How to study brain evolution

The study of brain evolution is a historical science, yet very little about brain evolution can be learned from the fossil record. Brains die and rot, and only the brain case, with luck, remains. Brain cases can tell us something about brain size, brain shape, and sometimes indicate major brain fissures. Brain size is easy to measure, and it is an important variable. Thus, much has been written about brain size (e.g., Jerison, 1973). Moreover, it is especially relevant to consider the implications of observed differences in brain size when studying the evolution of human brains, since our brains have greatly increased in size over the past 3 to 4 million years (Preuss and Kaas, 1999). But the great differences in the internal complexity of brains are not revealed by their external size and shape. Thus, most of what we learn about brain evolution will come indirectly through inferences based on observations made on the brains of the many extant (living) species (Kaas, 1997a). In studying human brain evolution, we have a rich opportunity in that living members of the primate order are highly varied in brain organization, and we have roughly 200 primate species to consider.

The comparative approach to studying brain evolution depends on examining the brains of the most relevant extant species, determining what feature or characters they share, and evaluating the probabilities that features are shared because they were inherited from a common ancestor or evolved independently (see Northcutt, 1984; Preuss, 2000; Striedter, 1998). To obtain a comprehensive understanding of the evolution of the human visual system and those of other primates will take much comparative research, but the approaches and methods of study are known. Fortunately, there has been considerable progress, and this progress allows some of the conclusions that follow here.

The basic premise of the evolutionary or phylogenetic approach is that the brain and other body parts of extant mammals are mixtures or mosaics of features that emerged at various times over the course of evolution. The distributions of any trait across the brains of extant mammals, and the phylogenetic relationships of these mammals, provide the critical sources of information about when a trait in any particular line of evolution emerged. For example, all placental mammals have a corpus callosum. No other mammals or vertebrates do. Thus, the corpus callosum emerged after the ancestors of placental mammals diverged from marsupials. This cladistic approach (Hennig, 1966) sounds easy, and it is in concept, but the difficulty lies in recognizing brain characters. If current researchers disagree on the existence of V3 in primates, how can we deduce the evolution of V3? As a further complication, even if the existence of a corpus callosum or V3 can be determined with a high degree of certainty, this is only a start, since the features and functions of these structures will vary across and within lines of evolution. A serious problem here is that comparative studies can be time-consuming and costly, and thus the need to be practical will often compromise the plan of investigation. Finally, many relevant species are either unavailable for study or can be studied only after natural death. This includes most of the 200 or so species of primates that are potentially available for study. Fortunately, much can be learned from histological studies of the brains of a few available specimens, and interpretations of comparative studies can be guided by more extensive information from more fully studied species. Other important insights will come from our emerging understandings of the mechanisms of brain development and the “rules” of brain scaling (Finlay and Darlington, 1995; Kaas, 2000).