How Bipedalism Has Affected The Human Skeleton
As you might expect, the human adaptation to bipedalism has had many effects on the skeleton in order to accommodate this altered stance and form of locomotion. There are many small features that have been altered in the skeleton: they all come together to produce an efficient form which can accommodate bipedal movement.
First of all, there are few features to do with the spine which allow the human skeleton to be bipedal. The modern human spine has dual curvature (both kyphosis and lordosis – these are demonstrated in the figure), which allows us to stand erect. This also means that we can conserve energy as it will require less to remain upright due to the fact that the spine naturally curves that way anyway.
Also, the foramen magnum is positioned in the centre of the basicranium (the bottom of the skull). This is a change from the foramen magnum being further back as it is in normal quadrupeds, as the upright spinal shape requires a more central entrance into the skull in order to also keep the head upright, in the optimum visual position. The location of the foramen magnum also means that the head is better balanced and that the facial muscles are not required to support the skull upright but instead are used purely for facial expressions.
The structure of the human foot is also significantly different in comparison to other modern mammals. Humans have evolved platform feet with enlarged big toes (we do not require the thumb-like toes that apes have as we are not arboreal) and large arches which act as propulsive levers and allow us to spring forward when walking. This, again, conserves more energy than if we had ape-like flat feet like the other great apes.
Other adaptations of the human skeleton to accommodate bipedalism include a shorter, broader, pelvis that allows the swinging motion walking requires with larger hip joints (than quadrupeds) which will support the greater mass that will be supported by them (as the weight is distributed on less legs than if we were quadrupedal). Longer legs give a bigger push when walking upright, working with the arched feet to propel the form forward while a valgus angle and lock knees increases balance. Lock knees (a large knee joint which doesn’t decrease its extension so much during movement) and a valgus angle mean that as we walk, movement of the centre of gravity is restricted so we do not walk rocking from side to side as chimps and other apes would do if they were to walk upright all the time (see gorilla video below).
Gorilla walking bipedally
Changes in physiology
Obviously, when the skeleton has to adapt to cater for a new form of locomotion, there will be changes in the physiology also. Bipedalism requires very strong leg muscles in order to propel the body forwards enough. The strongest muscles are concentrated in the thighs, and all the muscles in the legs will be much more developed than those in the arms, as they are used to a greater extent and are generally worked harder. Standing on two legs also requires an effective sensory control system that can maintain balance, and in humans there is constant balance adjustment to keep us upright and to stop us falling over when in motion. Most of this balance detection takes place within the ear, in a section of chambers called the bony labyrinth. This consists of three sections, two of which are known as “organs of balance”. As well as this, our visual input from our eyes can constantly update the brain on the position of the head, and specialised proprioreceptors in muscles and tendons tell our brain how stretched our muscles and tendons are, and therefore feeds us with information on how upright we are and at which angle we are standing at.
In order to walk, the centre of gravity must remain over the supporting leg during the swing phase (when the leg is not touching the ground and is moving) of walking. To ensure that the centre of gravity does not move, or at least to ensure that movement is limited, the pelvis has adapted specific pelvic angles. When walking, we must rotate our hips as we push up off the ground in order to maintain balance and to optimise stride length. Running is very similar, and is made easier by the adapted human foot. Before each step, energy is stored up in the ball of the foot and as the foot begins to spring off the ground, the energy is released and the individual will spring forward in a run.
Lastly, the main physiological change that accommodates bipedalism to its optimum is the structure of the respiratory system. For effective movement that does not leave us exhausted, we need to have a good aerobic system that allows oxygen to travel to our tissues and replenish their energy as we move. Due to the very large surface area found in the lung tissue and the effective circulatory system, humans have easily achieved this. However, even though we have a very good aerobic system, sometimes this is not enough when put under a great amount of pressure: we must also have a good anaerobic system. This is a system which will be needed during sprinting and in vigorous activity when the body cannot supply the muscles and tissues needed for the movement quick enough.
So, we can see that bipedalism has had a great impact on human life – not only on our body structure and how we travel, but how we learnt to gather food and interact with one another too. While no two scientists can completely agree on the reasons as to why bipedalism evolved in humans, it is clear that it has been a very useful skill in our adaptation to the ever-changing world, and one that has highlighted us as unique among the great apes.