In response to this shift in dominance, mammals and reptiles grew apart. We each grew into different niches and developed different brains in response. Not all of our brain grew to be different, though.
Despite the numerous differences that came to separate us from our reptilian ancestors, the lower parts of our brain stayed mostly the same. They gave us physiological baselines upon which the rest of the brain could be built. It’s the way in which these old systems interact with the new that gives us human happiness.
From Lamprey to Lizard
To understand what separates us from lizards, we have to know how the brain evolved. This understanding starts with the Cambrian Explosion.
The Cambrian Explosion, which was really more gradual than the name suggests, began around 540 million years ago. This “explosion” of vertebrate creatures in the fossil record kickstarted the early stages of vertebrate brain development.
We can trace our cerebral beginnings by examining the weird creatures that existed at the time. One such creature is the lamprey. Presumed not to have evolved much since the time in question, the lamprey has all the basal structures of our brain, yet none of the cool stuff like frontal lobes. Animals like the lamprey made up early vertebrate life.
Fast forward 200 million years, and a few of these lamprey-like fish began to move towards land. There was something alluring about life in the shallow waters. This curiosity eventually brought us amphibians. And, as a rule, amphibians have larger brains than their fish cousins.
It wasn’t long before amphibians began to march farther inland. Around 360 million years ago, some of them developed a thin film of water (called an amnion) that would surround their eggs. This amnion enabled amphibians to have their babies anywhere on dry land. The diaspora from the water that followed started us on the trajectory towards reptiles.
The journey onto land brought with it another burst in brain growth. In large part, this was due to the new physiological systems we needed to survive on land. We had to breathe air, communicate through air, and—perhaps most importantly—move around on land. We developed bigger brains to manage the transition.
Reptiles dominated the earth for hundreds of millions of years. This “Age of Reptiles,” as it’s called, made up the Mesozoic Era (which lasted from 252 to 66 million years ago). It ended when that giant meteor killed off the dinosaurs.
Birds and Mammals
Both birds and mammals evolved before this K-T extinction event. Birds split from the reptiles around 200 million years ago, and mammals 220 million years ago. Only birds’ brains, however, experienced any immediate growth when they split.
Like reptiles and amphibians, birds came to inhabit a new niche: air. While it didn’t happen for another 20–40 million years after they diverged from the reptiles, this environmental shift resulted in the development of bigger brains. There are several hypothesized reasons for this, most of which are similar to why the reptile cortex expanded.
Birds grew small and swift, providing them an advantage in catching prey. They also developed powered flight and endothermy, an ability to regulate their internal body temperature. These new physiological systems required more brain mass to control. And so, their cortex expanded.
Similarly, the first mammals acquired new traits that required new physiology. They grew fur, endothermy, and something called a neocortex—a six-layered sheet of brain that lies atop the brain stem. It differs from the allocortex, which only has three layers, specific to fish, amphibians, reptiles, and birds. It’s the neocortex that helps distinguish us from most other animals on this planet.
It wasn’t until primates emerged and the dinosaurs died that the neocortex really took off. It took an open environment into which these budding primates could radiate, adapt, and flourish. Once they were afforded this privilege, their neocortex grew substantially.
On Primates
Primates emerged as a distinct order around 80 million years ago. Nascent primates were mostly small and “adapted to the fine branches of bushes and trees.” This arboreal lifestyle brought with it another burst in cerebral anatomy. They developed an elaborate motor cortex, an exceptionally large visual cortex, and one of the largest neocortices of all mammals.
There are a few different reasons proposed for the growth of the neocortex. One of the more salient is the Machiavellian intelligence hypothesis (later rebranded as the “social brain hypothesis”), which suggests that the growth of the neocortex was due to our increasingly complex social lives.
The reasoning goes like this: the more complex the social situation, the more things you need to keep track of—possible competitors, mates, monkey friends that might attack you. And the more things you need to keep track of, the larger your brain needs to be.
Evidence for this hypothesis is everywhere. Brain size, for instance, tends to correlate with group size. This makes sense, given that larger groups are most likely formed by animals with better social skills. But the hypothesis also applies to mating strategies. Species that pairbond, for instance, have larger brains than those that don’t. This is another reason birds and mammals have larger brains than their lizard cousins: many of them pairbond.
Humans are the most social of social animals. Over our evolution, we moved from small bands of around 150 people each to large state societies or empires with hundreds of thousands to a few million people each. Now we live in a world that is almost entirely interconnected. It makes sense that we would have the largest brains.
The trend in brain growth, then, is that the more we grew to inhabit unfamiliar niches, the more our brains developed. But, more importantly, the way our brains developed was predicated on the environment we came to inhabit. Birds took to air, and their brains developed accordingly. Mammals got social.
The Basal Ganglia
Throughout all this cerebral growth, a few parts of the brain stayed relatively the same. Among these were the brain stem—a stalk of tissue that connects our brain to our spinal cord—and the basal ganglia, a set of nuclei that controls everything from eye movements to addiction and reward. The basal ganglia also happens to be a key component of human happiness.
When we look at the earliest manifestations of vertebrate life, we see that the basal ganglia served two primary functions: movement and action selection. Its main goal was to suppress one motor plan while amplifying another. If the plan was suitable, it would initiate movement. This ability, for the most part, has remained untouched since these primordial beginnings.
As vertebrates diversified into new habitats, the basal ganglia began to control more and more functions. It began to select, in other words, for actions that were unique to each species’ circumstance. It would help us find mates, walk on land, choose what to eat, and eventually climb into the trees. The term for this process is exaptation—the co-opting of one physiological trait to serve another purpose.
Somewhere in our vast phylogeny, the basal ganglia began to select for actions that were pleasurable. This makes sense, as pleasurable actions are often those advantageous to survival.
The above shift is what brought us the brain’s so-called “reward pathways.” These, like movement and action selection, operate via dopamine. When we get something we want—say, chocolate—certain nuclei secrete dopamine into the basal ganglia. Taking drugs will have the same effect. The result is that familiar experience of pleasure we get when we do or get something we want.
It’s not entirely clear when these pathways emerged, but we do know they came along relatively early. Lampreys, for instance, have the nuclei necessary for this experience—as do sharks. It’s reasonable to assume, then, that most vertebrates have pleasure circuitry at least somewhat similar to ours. If our systems are similar, the question becomes this: How do they work with the rest of the brain to give us alternative versions of happiness?
Human Happiness
Human happiness is characterized by a few interesting things. We might feel motivated, hyperkinetic, and some degree of pleasure. These feelings are all derived from the basal ganglia. Since other vertebrates—especially other mammals—have similar circuitry, we would expect their experience to resemble ours. The differences lie in what trigger this happiness.
The distinction between primary and secondary rewards helps to capture this. Primary rewards are things we intrinsically enjoy—things like sex and sugary and salty foods. Inevitably, primary rewards will vary across species. An affinity for salty foods, for instance, is more common in animals with bigger brains (which require a lot of sodium). It’s not unreasonable to expect that reptiles, amphibians, and the like wouldn’t find them as pleasurable.