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Anatomical changes

Why become a biped when it creates problems such as balance? A quadruped has balance between four legs like a table, the centre of gravity falls in the middle. In a biped they need to balance over on two legs when standing and one leg while walking. Therefore there must be structural changes to the body in the skull, spine, pelvis, leg and foot.


Make up of the spine

The spine is made up of 3 sections, the neck are the cervical vertebrae, then the thoracic vertebrae and at the bottom the lumbar vertebrae then in the pelvis are the sacrum and coccyx.


The vertebral column

The quadruped has a c-shaped curve in the thoracic region that means that their centre of gravity is in front of them, so when a primate stands on two legs they tend to fall forward. Bipeds have adapted and have an s-shaped spine created by the two secondary and opposing curvatures, this brings the centre of gravity towards the hips so it's between the legs.


Weight distribution in the spine

Due to the curves in the spine the weight of the biped bares down the spine and to the sacrum, where it passes to the hips and into the legs. The amount of weight increases down the spine and this is reflected in the size of the vertebrae, the cervical being the smallest and increasing down the spine to the lumbar region. Whereas in a quadruped the weight baring is equal along the spine and this is reflected in the vertebra's size. The impact of this is lower back pain particularly in pregnant women.


Foramen magnum and nuchal plane in quadrupeds

In a quadruped the position of the foramen magnum is at the back of the skull, this combined with the position of the nuchal plane- flat boney area where the neck muscles attach, indicates the angle that a quadruped holds its head. Forward as it leans forward on four legs.


Foramen magum and nuchal plane in bipeds

In a biped the foramen magnum is positioned on the bottom of the skull, demonstrating that the spine goes directly down directly below the skill. The muscles attached to the nuchal plane also show the neck is below the skull, not towards the back.


Make up of the pelvis

There are two innominate bones, that is made up of three bones, the ischium (bone you sit on), ilium (large hip area) and the pubis (bony portion of the pelvis in the pelvis region) that fuse during adolescence, and the sacrum.


Pelvis in bipeds

In a biped the pelvis is basin- shaped, with a short ilium that runs from the posterior to the anterior of the animal. The gluteal muscles, minimus and medius, which are attached to the ilium and rotated to the side of the biped, they connect at the top of the femur. The contraction of these muscles keep one from tipping forward while they walk. It also shortens and broadens the ilium, narrowing the birth canal.


Pelvis and birth canal of the quadruped

The quadruped has a long, flat ilium that is situated at the back of the animal. Chimpanzee and gorilla babies are born face up unlike human infants.


Birth canal of the biped

Efficient bipedalism requires a narrow pelvis, but this bust be balanced against the need for a birth canal wide enough for an infant. As brain size increased the baby was required to rotate during birth, human babies arrive facing downwards. The evolutionary drawback is that women now require help during birth.


The leg and angled femur

The board pelvis of the biped places the femur at the far side, but when you walk the foot must fall below the centre of gravity. A straight femur would, like that of a quadruped, would place the foot too far from the centre of gravity. The femur is angled from the hip into the knee it places the foot below the centre of gravity, this saves energy when walking


The leg and problems

This creates an issue with the knee as the muscular attachment on the femur also acts at an angle, when this muscle contracts on the front of the femur, it pulls superiorly (up) and laterally (out), moving the patella outward as it sits on the tendon of this muscle. To avoid dislocating the knee, the groove on the femur that the patella sits in is deep and the outside edge is enlarged in a biped.


Length of leg

Although relatively short in early hominids, the length of the leg has increased relative to the trunk length during human evolution. A longer limb length is favoured due to larger stride length and is more efficient at walking. This had evolved by the time of Homo erectus.


Composition of foot and toes

The foot is composed of the tarsals, which form the heel, metatarsals and phalanges form the toes. In bipedal walking the heel strikes first, followed by the rest of the foot. The main propellant force comes when the big toe pushes off the ground, and the toes bend backward therefore the big toe moves into line with the others and the big toe becomes much larger than the rest of the toes.


Shape of foot and arches

A bipeds foot is stouter and has arches that accommodate the extra weight of only two feet. The foot has two arches that act as shock absorbers; a transverse arch that runs from medial to lateral, and a longitudinal arch that running the length of the foot and formed by the metatarsals and tarsals.


The arm

One advantage to bipedal walking is that it frees up the arms to do other things, such as carrying objects and tool making. As bipeds do not use their arms for walking their orientation has changed throughout human evolution. Early hominids started with relatively long arms, a holdover from the suspensory ancestor. Over time the body proportions changed, the legs got longer as the arms got shorter, and became less robust since they don't bear weight anymore when when walking. Fine manipulations became more important the thumb became opposable and the phalanges became shorter.