BRAINS, BRAINS, BRAINS!!! Evolution of the Human and Mammalian Brain

Why is the brain so fascinating? As far as humans go, even though paleoanthropologists consider “bipedalism” to be the earliest characteristic which made the line of Homo unique, we all know that it’s our brains that make us special. It’s not only an incredible mystery as to why our intelligence appears to stand above the rest of our mammalian brethren, but it’s also our big-brained capacity for reflection and a drive for understanding that makes us want to dissect ourselves in the first place. Literally.

rembrandtdissection

Painting by Rembrandt, titled “The Anatomy Lesson of Dr. Joan Deyman” (1656).

I can’t quite profess to know exactly why the human brain is so fascinating except that 1) it’s really smart (though one might argue that common sense is frequently lacking!), and 2) it’s still kind of a mystery as to how we became so smart in the first place. Granted, we’re not the only mammals with big, highly-gyrified cortices and complex social hierarchies. Animals such as dolphins, whales, and elephants each share these traits with us. The last common ancestors that we and each of these mammals shared was long before any of our species lines became so highly encephalized, so the similarities seen between us can be considered Convergent Evolution, wherein the same or similar trait arises in two separate species and isn’t due to inheritance of the trait from a common ancestor.

gyrified_brains

An image of cerebral cortices from whale, dolphin, and elephant. Adapted from Butti et al. (2011).

Human ancestors evolved bipedally well before we became big-brained, tool-toting nerds. The line of Homo seems to have developed from the Australopithecines, a group of small-brained, bipedal hominins who lived in Africa. Though the evolutionary trek between them and us can sometimes seem a little sketchy depending on which paleonathropologist you happen to be listening to, it’s generally acknowledged that encephalization began picking up speed in Homo habilis and especially Homo erectus.

Homo_erectus_new

A reconstruction of Homo erectus, displayed in the Westfälisches Landesmuseum in Herne, Germany, in 2006. Image borrowed from here.

Unlike our reptilian ancestors, modern day reptiles, and birds, mammals have a decidedly layered cortex. Mammals appear to have about six layers, although their appearances can vary by location in the brain. These layers are numbered in a top-down fashion, with Layer I being the most superficial layer closest to the surface of the brain and Layer VI the deepest. Inputs from the information waystation of the brain, the thalamus, converge onto Layer IV, which is then sent up to Layers II/III and finally down to Layers V/VI which subsequently sends connections back to the thalamus. Meanwhile, Layer II is highly interconnected laterally within itself and Layer III is both interconnected locally and also sends connections long-distance across hemispheres to contralateral Layer III, forming the corpus callosum.

Overall, the cortex takes on both a vertical shape and also a vertically functional one. As you can see from the image below, cells in the cortex are arranged in columns, called “minicolumns” or “microcolumns”. This is partly a reflection of how neurons are born and travel into the overlying cortical plate, but this arrangement also predisposes to a functional verticality, such that neurons tend to wire and fire together within columns more preferentially than between columns, sending signals up and down the minicolumn. Interestingly, it was believed for a long time that this cortical columnarity was unique to mammals because the cortices of modern day reptiles and birds appear poorly layered and have only the equivalent of what would be considered mammalian layers IV-VI (the superficial layers of mammal cortex are a later development specific to our lineage) [1]. But Wang et al. (2010) have found that the auditory cortex of the chicken, used here as representative both of its species and its avian and reptilian heritage, does indeed exhibit vertical functionality even though its columnar organization may be less visually obvious. Using a tracer, they followed the various connections neurons made, finding that they tended to form contacts along a vertical orientation. So if the reptilian common ancestor which both mammals and chickens share likewise exhibited a vertical functionality to its amibiguously three-layered cortex, this may well have been the predecessor to the neocortical minicolumn.

autism-pamphlet-fig3

An image from my partner’s work, illustrating cortical columnarity at different time points of development. (A) 26 weeks gestation. (B) 32 weeks gestation. (C) 48 days old, postnatal. (D) 9 years old. Image borrowed from here.

One thing that’s truly special about the cortex is that it offers the rest of the brain, the parts receiving sensory information and outputting motor actions, to refine the incoming information and alter outgoing behavior so that the circuit doesn’t solely behave in an input → output fashion. It turns what would otherwise be a reflexive action into a more complex circuit whose output can be amplified, suppressed, or even completely postponed depending on what the organism deems suitable. And one of the nice things about having a bigger cortex, with its even greater number of circuits, is that it can provide an even more refined means for orchestrating behavior.

reflection

A simple model illustrating the benefit to cortical intervention in the monitoring of sensory-motor systems.

What makes the human brain unique compared to, say, a dolphin brain or an elephant brain? It certainly can’t be one solely of size, since many modern cetaceans (whales, dolphins) have an even greater brain size relative to body size [2]. And an extreme level of surface complexity also isn’t unique to Homo sapiens. For instance, dolphins have a neocortex with polymicrogyri which look more akin to that of the human cerebellum than a cerebrum.

dolphin_cortex

Dolphin brain. Adapted from Butti et al. (2011).

Why are humans so unique? Honestly, I can’t say I know the answer to that. Maybe it’s just sheer stupid luck to have developed the right combination of abilities, which many types of gyrified brains could potentially handle, but whose anatomies and circumstances didn’t allow them to happen.

Let’s review. For one, our bipedalism is a useful development, having allowed us to both travel long distances comfortably and having freed up our hands for more refined, useful activities. Related to that, opposable thumbs, which allow us to grasp objects and manipulate them better, have fostered our capacity for tool development. And not only our ability for understanding language but for having a throat, mouth, tongue, and lips which in symphony produce such a huge variety of sounds have been vital in our species’ success. And finally, put it all together and we have the ability to build, use, and communicate about important tools and ideas which have ultimately fostered our rapid societal progression.

Maybe there isn’t a single thing that’s truly unique about humans but a number of convergent abilities. Perhaps human evolution just happened to be in the right place at the right time.

3 responses to “BRAINS, BRAINS, BRAINS!!! Evolution of the Human and Mammalian Brain

  1. I have wondered whether the brains of so-called geniuses (e.g. Einstein) were portentous of a possible direction of human evolution? Would perchance know if that might be what “evolutionary neurologists” are thinking, as well?
    Enjoyed reading the post very much!

    • There have been a lot of speculations, mostly from people who like to think of bigger/more as some form of progressive evolution (although that’s a gross oversimplification as to how evolution works). I know my partner tried to get a sample of Einstein’s brain to study its minicolumnarity but Einy’s brain is too degraded, not good quality as far as postmortem tissue goes. Plus a lot of it has already been divided up and given away. But there is some thought that the mammalian cortex has expanded through the addition of more minicolumns, to which humans are no exception (see Pasko Rakic’s work on that topic). If I recall correctly, in some of my partner’s work in which he studied several well-known scientist’s brains, they did tend to have denser (i.e., possibly more) minicolumns, though effects of aging did make analysis more difficult. There is probably a limit as far as the number of minicolumns go in increasing intelligence before one starts creating disability. Each minicolumn must establish thousands of contacts, so you can imagine just the addition of a few columns can vastly increase the requisite interconnectivity of the brain. Problem being that as local connections are increased, there seem to be a concomitant decrease in long-range connectivity (not quite sure why, perhaps some sort of balancing within the system, probably also having to do with cell-to-cell competition). My partner’s work goes much more in depth on that topic.

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