Neocorticalization

Neocorticalization is a concept in comparative neu-roanatomy, based on comparisons among living species. For example, it describes the fact that primates have relatively and absolutely more neocortex than insectivores. One makes the statement: Primates are more neocorticalized than insectivores. Of course, living insectivores did not evolve to change into living primates under natural selection. But there is an evolutionary translation of the statement: The ancestors of living insectivores were members of an order (or other taxon) of mammals that probably included at least one species from which primates evolved. This species has not been identified, but as evolutionists positing a relationship between insectivores and primates as ''sister'' groups, we have to assume that it existed. Neocorticalization is then understood as part of the differentiation of daughter species from parent species: One daughter species of a fossil insectivore, which had relatively more neocortex than the parent species, was the ancestral primate species. These statements are almost parodies of evolutionary analysis, but they approximate a correct analysis.

The concept of neocorticalization can also be used in another sense—as describing the history of a trait in successive populations of species. If we sample a broad range of species across geological time and determine that later species had relatively more neocortex than earlier species, we could state that neocorticalization had occurred, even though we would not be able to determine its phylogenetic history. Such a discovery would be enough to suggest that there was a selective advantage in an increase in neocortex. The analytic model to be applied might be an elaboration of simple models of phenotypic evolution within lineages. It would require theorizing about selective advantages above the species level—about broad evolutionary ''landscapes'' that are contexts for interaction among species.

Neocorticalization in this sense can be quantified as a feature of the history of the mammals. It is more or less evident from a simple inspection of endocasts, but the quantitative effect can only be demonstrated with the help of some statistical analysis. I present such an analysis in Fig. 7. For the analysis, I measured the planar projection of neocortex and olfactory bulbs in 59 species of living and fossil ungulates and carnivores. The sample included 38 Carnivora, 7 Creodonta, 4 Condylarthra, 5 species from extinct South American (neotropical) ungulate orders, 1 Eocene perissodactyl (Hyrachyus, an ancestral rhinoceros), and 4 progressive ungulates (artiodactyls). The complete sample consisted of 35 fossil species and 24 living species.

The results were analyzed as neocortical and olfactory bulb quotients, determined by partialling out the effect of body size much as in encephalization quotients. A quotient of 1.0 means that the size was as expected for a species at the centroid, 0.5 means it was half as big, and so on. As presented in Fig. 7, the results lead to four conclusions. There is first the fact of neocorticalization. Later species tended to have relatively more neocortex than did earlier species. This is the meaning of the significantly positive slope of the regression line of the neocortical quotient against geological time.

Figure 7 (Top) Change in relative neocortical surface area (neocortical quotient) as a function of geological age. ''Progressive'' change noted here (positive slope of regression line) indicates increased neocorticalization over time. Each point is a species. (Bottom) Absence of change in relative surface area of the olfactory bulbs as a function of geological age. (Redrawn with permission from Jerison, in Jones and Peters, 1990).

Figure 7 (Top) Change in relative neocortical surface area (neocortical quotient) as a function of geological age. ''Progressive'' change noted here (positive slope of regression line) indicates increased neocorticalization over time. Each point is a species. (Bottom) Absence of change in relative surface area of the olfactory bulbs as a function of geological age. (Redrawn with permission from Jerison, in Jones and Peters, 1990).

Second, species from archaic orders (Fig. 7, filled symbols) tended to have less neocortex than did species from progressive orders (Fig. 7, open symbols). Thus, 3 of the 4 archaic ungulate species, 5 of the 7 archaic carnivore (creodont) species, and 4 of the 5 Neotropical ungulate species (also archaic in that their orders are extinct) are below the regression line. Twelve of the 16 archaic species thus had less neocortex than would be predicted for their geological age by an unbiased regression analysis. For those who enjoy playing with statistics, a chi-square analysis contrasts this with an expected even split: y2 = 4, df = 1, p o 0.05.

The third result in a comparison between progressive and archaic species limited to fossil Carnivora versus Creodonta. It is in two parts. First, the Carnivora points appear generally to be higher than those of the Creodonta. Second, the Carnivora points seem to show more ''progress'' over time than do those of the Creodonta. It is not possible to test the first part properly, because the species are from different geological times, and there is no obvious way to control the time variable. The second part, however, can be tested by simple regression analysis. The correlation between age and neocortical quotient for 15 Carnivora species was r = 0.72 (p<0.01). For seven Creodonta it was r = 0.42 (p>0.05). Only the Carnivora were demonstrably progressively neo-corticalized.

The result is important for evolutionary analysis of the relations between true carnivores and creodonts. The argument is summed up by R. L. Carroll as follows: ''Romer (1966) and Jerison (1973) stigmatized the creodonts as archaic and small brained, but Radinsky (1977) demonstrated that relative brain size increased as rapidly among creodonts as it did in the early members of the Carnivora, together with an increase in the extent of the neocortex.'' The quantitative analysis supports Romer's view as mentioned by Carroll. (My contribution in 1973 was mainly to quote Romer and to provide very limited quantitative data. The current confirmation of our older view is possible because of the additional data collected by Radinsky, which permitted a statistical test.)

The final conclusion is that unlike neocortex, the olfactory bulbs did not change in relative size with the passage of time. The correlation between the olfactory bulb quotient and geological time was r = 0.1, which is not significantly different from zero. This is an important point because it shows that this approach is fine enough to discriminate between the presence and absence of change. It also helps quash a myth about what is ''primitive'' and ''progressive'' in brain evolution.

Although careful students do not make the error, some neurobiologists assume that having large olfactory bulbs is a primitive mammalian trait and that the olfactory bulbs became relatively smaller as the mammals evolved. Fig. 7 corrects this error by showing that olfactory bulbs have been a stable feature of the brain in Tertiary carnivores and ungulates. The misconception is a bit of primate chauvinism, as it were. Primates (at least the anthropoids) are neocor-tical specialists, but they are deficient mammals in olfactory development. A reduced role for olfaction is part of the adaptive mosaic of the adaptive zone of simian primates and is not a broad feature of mammalian evolution. (It also complicated the factor analysis presented earlier.) Neocorticalization, on the other hand, appeared as a general trend in many mammalian groups and its relative absence in the insectivores and many marsupials is correctly recognized as a primitive feature in these groups.

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  • Lara
    What is neocorticalization?
    7 years ago

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