Conclusions

First on neocorticalization. The fossil evidence indicates that there was neocorticalization in mammals and that it can be detected even in samples as small as 15 species of Carnivora. Evolution of the carnivores involved neocorticalization in another sense, in that the two great orders of Tertiary carnivores (Creodonta and Carnivora) differed in the extent of neocorticali-zation. This could have been a factor in the survival of true carnivores. In any event, the history of neocorti-calization indicates that there was almost certainly some benefit derived from the expansion of neocortex. The fossil evidence therefore confirms conventional wisdom in neurobiology that it was a progressive thing for mammals to evolve neocortex and (perhaps within limits) more is better.

If the conclusions on neocortex are expected, those on the olfactory bulbs are not. These structures are surprisingly constant features in mammalian brains. There is no reason to have predicted that they would not evolve to large or small size relative to other parts of the brain, depending on the extent of olfactory specializations in particular species. But according to current evidence from fossil brains, the olfactory bulbs have been constant and relatively unchanging features that make a brain a mammal's brain. They are not unusually enlarged in any species; most mammals are olfactory specialists. Evolutionary changes in the olfactory bulbs occurred mainly in a negative way, by reduction. The reduced state of olfactory bulbs in humans and other primates (and their complete absence in some cetaceans) merely reflects the extremes of diversity that are possible as the brain evolved to control the activities of mammals in the variety of niches in which they function.

From these and related data it seems likely that encephalization in mammals was driven by neocorti-calization. One mammalian trend was toward enlarged neocortex, and since neocortex is a fairly fixed fraction of total brain size the enlargement of the brain was presumably correlated with the increased size of neocortex. Because neocortical function is deeply involved with cognitive functions—knowledge of ''reality,'' expanded neocortex would be associated with more elaborate cognition, a major suggestion about the evolution of mind.

It would be appropriate to look more closely at neocortical functions and assume that the evolutionary advantage conferred by these functions was the engine driving progressive brain evolution in mammals. However, it would be a mistake to make much of such an idea of progress. It is true that neocorticalized species are more prevalent now than in the distant past. It must also be true that some fitness is associated with this aspect of the brain's evolution. But there are many successful living species that are at a very ancient grade of mammalian neocorticalization. Hedgehogs in Europe and opossums in America are outstanding examples because they are very fit in the evolutionary sense. They may litter our highways because of their "stupidity" in refusing to yield the right of way to cars and trucks, but the litter is part of the evidence of their reproductive success. And they manage this at a grade of neocorticalization and encephalization that some mammalian species reached 150 Ma.

The analysis of neocorticalization suggests that comparable advances occurred in birds with the expansion of their forebrains. It is certainly true that the avian forebrain is much enlarged compared to that of reptiles, and the grade of encephalization in birds is probably related to forebrain enlargement. The optic lobes (midbrain, homologous to the superior colliculi in mammals) also seem much larger in birds than in reptiles, at least to an analysis by eye.

The final conclusion, however, must be that animals do not live by brains alone. The majority of living vertebrates get along with about as much brain as was present in their earliest ancestors. Adaptation to one's niche can be accomplished in many ways, and to adapt behaviorally by brain enlargement is expensive energetically. The brain is profligate in its use of energy, and almost any other solution to an adaptational problem is less costly. In those groups that adopted encephalization as an adaptive strategy, however, there was evidently a real gain in fitness. Encephaliza-tion appeared in many very distantly related species of birds and mammals. It is a general rather than specific adaptation for increased total information processing capacity. It is sometimes considered as the brain correlate of intelligence. If that is true, it must mean that there are many intelligences that evolved, since encephalization is an overall sum of enlargements of constituent regions within the brain. Because the different regional enlargements are correlated with different behavioral capacities, there would be different kinds of intelligences in different species.

See Also the Following Articles

BRAIN ANATOMY AND NETWORKS • BRAIN DEVELOPMENT • CIRCADIAN RHYTHMS • INTELLIGENCE • LANGUAGE ACQUISITION • LATERALITY • NEOCORTEX

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