorganized the internal organs and the biomechanism of the trunk through a double S shaped vertebral column. The double S shape of the vertebral column is also an efficient shock absorber. The entire skeletal system thus efficiently shifts the weight of the upper body to the sole of the feet. Long load bearing legs are also very nicely optimized for bipedalism.
Moreover, all the large joints of the lower limb are aligned in a vertically oriented straight line.
Human Evolution to Bipedalism: Hip and Pelvis
Human evolution to bipedalism also turned hip joints larger than before to support the entire body weight in a better way. It turned its shape shorter and broader, bringing the vertebral column closer to the hip. This arrangement provided a more stable base for the support of trunk while walking upright. Also, the ilium broadened in shape turning the walls of the pelvis adjusted to face laterally.
Also the size of gluteus muscles increased enabling homo sapiens to stand on one leg with body balance intact. The sacrum turned broader, thus increasing the opening of birth canal.
Three pelvic bones fused into one as the hip bone forming a part of the hip region. The hip joint joins the head of the femur to acebulum of the pelvis. The joint supports the weight of the body in its static as well as in its dynamic state. Thus the joint retains balance and maintains the pelvic inclination angle.
Human Evolution to Bipedalism: Vertebral Column
Human evolution to bipedalism reshaped the vertebral column to what we see it like today.
The vertebral column of the homo sapiens in its natural posture has a forward bend in its lumbar region. Above this it also has a backward bend in its thoracic region. In fact without its forward bend in its lumbar region, the body would have kept leaning forward.
The forward bend in the lumbar region and the backward bend in the thoracic region keep the body straight. They do so by keeping body’s center of gravity directly in line with its feet vertically.
The vertical column makes homo sapiens a vertebrate having a long chain of vertebrae with intervertebral discs between them. In fact, they are named by their region in the column and their position in their region. The vertebral column also has a spinal canal for housing the spinal cord.
There are 33 vertebrae in the vertebral column. The upper 24 of them are articulating and separated from one another by intervertebral discs. The lower 9 of them are fused in human adults, 5 in sacrum and 4 in coccyx or tailbone. Also, ligaments at the front, at the back and in between the vertebrae lengthen the vertebral column. In fact these ligaments join the spinous processes, the transverse processes and the vertebral laminae.
All 33 vertebrae are divided into five regions…
- Cervical spine has 7 vertebrae
- Thoracic spine has 12 vertebrae
- Lumbar spine has 5 vertebrae
- Sacrum has 5 fused vertebrae
- Coccyx has 4 fused vertebrae
Atlas and axis are the topmost vertebrae in the cervical region supporting the head.
Vertebral body and vertebral arch are the two parts of a vertebra placed anteriorly and posteriorly respectively. The two parts in contact with each other enclose vertebral foramen through which the spinal cord passes.
The vertebral arch has two pedicles and two laminae. The arch performs seven processes out of which 4 are particular, 2 are transverse and one is spinous (aka neural spine). The spinous process comes out the back and the two transverse processes come out one left and the other right. Also, vertebrae are joined by a thin portion of the neural arch. It’s called the pars interarticularis with facet joints allowing a restricted movement. Each pair of two vertebrae contain two small holes between them. These holes are named intervertebral foramina. Spinal nerves go out of the spinal cord to the organs through these holes.
The vertebrae are named with the help of their regions and numbers from up to down…
- Cervical spine 7 vertebrae (C1 – C7)
- Thoracic spider 12 vertebrae (T1 – T12)
- Lumbar spine 5 vertebrae (L1 -L5)
- Sacrum 5 fused vertebrae (S1 -S5)
- Coccyx 4 fused vertebrae (Tailbone)
As a result of human evolution to bipedalism, the vertebral column got its curves at numerous places to make body stable in upright position.
