Figure 3 Correlated evolution among major brain structures. Whichever explanation is correct, the patterns of covariation indicate that, even at this anatomically crude level, brain structure evolution involved a complex set of relationships among individual structures. The chain of structures along the main axis in Fig. For example, the main part of the diencephalon, the thalamus, is the site of many major relays to and from the neocortex. The only difference between the patterns for the two orders is that cerebellum size correlates with neocortex size in primates, and with diencephalon size in insectivores.
Extensive connections exist between neocortex and cerebellum.
Given that many of these are relayed through the diencephalic thalamus, there is likely to be a tight three-way evolutionary relationship amongst neocortex, cerebellum and diencephalon. The major brain sub-divisions in Fig. We therefore tested for correlated evolutionary change in the individual components of specific functional units or systems. Data are available for six different systems, five in each order, yielding ten separate tests in all. Hence, components of functional systems have highly specific evolutionary relationships, not attributable to their membership of more global systems such as the whole brain or the limbic system.