Ruben Baler, Ph.D.'s  picture
Ruben Baler, Ph.D.
Health Science Administrator, Office of Science Policy and Communications
National Institute on Drug Abuse
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Evolving Our Brain

Evolution has shaped the human brain layer by layer, like an onion, as we have become instinctual, emotional and rational beings

The Magic OnionThe brain that we lug around without paying it too much attention is believed to be the most complex structure in the universe. Not surprisingly, arriving at this level of complexity took billions of years of blind genetic tweaking and merciless Darwinian competition. If we zoom into the neuronal circuits and clusters that make up the brain’s systems, we’ll find them laid out like the concentric layers of an onion, functional layer upon functional layer. This arrangement is the result of the relentless evolutionary push to continuously improve on an animal’s ability to navigate its complex and ever-changing environment. This onion reveals no preconceived design—far from it—but the unavoidably messy piling up of new over old structures, forced to work together for the common good.

The core of the onion: autonomous systems. Along the oldest, deepest layers, in areas like the medulla and the pons, lay the autonomous systems. They are in charge of the most basic survival routines like swallowing, vomiting, heart beating and respiration, which are so indispensable that they run, for the most part, on autopilot. Accordingly, they are very hard, though not impossible, to modulate voluntarily. Our human species shares these circuits with fish and reptiles.

Next layer of the onion to grow: instinct. Our chances of survival were greatly enhanced with the arrival of instincts. Vertebrates benefited tremendously from these pre-programmed circuits that expanded their behavioral repertoire and increased their fitness. Instincts mediate behaviors so crucial to survival that they are encoded as such in the genes themselves, hard-wired in the brain during an animal’s development and not changed by later experience. The hypothalamus, which lights up like firecrackers upon exposure to a live predator, plays a major role in these stereotypic responses that include prompting an animal to seek food, shelter and mates; running instinctively when sensing danger; or allowing a baby to tightly wrap his fingers around his mother’s pinkie. Very useful accessories, but evolution was not about to stop there.

Let’s examine the next layer: emotion. An amazing breakthrough happened in mammals, with the laying out of a new set of neural substrates capable of generating emotions. These paths grew together in a circuitry that occupies several brain regions, including the basal ganglia, hippocampus, cingulate gyrus and amygdale. This collection of brain regions is often referred to as the limbic system, because like an arm (a limb), it surrounds in its embrace the more primitive survival and instinctual regions of the brain, affording us primates a whole new set of behavioral tools for interacting with the world. Accordingly, life became more interesting and vastly more complicated. For example, the instinctual drives to have sex and to run away from danger would be inextricably linked to powerful feelings of affection and fear, respectively. The influence that this new circuitry exerts over our decisions and behaviors is enormous. The neural substrates of emotions are strongly influenced by developmental factors. Perhaps because of its more recent origins, the emotional circuitry is even more flexible and responsive to external influeces than earlier circuits. This is why, for example, prolonged exposure and other cognitive behavioral therapies (CBT) can be used to treat post-traumatic stress disorder (PTSD) with some success.

The outermost peel (aka, the neocortex) was overlaid atop an already crowded swarm of networks. In primates, it exploded into two huge hemispheres that completely enveloped the older parts of the brain. This new shell serves as the testing grounds for our still developing reasoning abilities. Because they are so recent, neocortical functions are the most flexible and sensitive to the impact of social and environmental experience. Since the neocortical outputs are closer to conscious experience, it is hard to recognize that the neocortex is constantly competing against earlier webs of well-established brain circuitry, and that its contribution to our motivations and actions are likely less than what we’d expect.

We humans are the flavorful mash-up of all these brain layers: we are automaton, instinctual, emotional and rational creatures. We are like magic onions, proud of our paper-thin skin but only vaguely aware of the thick and deeper layers of our brains. All of these different layers play critical roles in shaping our interactions with the world around us, even as they continue to search for more efficient ways of talking to each other; but, alas, this will take millions of years. In the meantime, we can try and help the biological process along by combining our brain’s flexibility with the lessons of modern neuroscience to nurture and develop our human potential – just as the sensible application of food science could lead to, say, better onions.

LEArn More:

“How Your Child Got Her Brain,” by Ruben Baler

“Understand Your Teen’s Brain,” by Timothy Myers

Related Reading:

“In Pursuit of a Mind Map, Slice by Slice,” New York Times

“Complex Synapses Drove Brain Evolution,” ScienceDaily

The Deciding Factor, NOVA podcast