6. The Heritages of the Chordate Phylum

6.3 Levels of encephalization of the vertebrates

Learning, with its corollary, memory, is the incorporation of individual experience into adaptive responses to challenges of the environment. They both represent an important evolutionary step, which allowed animals to adapt to environmental conditions not anticipated by the genetic code, which programs the brain for innate responses. The size of the brain is important to achieve this freedom from innate responses. The size of the brain of chordates is an index of the intelligence of the species. Humans possess unique mental skills as the ability to read, write and solve problems. These skills are made possible by the cerebral cortex, which is a thin sheet of neurons on the surface of the brain. Although all vertebrates have cerebral cortices, the cerebral cortex of humans has undergone a vast expansion in size during evolution. The increased size of the brain during evolution results primarily from a fantastic expansion of its surface area, with folds of the cortical surface (the hills are known as gyri and the valleys as sulci) providing a means to increase the total cortical area in a given skull volume.

Brain sizes increased in a discontinuous way during the evolution of the chordates.

6.3.1 The brain of a protochordate

We are so used to the voluminous human brain that it is a little puzzling to see how poorly developed was the brain of primitive chordates. Figure 6.5 illustrates the brain of Amphioxus, a protochordate. The nervous material making up the brain is located on top of the chord. This chord stops just behind the hypophyse. Additional nervous material could develop in front of the chord because the secondary mouth of the deuterostomes has moved to a ventral position.

Figure 6.5. Schematic view of the brain of Amphioxus. The slide is made longitudinally, along the median axis of the animal. The prosencephalon (PROS) is located in front of the chord above the hypophysis. It is composed of the median diencephalon (dienc) and the two lateral olfactive lobes (tel), of which the right one is shown. The olfactive lobes communicate with the diencephalon by the Monro Holes.


The telencephalon is represented by the first and second ventricles, which are two lateral olfactive lobes. These communicate with the third median ventricle by the Monro holes. The diencephalon is mainly concerned with the formation of eyes. Two lateral eyes, on the bottom, are communicating with the brain through the chiasm. Two median eyes, on the top, are formed from evaginations called the “parietal organ” and the epiphyse. The epiphysal eye is playing a role in mimetism by the lamprey: according to the light the animal receives, its body will take a determined color. The mesencephalon contains the optical roof where the optical nerves end up. The metencephalon contains the cerebellum (fourth ventricle), which controls and coordinates the movements of the body.

This primitive chordate brain will undergo various changes and developments in the course of evolution towards higher living forms. This development will, however, not be a uniform progress affecting all parts of the brain to the same extent in different animal classes. The brains of fishes, amphibians, reptiles, birds and mammals will show large differences in anatomy, which in turn bear on their intellectual capacities. The size of the brain is naturally related to the body size. When allowance for this relation (brain size/body size) is made, several levels of encephalization among the jawed vertebrate species are recognized (fig. 6.6).


Figure 6.6. The increase in size of the brain varied in different vertebrate classes and orders, at different times. The lines drawn are, of course, only averages because great variations occur within each group. Also, there are gaps in our knowledge about the subject and broken lines represent these gaps. Within the mammalian group, dolphins and primates reached the highest level of encephalization. Dolphins were well ahead in this achievement.

6.3.2 The “lower vertebrate” group

It appears hardly credible, yet the relative brain size of bony fishes, amphibians and reptiles is the same. This holds for living and fossil representatives of these animal classes and includes the mammalian-like reptiles. All members of these vertebrate classes can be lumped into one big “lower-vertebrate” group. This group appeared as early as 400 million years ago and has remained successful until now, snakes being presently in full bloom. Vertebrates can live and prosper with an insignificant level of encephalization and do not need intelligence to survive and invade new biotopes. The passage of fish to amphibian occurred 350 million years ago and illustrates the conservatism manifested in brain evolution. The invasion of terrestrial niches by descendants of crossopterygians required from them only to be fish that move on land. This demanded only minor alterations in the patterns of neurological and behavioral organization and explains why batrachians are not more intelligent than bony fishes. The subsequent adaptive radiation of reptiles was accomplished without any major advances in encephalization, nor regressions. Dinosaurs were not small-brained. With respect to brain size, they were normal reptiles. The mammal-like reptiles, i.e. the terapsidian synapsids (see fig 5.19), were not significantly different with regard to the volume of the brain. In this reptilian order, there simply existed a potential for a magnified encephalon, due to the opening of the axial skull.

6.3.3 The birds

The two vertebrate classes that have a brain size superior to that of reptiles are the mammals and the birds. Some birds enjoy binocular vision. This convergence of the eyes into a frontal position is accompanied by a rearrangement of the optic nerve fibers in the brain. This may be accompanied by an increase in the size of the brain. The increased encephalization of birds is accompanied by an increase in intelligence as demonstrated by their successful performance of difficult intelligence tests. Yet, our feeling of alienation in the presence of members of this class, even the most intelligent such as crows, parrots and canaries, is justified. Intelligence as apparent in the behavior of mammals like cats, dogs or chimps, is a little-used capacity in birds. Birds descend from dinosaurs. The brain of reptiles is adapted to control automatic responses to specific patterns of stimulation. It has few requirements for plasticity and flexibility and the birds seem to have simply perfected this fixed-action pattern as their basic response to the requirements of their biotope. They lack flexibility and powers of adaptation.

6.3.4 The mammals Archaic mammals: nocturnal life

The mammalian-like therapsids were the dominant forms during the late Paleozoic era. When the dinosaurs appeared, competition for the normal niches of land reptiles was lost and the therapsids disappeared. These niches were for large diurnal animals that used daylight vision as their most important sense. The earliest mammals deriving from the therapsids must have been modified in a direction that would allow their survival as small nocturnal animals feeding on fruits and insects. This necessitated an adaptation to night vision and the development of a refined auditory and olfactory system. This development is sufficient for the evolvement of an enlarged brain.

