There is little doubt that the vertebrate brain is the most complex structure we know. As with any complex structure, there is the immediate question about its origins. How could such a complex design develop from the simplest multicellular animals? This problem has pervaded the study of evolutionary biology since its beginnings. Although Darwin (1859, 1871) proposed an impeccable mechanism (natural selection) for the gradual transformation of species including human origins, even he sometimes expressed certain doubts about the origin of highly complex structures. This issue has been highly debated both within science and outside it. For instance, a rebirth of the old religious argument of intelligent design has gained unexpected strength in the last few years. In essence, this argument follows Paley’s (1802) claim that if we find a clock that has been thrown away we cannot consider that it was created on its own, but rather has to be the consequence of conscious design. Today, creationists have developed a modern version of this argument, that of “intelligent information” (Denton 2002). For example, after sequencing the human genome in 2001, one of Celera Genomics top computer scientists claimed that this complexity suggested design. Although he clarified not to be thinking of God, he asserted that “there’s a huge intelligence there” (quoted in Witham 2002, p 9). As Witham (2002) says, modern computer-literate believers may soon ask the question of whether the universe is self-running or functioning on DOS, a Divine Operating System.
In this volume, we have decided to tackle the problem of the origin and evolution of the vertebrate brain, from the simplest nervous system-like elements that we can observe in nature. In doing so, we expect to establish a continuity between the simplest stages and the elaboration of the highly intricate neuronal network that is the mammalian cerebral cortex. For lack of space, we will have to leave aside several other brain structures such as the basal ganglia or the cerebellum, as well as components perhaps comparable to the mammalian cerebral cortex in other vertebrates; as will be seen, the cerebral cortex alone is sufficient to fill up quite an extensive review. Our main point will be to present a case for continuous evolutionary transformation of the central nervous system, from its very origins to the elaboration of the most complex structure that exists on earth.
