Brains, Mammals, and the Brains of Mammals

Characters from deep history

We have been around for three and a half billion years, more or less, and we’ve taken on a great number of forms. Every single one of us is part of a long, uninterrupted line of evolutionary success. Our ancestors were, de facto, successful. If they hadn’t been, we would not be around to speak of them. If they hadn’t been successful, they would not now be ancestors.

Evolutionary innovations are many, and some that all modern people would recognize include bones and brains, hearts and hands. Let us consider brains.

Before brains evolved, our ancestors had multiple centers in their bodies in which they processed neural information. Brains focused those inputs in a single center, organized them, and sent messages back out: sensory inputs, and motor outputs, now concentrated in one place. Not long after brains evolved—or not long after on geological time scales at any rate—we encased those valuable brains within a protective box. We call that protective box a cranium, and those of us who have craniums are called, appropriately enough, the Craniates.

At the earliest point that our ancestors were Craniates, however, they did not yet have bones, or jaws, and certainly no hands. But they did have those precious brains, protected in those still cartilaginous craniums.

Jaws evolved before bone or hands. Jaws was also a 1970’s shark, and was aptly named. Sharks have big powerful jaws with piercing teeth, teeth that are attached to a cranium made of cartilage. Sharks have no bones. (Sharks also have no hands, but you knew that.)

In due time, bone evolved, and the cranium became bony. But the even more dramatic changes were happening inside. The brain itself was becoming more modular, and more specialized. The “tripartite” brain—a brain broadly in three parts, fore, mid, and hind—develops in one long line, a line contiguous with the spinal cord, but unlike the spinal cord, the brain curls in upon itself, becoming something like a ball of neurological tissue, some of the oldest and most fundamental parts on the inside and the bottom, many of the newer, flashier parts on the outside.

Close to two hundred million years ago (with forgiveness requested for the inherent imprecision and occasional inaccuracy of historical reconstruction), mammals evolved.¹ We are mammals, but we have changed mightily since those earliest ones, just as those first mammals had changed mightily since their ancestors had no bones or brains or jaws.

All mammal species share our eponymous trait: mammary glands. With these mammary glands we feed our young. Their presence thus guarantees parental care which, in turn, paves the way for maternal love. But these are hardly the only traits that mammals evolved.

Mammals run hot (we are endotherms); we stand up tall (with our parasagittal gait, a gait that is further modified in bipedal humans); and we can move fast for sustained periods of time. This last ability, our speedy endurance, is facilitated by several additional innovations of mammals: We have four-chambered hearts (which keep our oxygenated blood fully separated from that which is deoxygenated); when we move, we undulate up and down, rather than from side to side (so one lung is not squeezed with each step that we take); and we have diaphragms, the band of muscle below our hearts and lungs, which helps organize and power our breath. 

Mammals are insulated with fur or hair; we have an excellent sense of smell; and we have fantastic hearing. Our middle ears contain bones—the malleus (hammer), incus (anvil) and stapes (stirrup)—which were part of the jaws of our ancestors. Those bones have moved over time, dramatically changing shape, position, and function, and whereas earlier vertebrates had multiple bones in their lower jaws, we have just one, our strong and sturdy mandible. We have—stay with me here—teeth set in sockets and attached to the jaw via ligaments rather than fused directly to the bone. This arrangement allows us to have tremendous bite force. Other than us, the only other critters that have teeth like ours are crocodiles and their closest relatives.² Unlike crocodiles, though, which sport mouths full of similarly pointy, piercing teeth, we are heterodont—different teeth—and our incisors, canines, premolars, and molars are best for different things. It thus makes no sense to wonder “are incisors better than canines?” The question must include mention of what job you are hoping to accomplish.

Mammals also have an elaboration of our kidney called the loop of Henle, which concentrates urine for excretion, and conserves water. We have REM sleep. And the vast majority of us have gestation and live birth (but see the duck-billed platypus and echidnas for exceptions).

And—returning to brains now—mammal brains are large and supremely well connected on the inside. Long before we were mammals, but after we already had brains, those brains were becoming specialized into smaller, modular units. From small, physically disparate neural centers, we evolved a central brain that has specialization within it. Specialization fails if the various specialists can’t connect with each other, though, so our modular brains also became adept at communicating across borders.

There is a tension in any system as it becomes complex: how to maintain the integrity and nimbleness of small units, while developing communication and feedback between each or all of those units. Cohesion is necessary, but sometimes—maybe even often—it is also useful for subunits to work in isolation, without interference from admin. And yet no individual subunit can make a go of it on its own—they are all subjects of the larger brain of which they are a part, just as that brain is a subject of the individual in which it is housed. At the end of the day, any one of the subunits may be extraordinary, but they are nothing without the brain to which they belong.

Our cerebral hemispheres, the giant paired cauliflowers that we think of when we think of what a brain looks like—that’s the forebrain, curled around the rest of the brain, obscuring the brain’s other parts. In deep antiquity, the forebrain was a place where smells were processed and interpreted, but it has expanded and been co-opted into new functions, now being a command center for memory, planning, and scenario building. The forebrain is where we consider the past, plan the future, and integrate what we know with what might be possible.

Again, though, for a mammalian forebrain to be successful in its jobs, two seemingly mutually opposing things must often be true: it must have the freedom and autonomy to work things out without interference, and it must be connected to the other parts, in order to strategize, organize, and implement.

And within the forebrain, we have an obvious split: the cerebral hemispheres literally have two sides, left and right. This offers the possibility for one side to not know what the other is doing, but those hemispheres must be connected so that, when called upon to do so, the two halves can coordinate. The innovation in the mammalian brain that connects the two hemispheres of our neocortex is the corpus callosum.³

This thick band of nerve fibers that runs between the two halves of our brain neatly illustrates the value of both specialization and integration. Does it put the lie to the value of lateralization—having two halves rather than a single integrated whole in the first place? It does not. It reveals an inherent trade-off in a complex system. A single static state or answer will rarely suffice. It makes no sense to ask, “which is better, left hemisphere or right hemisphere?”, just as it makes no sense to ask, “which is better, more connected or less connected hemispheres?” If always being connected were better, why did selection split our hemispheres in the first place? If less connection were always better, why is there so much variation in the size of corpus callosums, even within species?

It is true, for instance, that the shape of a man’s corpus callosum tends to be different from that of a woman’s.⁴ This suggests different kinds and degrees of connectivity between particular parts of the cerebral hemisphere in men versus women (suggests, but does not prove). In turn, that suggests that there is no single “best” shape of human corpus callosum; rather, that how effective a given corpus callosum is depends in part on what else your brain is doing.

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Our earliest ancestors were single cells, barely differentiated from one another at all. No brains or bones; no jaws or hands. And now here we are, with all of those things and more: cuisine and consciousness, language and laughter. There have been, in our path that is billions of years long, some missteps. We are far from perfect. But we are very, very good.

1. Foley, Springer and Teeling, 2016. Mammal madness: is the mammal tree of life not yet resolved? Philosophical Transactions of the Royal Society B: Biological Sciences, 371(1699): 20150140.

2. LeBlanc et al 2017. Evolutionary implications of tooth attachment versus tooth implantation: A case study using dinosaur, crocodilian, and mammal teeth. Journal of Vertebrate Paleontology, 37(5), p.e1354006.

3. Exactly when and in what clade the corpus callosum evolved is a matter of active debate.

4. Arnold 2017. Sex Differences in the Age of Genetics. Hormones, Brain, and Behavior, 3rd ed, vol 5: 33 - 48.