In the earliest stages of evolution within the oceans animals depended on organic nutrients and thus had to move around to find it. Muscles need a firm support to be attached to so a primitive spine developed. While animals remained in the water the spine consisted of hollow tube filled with a flexible material which would allow side to side as well as forward movement. As the animal evolved into a larger and more complex creature it needed a stronger spine. This was first made of cartilage and then bone. Since bone on its own is too rigid, cartilage segments give it a necessary flexibility as well as a useful jointing function.
Getting on to land and then off the ground required the fishy fins to be strengthened and filled out so these organs developed an internal bone structure. The frontal fins became detached from the head and were thus able to move independently. In the early stages both the spine and the rear fins worked together to allow locomotion. As these rear limbs became more adaptable they became stronger and longer and completely responsible for movement while the spine itself became shorter and stronger.
Our very earliest ancestors were possibly somewhat like a sloth but then gradually gained an ability to move more quickly along branches then to grasp the branches as well to aid in locomotion so their trunks became shorter and their limbs became longer and stronger. Those animals that moved down to the ground to go further afield for food became at times two-legged and eventually adopted this method of movement more completely.
To stand easily on two legs requires several changes to occur. The pelvis that attaches our hips to the lower spine is needed to provide a floor for the stomach. At the same time being wider helps to give us a better base for balance. The hips, pelvis and knees needed to straighten out to allow the body to become completely upright but this was not easily accomplished so it became the task of the spine to be responsible for the rest of the straightening.
There are three main sections making up the spine. At the top is the neck or cervical region made up of seven blocks of bone or vertebrae, Below the neck in the middle is the chest or thoracic region which has twelve vertebrae and each of these bony blocks has curved ribs attached to them. Finally at the base is the lower back or lumbar region having 5 vertebrae. This lower section has a reverse curve which is allows for some straightening of the whole body. To assist in this straightening process the neck also has a reverse curve. These two curves allow us to stand upright with a reasonable degree of balance and most important enabling our head to face directly forward which is also an important determinant of good balance.
As you observe the spine from the top to the bottom you will notice that the vertebrae gradually become larger. This allows the spine to carry an increasing amount of weight. As mentioned above both the lumbar and cervical regions are curved and bend backward. The curves help reduce the load on the lower region of the spine. In addition the backward bend at neck allows us to look straight ahead and so saves us from falling flat on our face - hopefully!