Here is a list of curves in the vertebral column with their vertebrae in them…
- C2 – T2: convex forward (lordotic curve)
- T2 – T12: concave forward (kyphotic curve)
- T12 – S1: convex forward (lordotic curve)
- S1 – Tailbone: concave forward (kyphotic curve)
Human Evolution to Bipedalism: Skull
Human evolution to bipedalism also reshaped the human skull. Here is what we see it like at present…
The skull of homo sapiens is freely balanced on the vertebral column at its first vertebra named Atlas, coded C1. In fact occipital condyles maintain its balance dynamically. Owing to this, head can be postured erect without the prominent supraorbital ridges and strong muscular attachments like in apes. As a result, occipitofrontalis (frontalis and occipitalis) muscles act only for face expressions and for central fixation. The spinal cord as an extension of medulla oblongata enters the vertebral column through foramen magnum, an occipital bone opening.
Human brain size has also played a very big role in human evolution to bipedalism. It’s 3-4 times larger for its body size than that of its nearest evolutionary relative, chimpanzee.
The skull acts as a protective cavity for the brain. It also houses most of the sense organs including eyes, ears, nose and tongue. In fact, it has two parts to it, neurocranium and membraneous viscerocranium with mandible as the latter’s largest bone. Numerous flat bones fuse together as sutures to give the skull its shape that it has in the humans. Surprisingly mandible is the only major movable bone that engages with the neurocranium via temporomandibular joint.
In all, the skull has 22 bones in it. Out of these, eight are the cranial bones:
- Occipital bone
- Temporal bones (2)
- Parietal bones (2)
- Sphenoid bone
- Ethmoid bone
- Frontal bone
There are 14 more bones in the facial skeleton…
- Inferior nasal conchae (2)
- Nasa bones (2)
- Maxilla (2)
- Palatine bones (2)
- Zygomatic bones (2)
- Lacrimal bones (2)
Also, there are paranasal sinuses (air-filled cavities lined with respiratory epithelium) and numerous foramina (openings in the skull).
7 Holes That Human Evolution to Bipedalism Left in The Human Skeleton
Although human evolution to bipedalism worked miraculously well yet it left 7 holes in our skeleton. The reason was that bipedalism turned the orientation of the vertebral column from horizontal to vertical. Thus the vertical orientation imparted more freedom to the vertebral column which it didn’t have in its horizontal orientation. The horizontal orientation was controlled by a guide between the neck and the pelvis keeping its posture naturally intact.
With freedom came the responsibility to posture the vertically oriented vertebral column with skull resting on it naturally intact. But the increased downward pull from gravity on both, vertebral column and skull, tended to give way to this pull. It took place through dropping both of them down on their joints in the longitudinal plane. Meanwhile the brain size had also increased tremendously giving rise to another entity called mind to cognize psychedelic perceptions. As a result, the mind gave birth to a complex set of positive and negative emotions in pairs. These emotions tended to tuck the downward pulled skeletal joints ‘in’ in the transverse plane.
A List of 7 Holes That Human Evolution to Bipedalism Left in The Human Skeleton
We, the bipedal Homo sapiens, gave way to these downward pulls and inward tucks. Further, with the passage of time, we made it a habitual way of posturing our skeleton. These were the 7 holes that human evolution to bipedalism left in the human skeleton listed below.
- Forming Pelvic Tilt at Tailbone aka Coccyx through Dropping Coccyx Down
- Dropping Sacral Vertebrae Distorting The Lower S of The Double S Shape of Vertebral Column
- Dropping L1 Vertebra And Tucking It In, Compressing Thoracic Diaphragm throughout Its Length
- Turning The Upper S Shape of C8 – T1 Vertebrae to An Inverted C Shape
- Turning The Upper S Shape of C5 – C6 Vertebrae to An Inverted C Shape
- Pulling Frontalis Muscle Down in The Skull
- Pulling Temporalis Muscle & Temporomandibular Joint Down And Tucking It ‘In’ in The Skull
These 7 holes have done a lot of harm, not only to our health but also to our sociocultural institutions. These harms have been there despite all the benefits that human evolution to bipedalism has tremendously blessed us with.
We will look into their implications in our blog posts to come!
Check our home page at Skeletal Leap: The Next Homo Sapiens.
You can also check the blog posts on human evolution in our Human Evolution Blog.
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