An Anomaly in Vertebrates’ Evolution as Limit of Human Evolution

Abstract

During several years we have studied the so-called “intracranial system”, that is the system constituted by a container, rigid and not expandable (skull and dura mater), and by non-compressible contents, represented by the brain parenchyma, cerebrospinal fluid, and blood. We studied this system in different animal models, <i>in vitro</i> in a physical model, and in patients. While the circulatory system is different in the various types of vertebrates, less complex in fishes and more complex in birds and in mammalians, the intracranial system is always the same, constituted by the same things, skull, dura mater, subarachnoid spaces, ventricles, parenchyma, blood, and cerebrospinal fluid. All these things follow the same rule, that is the Monro-Kellie doctrine. In other words, there is an exact balance between the blood flow inlet and the blood flow outlet for each cardiac cycle. This constitutes, for each instant considered, the constancy of the intracranial blood volume. This phenomenon is always the same in all the species examined: this fact forced us to ask what the real significance of this system in the evolutive field was. Why the circulatory system follows the laws of the evolution, from less to more complexity, and the intracranial system seems to escape from these same laws? The question has two answers: the first relates to the function of this system. Being to be enclosed in a rigid box permits to minimize the necessity of more space to compensate the spontaneous increase in blood volume due to the dilation of the arteries during the systolic period of the cardiac cycle. The second one is relative to the human birth: the birth of a human being is not possible without the protection furnished by a rigid skull.