Evolution and the brain: what is novel?

The geneticist Theodosius Dobhansky famously stated that in biology nothing makes sense unless it’s in light of evolution. The same applies to neuroscience, a biological science. But evolution poses a conundrum. Vertebrates have been evolving for over 500 million years[1]. A telencephalon, a midbrain, and a hindbrain are part of the general plan of their nervous system. Structures like the amygdala and the striatum are found in animals as diverse as a salmon, a crow, and a baboon. Thus, many parts of the brain are “conserved”. But, then, what is novel? Something must be new after all.

From Pessoa, L., Medina, L., Hof, P. R., & Desfilis, E. (2019). Neural architecture of the vertebrate brain: Implications for the interaction between emotion and cognition. Neuroscience & Biobehavioral Reviews, 107, 296-312.

In chapter 9, we described how homology refers to relationships between traits that are shared as a result of common ancestry. The leaves of plants provide a good example[2]. The leaves of a pitcher plant, Venus flytrap, poinsettia, and a cactus look nothing alike, and in fact have distinct functions. In the pitcher plant, the leaves are modified into pitchers to catch insects; in the Venus flytrap they are modified into jaws to catch insects; in the poinsettia bright red leaves resemble flower petals and attract insects and pollinators; cactus’ leaves have become modified into spines, which reduce water loss and can protect the plants from herbivores. Yet, the four are homologous given that they derive from a common ancestor.

A structure adopts new functions during evolution, while its ancestry can be traced to something more fundamental[3]. Take the hippocampus of rodents, monkeys, and humans. There is copious evidence indicating that the area is homologous in the three species, that is, that is a conserved structure. But does this mean that it performs the same function(s) in these species? Does it perform some qualitatively different function(s) in humans, for example? To many neuroscientists this sounds implausible. However, the possibility need not be any more radical than saying that the forelimb does something qualitatively different in birds compared to turtles, say. If common ancestry precluded new functions, no species could ever take flight!

The ongoing discussion is particularly pertinent when we think of emotion and motivation, because researchers invoke “old” structures when studying these mental phenomena. Regions like the amygdala at the base of the forebrain and the periaqueductal gray in the midbrain in the case of emotion; the accumbens (part of the striatum) also at the base of the forebrain and the ventral tegmental area in the midbrain in the case of motivation. Because these regions are deeply conserved across vertebrates, they function in a similar way, or so the reasoning goes. If we entertain these areas in rodents, monkeys, and humans, closer as they are evolutionarily, the expectation would be that they work in largely the same manner. But rodents and primates diverged more than 70 million years ago. Are we to suppose that no qualitative differences have emerged? This seems rather implausible. (In Chapter 9, we briefly reviewed some of the differences in the amygdala of rats, monkeys, and humans.)

The argument made in this book is that we should conceptualize evolution in terms of the reorganization of larger-scale connectional systems. Instead of more cortex sitting atop the subcortex in primates relative to rodents – which presumably allows the “rational” cortex to control the “irrational” subcortex – more varied ways of interactions are possible, supporting more mental latitude.

The brain doesn’t fossilize. Unfortunately, with time, it disintegrates, leaving no trace. So we simply don’t have a way to know exactly what the brain of a common ancestor looked like. Without fossil remains, scientists tend to think of the brain of a common ancestor of rodents, primates, and humans as something like the current brain of a mouse, as this animal is the “most primitive” one. But a mouse encountered today has had 70 million years to evolve from the ancestor in question, and thus specialize to the particular niches it inhabits now.

Evolution is as much about what’s preserved as what’s new. Ever since science was transformed by the independent work of Charles Darwin and Alfred Russell Wallace in the late 1850s, biologists have sought to determine “uniquely human” characteristics. This has led to a near-obsession to identify one-of-a-kind nervous system features, from putative exclusively human brain regions to cell types. The cortex, in particular, has attracted much attention. We described in Chapter 9 how much of the pallium of mammals is structured in a layered fashion, a quality that is not observed in other vertebrates. Well, not exactly, as some reptiles (such as turtles) have a dorsal pallium that is cortex-like, with three bands of cells. Mammals, however, have parts of the cortex that is much more finely layered, with six well-defined zones. In fact, six-layered cortex is often referred to as “neocortex”, with the “neo” part highlighting its sui generis property (in the book, the more neutral terminology “isocortex” was adopted for this type of cortex).

I offer that the concept of reorganization of circuits is a much more promising idea. That is to say, what is unique about humans is the same that is unique about mice, or any other species: their circuits are wired in ways that support survival of the species. This is not to deny that some more punctate differences play a role. But whatever the differences are, at least considering primates with larger body sizes, they are not staring us in the face – they are subtle. For example, all primates exhibit an isocortex that is massively expanded[4]. Primates also have prefrontal cortices with multiple parts, including the lateral component, which neuroscientists often link to “rational” capabilities. More generally, direct evidence for human-specific cortical areas is scant[5].

Let’s go back to Dobhansky’s call to consider biology in light of evolution – always. Biologists would vehemently agree. But evolution is so egregiously complex that the suggestion doesn’t help as much as one would think. Verily, what we observe in practice is that neuroscientists who don’t specialize in studying brain evolution are time and again cavalier, if not outright naïve, about how they apply and think of evolution. By doing so, our explanations run the risk of becoming just-so stories[6].

[1] For a framework on vertebrate evolution, see Pessoa, L., Medina, L., Hof, P. R., & Desfilis, E. (2019). Neural architecture of the vertebrate brain: Implications for the interaction between emotion and cognition. Neuroscience & Biobehavioral Reviews, 107, 296-312.

[2] https://evolution.berkeley.edu/evolibrary/article/0_0_0/lines_04

[3] Sentence closely borrowed from Murray et al. (2016): “a structure adopts new functions during evolution, yet its ancestry can be traced to something more fundamental”. Discussion of the hippocampus until end of the paragraph also from them.

[4] Striedter (2005).

[5] Striedter (2005).

[6] The Wikipedia page on just-so-stories is actually pretty decent: https://en.wikipedia.org/wiki/Just-so_story.

Brain structure and its origins

I just finished reading most of this wonderful new book by Gerald Schneider from MIT (who was already describing two visual systems in 1969): Brain Structure and Its Origins: in Development and in Evolution of Behavior and the Mind.

9780262026734Comparative neuroscience is seldom ever touched in standard textbooks, which is really unfortunate because so much of how we think about the brain has to do with some “folk neuroscience” ways of thinking. This can hardly be ignored when we start studying emotion, motivation, reward, all those good things that gained so much traction in neuroscience in the past two decades. And now are mainstream.

No ones’ work, or book, is perfect of course. My main problem — perhaps not surprisingly — with the book is its treatment of the “limbic system”. Although it is grounded in comparative neuroanatomy and thus much better than other treatments of the purported system, the treatment is problematic for lots of reasons. The presentation is miles better than what would be found in a medical neuroscience textbook, but discussing an emotion circuit of Papez is just unfortunate.

But otherwise, what a great book. I wish I had learned about the brain from a book like this, where was it all along?!