Brain changes with evolution

Chapter 4 endnote 61, from How Emotions are Made: The Secret Life of the Brain by Lisa Feldman Barrett.
Some context is:

So how do brains evolve? They reorganize as they expand, like companies do, to keep themselves efficient and nimble

Scientists still debate whether the human brain has evolved unique regions over the millennia. (Frankly, it’s very difficult to “line up” the brains of two different species and see what’s left over.) But there is no anatomical justification for a story where cognition in the cortex regulates emotion in the “limbic system,” nor where cognition and emotion interact to produce behavior. Some scientists like the neuroscientist Barbara Finlay argue that no new regions have developed since the original nocturnal mammals evolved, from which all modern mammals derive. Other scientists argue that they have identified entire brain regions as evolutionarily new in humans. For example, the neuroscientist Bud Craig has written that macaque monkeys have no ventral anterior insula, and the evolutionary biologist Todd Preuss argues that rats and other rodents lack a lateral prefrontal cortex.

Setting the question of new brain regions aside, it is possible for brains to change in other ways during evolution. Here are three examples:

Individual regions may become more or less complex.

Neurons in the cortex are arranged in layers (when viewed in cross-section, they look as if they are arrayed in columns). As the mammalian brain evolved, primary sensory regions developed a distinct middle layer of cortex (called “layer IV” or the “granular” layer) with projections from the thalamus that bring sensory input from the world to the brain. Opossums and other small brained marsupials have no middle layer in their primary visual cortex (V1). Rats have a very small middle layer. Monkeys have a well-developed layer, but not as much as humans and other great apes. Conversely, sometimes regions become simpler—for example, the olfactory bulb is smaller and simpler in humans than in chimpanzees and other primates.

Layer 4 is called the granular layer because it contains neurons that, when stained, look like granules. According to the neuroscientist Barbara Finlay, evolving a layer 4 introduces an extra processing step in a column of neurons that allows better integration of information, because most of the neurons in Layer 4 are stellate neurons that project locally to other neurons close by.

The number of connections can increase.

A human brain is pretty similar to a chimpanzee brain, only bigger and rewired. Human brains are three times larger than chimpanzee brains, sharing about 98% of the same genes but less (about 96%) other genetic material, and differ primarily in the density of connections between neurons in the middle and at the front of the brain. This is where the interoceptive and control networks are located (as well as the language network, which overlaps a good deal with portions of both networks).

Networks can change their function.

The brain regions important for language in humans are also found in macaque monkeys and rats who don’t have language. Chimps have an intrinsic network that looks very similar to humans’ language network, even though chimps cannot speak or understand language (although there is an ongoing debate about their ability to use simple symbol systems to communicate). Chimps can learn individual words but they have nothing like human language skills.

Rats also appear to have something like a default mode network (part of the interoceptive network), as do macaque monkeys and chimps, but it seems fairly certain that this network is performing different functions in humans when compared to these other animals, in part because the microstructure of the human brain (particularly in the default mode network) has changed with evolution. At the front of the brain (in language, control and interoceptive networks, including the default mode and salience parts of the interoceptive network), there are substantial wiring differences between human and chimp brains (with human brains having denser connectivity). There are also differences in the wrapping around neurons (to speed information transfer between neurons, called myelination), dendritic densities (increasing the strength of connection at synapses, to help transmit the electrical signal to the neuron’s cell body), neurotransmitter concentrations (the chemicals that sit in the synapses between neurons and allow them to transfer information), the number and type and distribution of glial cells (brain cells that help to regulate how efficiently neurons pass information to one another, along with other things like brain metabolism).