Comparisons of vertebrate, mammalian central nervous systems to those of non-mammalian vertebrates shows that both types of brains have very similar patterns of cell groupings and connections. While brains of different species are specialized for certain types of tasks that allow the animal to survive in their particular niche, it is fascinating to realize that all vertebrate central nervous systems have a common organization.

Thus, whether looking at a cat, a rat, a human, a monkey, a bird or a fish, if you understand the organization of the brain of one species, the brain of another vertebrate should seem very familiar to you. Even the brains of animals that do not share a common ancestor may be very similar because animals share a common environmental challenge. For example, animals that need to root for food such as sharks, alligators and pigs will all have a great deal of sensory innervation to the snout area. Animals that use olfaction to find food, prey, mates and avoid predators will have a highly developed olfactory system.

One of the persistent ideas in biology and comparative psychology has been that evolution is a progressive process so that we can arrange species into a hierarchy from “lower”, less complex species all the way up to “higher” more complex species, with humans occupying the highest rung of the ladder. The implication of this is that lower, less complex forms came first and there has been a progressive evolution to produce the ‘most evolved’ form of animal, the human. Philosophically, this is a very comforting thought for humans, since this philosophy suggests that the meaning of human existence is to be the most developed, complex and intelligent species on the earth. However, current thought suggests that this kind of thinking is anthropocentric and has no scientific value. It is a human concept, not a biological principle. Thus, it is not appropriate to refer to animals as ‘higher’ or ‘lower’ on the evolutionary scale. What is appropriate is to talk about some characteristics in a species being more ‘primitive’, with the word primitive meaning that that characteristic developed longer ago than some other characteristic. Indeed, we can even challenge the idea that primates are more complex than other species. Even a fairly cursory look at a number of what we think of as less complex species indicates that many non-mammals have more sensitive olfaction (including smelling molecules that primates cannot), more sensitive visual systems (including detection of colors, UV, infrared and polarized light that primates cannot see), and that some species generate electrical fields that cannot be detected by mammals. Thus, complexity is not only found in mammals.

All that said, when looking at reptiles, fish, birds and mammals, there seems to be a general trend (with some overlap) for vertebrates to develop larger brains as we move from more primitive vertebrates to less primitive vertebrates. What is clear, is that a less complex CNS can work well for animals that occupy less complex environmental niches, while some vertebrates have evolved more complex brains to allow them to utilize more complex and variable niches in the environment. The development of a more complex brain can simply be thought of as an adaptation, since cognition (such as learning, memory and decision making), enhanced sensory ability and improved motor control all follow from developing a more complex CNS. In turn, these abilities aid in survival. Adaptations for larger, more complex brains have resulted in:

  increased numbers of neurons
  more extensive dendritic organization
  more synapses
  more efficient arrangement of neurons