It is very likely that, at some point or other, you have wondered why evolution has not given us the possibility of having eyes in the back of our heads. Or, what is the same, on the nape . The truth is that we can find two reasons that could explain it easily, and without too many complications.
From a biological point of view, eyes are tremendously expensive to manufacture , and placing them at the back of our heads would not fit well into the very primitive body plan that mammals inherited.
In fact, even for those mammals for whom rear vision becomes a fundamental trait for their own survival, such as rabbits, for example, they only have eyes capable of looking to the side, so that the view backwards it is only peripheral (enough to alert them when danger exists or is approaching).
Spiders, for example, which are characterized by having many eyes (between six to eight small eyes), only have them looking forward.
According to experts, rear vision is only useful when potential threats are truly visible . And, during human evolution, this was not the most common case. Evolution is supposed to work towards particular goals in a more or less planned way, but this is not the case.
Genetic mutations happen randomly. And then we find those that are subject to selection pressures, which can end up allowing a certain change or mutation to become more general within the gene pool, as long as they are beneficial, or even eliminating them if they are not so beneficial.
A second factor is found in the fact that most of the traits, especially the more complex ones –as could be the case of sight–, do not appear or are completely formed in a gene pool, but rather tend to develop over generations, starting from pre-existing structures and features . In the case of the extremities of vertebrates, we can see how an extraordinary variety of functions and forms have developed, starting from the same basic structure.
It would seem that it is relatively easy to adapt an existing structure to new uses, as for example is the case with additional pairs of limbs (so that they can have arms and wings), but it is much more difficult to create completely new structures.
In the case of birds, many have their eyes positioned towards the side of the skull, which offers them the possibility of having a fairly wide angle of vision. While, in the case of some specific birds, such as the woodcock (Scolopax rusticola), they are located in a way that they can truly achieve a 360-degree vision. But this comes at a cost in terms of loss of binocular vision (or stereoscopic vision).
In short, having eyes on the back of the neck would not be so useful because we need to know where we are going , so having them looking forward is essential. Another thing would be if, in addition, we had another pair of eyes located at the back of the head. In this case, possibly our brain would have to combine these two completely different visions into one, including the opposite directions of movement.
