Are the main grades of brains different principally in numbers of connections or also in quality?

Summary

From left to right, no and yes. The premise is underlined that grades of complexity of brain and consequent behavior have evolved over an extremely wide span, if we compare major taxa from jellyfish to cuttlefish — not linearly, steadily or invariably and not necessarily correlated with survival value or improved adaptation. This relatively rare form of evolution, unequivocal only between some phyla, some classes and some orders, can be called vertical gradation and should be distinguished from lateral radiation that leads to species, genera and many families, dominated by adaptive specialization to a particular habit of life. More complex brains may have quantitatively more connections but are principally recognized by qualitatively different components. They can logically be called more advanced or “better” in this dimension, without implication of moral value or improved adaptation. Complexity is usefully measured by the number of kinds of components of structure, function and behavior, a usage quite distinct from a common application to highly specific or amazing achievements.

The inverebrate groups are reviewed to underline the saltations in grade of complexity of brains and behavior between many phyla and classes and some orders. The proposition is asserted that neuroscience has relatively neglected the study of such differences, to discern them, quantitate and evaluate them, using modern methods in each relevant domain — anatomy, physiology, chemistry, and behavior. It will be important to complete the detailed characterization of the major grades of complexity found among animals. This agenda is likely to uncover new principles in brain operation and in the biology of diversity.