The final—and most important—problem with this mistaken view is the implication that anatomical evolution proceeds in the same fashion as geological strata, with new layers added over existing ones. Instead, much evolutionary change consists of transforming existing parts.
- From https://journals.sagepub.com/doi/full/10.1177/0963721420917687#con1
I have definitely made this mistake, many many times.
I'm not sure yet the implications of recognizing instead that "all vertebrates possess the same basic brain regions, here divided into the forebrain, midbrain, and hindbrain;" in some ways it seems like a nuance, but in other ways I think it'll shift how I see things and talk about things.
more quotes in case you don't read the article:
neural and anatomical complexity evolved repeatedly within many independent lineages
the correct view of evolution is that animals radiated from common ancestors (Fig. 1c). Within these radiations, complex nervous systems and sophisticated cognitive abilities evolved independently many times. For example, cephalopod mollusks, such as octopus and cuttlefish, possess tremendously complex nervous systems and behavior (Mather & Kuba, 2013), and the same is true of some insects and other arthropods (Barron & Klein, 2016; Strausfeld, Hansen, Li, Gomez, & Ito, 1998). Even among nonmammalian vertebrates, brain complexity has increased independently several times, particularly among some sharks, teleost fishes, and birds (Striedter, 1998).
The idea that larger brains can be equated with increased behavioral complexity is highly debatable (Chittka & Niven, 2009).