The brain taking shape

The basic developmental stages of the brain and spinal cord – from a simple neural tube to the three main brain regions (forebrain, midbrain, hindbrain) – are common to all vertebrates, from fish to amphibians, to reptiles, birds, and mammals (including humans).

This similarity of the early stages of brain development across species is an example of ‘evolutionary conservation’ – a key theme in evolutionary and developmental biology. Only as development progresses do the specialised structures become more distinct across different vertebrate species.

However, regions derived from similar early brain structures, i.e., forebrain, midbrain, or hindbrain, still correspond to similar, increasingly complex functions.

For example, as in humans, the hindbrain controls basic functions like breathing, reflexes and fine movements. The midbrain plays key roles in sensory perception and attention. The forebrain controls higher-order cognitive functions, like learning, memory, and sensory processing.

There is clear functional importance for different brain regions growing to different sizes and shapes, with larger and more complex areas usually indicating more specialised functions.

For example, species in which the sense of smell (olfaction) is more important, such as pigs, also have relatively larger brain areas dedicated to the processing of olfactory information. On the other hand, species that rely more on visual cues, like humans, have enlarged brain areas dedicated to vision.

Humans also have a uniquely enlarged cerebral cortex (the wrinkly surface covering most of the brain) thought to be the key to our remarkable cognitive abilities. Thus, refinements in the shaping of a shared basic brain plan across vertebrates underlie the astounding diversity of form and function between brains of different species.