The optics of the eye and the arrangement of the retinal elements provide a grid that labels the location of stimulated retinal cells. Tri-dimensional shapes located in space are thereby well perceived. However, archaic mammals leading a nocturnal life needed more to be successful and survive. They also had to be able to rely on an excellent auditory system. The decisive character that allows the paleontological recognition without ambiguity of an archaic mammal from a reptile is the direct articulation of the dental bone to the temporal one in the mammal. In reptiles, even the most mammal-like, this articulation uses two bones that, in mammals, are located in the ear where they form the incus and the malleus.

The auditory system must in addition be backed up by a good olfactory system. For these sensory systems to be useful in the appreciation of the external world, at least two conditions should be met. First, the auditory and olfactory systems must, like the visual system, be able to efficaciously handle the incoming information. The location of noises in space required the development of nervous structures. For the visual system, the first development occurs in the eye, at the level of the retina (fig. 6.7).


Figure 6.7. The circuitry of abstraction begins with receptive fields in the retina. These fields communicate with ganglion cells. Each row of receptive fields converge through the ganglion cells to one simple cortical cell. The simple cortical cells converge to one complex cortical cell. In mammals such as cats and monkeys, straight-edged lines are best noticed at the conscious level. In humans, there is evidence that curvature also is a specific feature of visual perception. This feature would depend on exposure to such curvature early in life. The human brain is viewed from below. About half of the optic fibers emerging from each eye remain uncrossed at the optic chiasm. Simple cortical cells are connected to receptive fields located in both eyes This binocular vision allows good perception of three-dimensional forms and a good evaluation of distances.

For the auditory and olfactory systems, such a development could not take place in the peripheral organ itself because the ear and nose are bony structures. There is no space in the ear or in the nose to accommodate the required additional nervous elements. What happened was a development of those parts of the brain involved in audition and olfaction. The first expansion of the vertebrate brain beyond the level of the reptiles may thus have been initially only the easy solution of a packaging problem. Perhaps man would have reached even a greater intelligence if the chordate eye had not been so malleable and able to accommodate additional nervous elements.

The second imperative to which the development of an auditory and olfactory system had to obey was the meaning of the information collected for the animal using these systems. If no integration occurs between the information received from the visual, auditory and olfactory systems about one single entity present in the environment, biological intelligence and perhaps even chances of survival will be poor. The noise, aspect and odor perceived must be integrated and given a common label so that all incoming information may be related in space and time to the same object, especially if it concerns a prey or a predator.

This construction of nervous systems for the handling of incoming information from a single source captured by different modalities and labeled in a simple, consistent way as coming from the same object in space at a particular time, will ultimately lead to the human conscious experience. The archaic mammals that appeared between 200 and 160 million years ago had a relative brain size about four-fold bigger than that of the reptiles. Once this increase in brain size was achieved, mammalian encephalization remained at a steady level at least 110 million years. This prolonged stability suggests a successful response to the selection pressures of a stable new ecological niche available from the Upper-Jurassic period (150 million years ago) to the late Eocene period (40 million years ago). The insectivores

The further encephalization of the mammals beyond the archaic level involved adaptations to niches that became available as a result of the elimination of the dinosaurs. With their extinction, mammals began the occupation of daytime niches. This occupation was heralded by an initial adaptive radiation that simply involved an increase in body size. This first stage composed the archaic mammalian divergence that involved no further encephalization. However, readaptation to daytime activities requested the evolvement of a daytime vision. This vision had to evolve, based on mechanisms different from those of earlier reptiles, because these mechanisms had been lost during the nocturnal phase. The new mammalian daylight visual system called for further encephalization. The main problem at that stage was to encode the spatially integrated information in a temporal code, i.e. gain a memory of a sequence of events. The initial step in that direction was made by insectivores. These appeared 50 million years ago and showed a 2 to 3 fold increase in brain size over that of architherians. This encephalization was maintained by insectivores at that level ever since. Evolved mammals

Some mammalian orders increased, thereafter, their relative brain size in an orderly fashion (e.g. carnivores and ungulates i.e. horses, antelopes, camels, tapirs, etc.). This gradual increase in encephalization occurring in ungulates and carnivores throughout the last 50 million years is expected if this encephalization evolves as other traits do: the invasion of new niches by successive species of carnivores and ungulates must have included many niches in which there were selective advantages for species further encephalized. As more of these niches were invaded, there would have been a diversification with some small-brained species persisting while others gained a relatively larger brain-size. Prosimians, cetaceans and hominoids

The cetaceans, prosimians (see fig.5.26) and hominoids are three animal groups that effectuated additional encephalization quantum leaps. For them, a different evolutive pattern must be invoked. In their case, the penetration of an adaptive zone was probably followed by the rapid attainment of a particular grade of encephalization and this grade was thereafter maintained steady.

At the present time, encephalization is maximum for dolphins and man, which both intensely rely on sound and hearing as means of communication. The large brain size of the dolphins was already acquired 18 million years ago. The dolphins and whales derive from artiodactyls (close to the hippopotamus) while the seals derive from an ancestor close to the wolf. Both lost the capacity to recognize colors except green. This loss was perhaps the price that had to be paid to develop other, desirable, capacities necessary to exploit the alimentary bounties of the oceans. The adaptation of mammals to the high seas required four adaptations. The first was locomotion, with the morphological adaptations needed to pursue efficaciously their preys, the second was the supply of fresh water, because the mammals cannot survive on salt water, the third was communication with their congeners in water and the fourth was location of their preys in muddy waters.

The evolution of the hominid brain was completed only in the last million years.

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