Quantitative Analysis Living Vertebrates

Darwin's theory of natural selection emphasized evidence of selection as practiced by animal and plant breeders. The term natural selection implied that nature, like breeders, worked to select the "most fit" individuals relative to some criterion. In a breeder's case, the criterion might be plumpness, large size, docility, and so forth. Nature had other criteria, and animals well endowed on nature's criteria would be more likely to survive to produce more offspring than would less well-endowed animals. The unnatural docility of domestic animals suggested some deficit in their brains.

Darwin could explore the relationship by comparing brain size in wild and domesticated populations of animals known to be related to one another. In what was probably his only contribution to neurobiology, Darwin was the first to observe that the brain in domesticated rabbits was smaller than that in their wild cousins. This is evidently a general principle on the effect of domestication on brain size. It may even be true for human brain evolution if we think of ourselves as domesticated and our ancestors as savage or feral, although the available sample size is too small for a clear test. The earliest Homo sapiens were the neandertals, and they were slightly larger brained, on average, compared to their living conspecifics.

A. Uniformities in Structure in Living Brains: Allometry

Darwin's publications inspired several generations of comparative neuroanatomists to provide detailed pictures of the diversity of brains. The effort included studies of brain size and of brain-body relations, some of which stand up under appropriate analysis today. In this tradition, Brodmann tabulated data on brain surface area in mammals, and his results are included as 33 of the 50 data points in Fig. 2A. The validity of his work is attested to by its consistency with data from more recent studies that used different methods of measurement. A single regression line fits the entire data set remarkably well. Both Fig. 2A and Fig. 2B are examples of uniformities of organization of the brain in mammals.

Analyses such as those in Fig. 2 are "allometric" in that they display the relationship between different morphological features, such as height and weight, as they can be measured in any animal. The relationships displayed in Fig. 2 transcend species and are so strong that they appear to reflect a fundamental feature of the body plan (Bauplan) of mammals. Evolutionists call shared ancestral features plesiomorphies, and although the term is not usually applied to functional relationships such as those shown in Fig. 2, the idea fits. The relationships should be thought of as representing a primitive feature in mammalian evolution.

Because of its unusually diverse sample of species, Fig. 2A provides important justification for using total brain size as a statistic that estimates the total neural information processing capacity of a brain, between species, in the mammals as a class. To understand this further, consider some candidates for the role of processing unit. A frequent candidate is the cortical column, and its cross sectional area appears to be relatively uniform across species. The neuron is another candidate, and the number of neurons under a given surface area of neocortex is more or less constant across species. Finally, the synapse is a candidate. Braitenberg and Schiiz observed that the number of synapses per unit volume of cortical tissue is

Invertebrate Mammal Human Brain Size

10-1 10° 101 102 103 104 10-1 10° 101 102 103 Brain Weight (grams) Brain Weight (grams)

Figure 2 (A) Cortical surface area as a function of brain size in 50 species of mammals, including orders Monotremata, Marsupialia, Artiodactyla, Carnivora (including pinnipeds), Cetacea, Perissodactyla, Primates, and Xenarthra. Minimum convex polygons enclose individual human (n = 20) and dolphin (Tursiops truncatus: n = 13) data and indicate within-species variability. Some species are named to suggest the diversity of the sample. (B) Cerebellar volume as a function of brain size in 76 species of mammals (insectivores: 26 Insectivora, 2 Macroscelididae, and 3 Scandentia; primates: 18 Prosimii, 27 Anthropoidea (redrawn with permission from Jerison, 1991).

10-1 10° 101 102 103 104 10-1 10° 101 102 103 Brain Weight (grams) Brain Weight (grams)

Figure 2 (A) Cortical surface area as a function of brain size in 50 species of mammals, including orders Monotremata, Marsupialia, Artiodactyla, Carnivora (including pinnipeds), Cetacea, Perissodactyla, Primates, and Xenarthra. Minimum convex polygons enclose individual human (n = 20) and dolphin (Tursiops truncatus: n = 13) data and indicate within-species variability. Some species are named to suggest the diversity of the sample. (B) Cerebellar volume as a function of brain size in 76 species of mammals (insectivores: 26 Insectivora, 2 Macroscelididae, and 3 Scandentia; primates: 18 Prosimii, 27 Anthropoidea (redrawn with permission from Jerison, 1991).

constant across species. There are qualifications to these generalizations, but they are reasonable first approximations. They reinforce the conclusion about the use of brain size as a statistic.

Figure 2B is another allometric analysis. It demonstrates the uniformity of cerebellar size in mam-mals—that, independent of species, if you know the size of the brain you can make a very good estimation of the size of the cerebellum. Although this analysis does not have the obvious theoretical significance of Fig. 2A, it demonstrates the fundamental orderliness of the construction of the brain. It validates the use of brain size as a statistic to estimate the size of other brain structures. To the extent that these other structures can be assigned special functional significance, one may be able to use quantitative data on brain size to assess the evolution of brain functions. This has been done by M.A. Hofman and by me for the analysis of the control of social behavior and of other neocortical functions. In general, anatomists emphasize the differences among the species that they examine, but I have been even more impressed by the uniformities. The example in Fig. 2B of the relationship between cerebellum and brain size is just one of these. Table I summarizes a multivariate analysis of an entire data set: 12 morphological measures in 76 species.

The most important fact in Table I is that just two factors were enough to account for all but 1.5% of the variance, and that 86% of the variance is explained by a single "size" factor. The size factor is a "general" factor in the sense that it is strongly represented in all the brain structures with the exception of the olfactory bulbs, and it is also represented in body weight. The loading of cerebellum on this factor (0.983, accounting for 97% of the variance in cerebellar volume) in conjunction with the even higher loading of total brain weight reflect the high correlation shown in Fig. 2B. All the measures with higher loadings would have produced bivariate graphs such as Fig. 2B. The mammalian brain hangs together well, and when one part is enlarged the rest of the brain tends to be correspondingly enlarged.

Perhaps the most surprising extension of this conclusion is the tentative discovery that even the size of prefrontal neocortex, the ''executive organ'' of the mammalian brain, appears to be determined by the size of the whole brain in mammals. The conclusion is tentative because it is based on evidence from only four primate species (marmoset, rhesus, orang, and human) and the laboratory rat, but there was an almost perfect correlation between neocortex volume and brain

Table I

Factor Loadings and Percentage Variance Explained by Two Principal Components (Factors) in Brain and Body in 76 Species of Mammals0

Table I

Factor Loadings and Percentage Variance Explained by Two Principal Components (Factors) in Brain and Body in 76 Species of Mammals0

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