The Tell-Tale Brain: A Neuroscientist's Quest for What Makes Us Human

I would even suggest that phase transitions may apply to human origins. Over the millions of years that led up to Homo sapiens, natural selection continued to tinker with the brains of our ancestors in the normal evolutionary fashion—which is to say, gradual and piecemeal: a dime-sized expansion of the cortex here, a 5 percent thickening of the fiber tract connecting two structures there, and so on for countless generations. With each new generation, the results of these slight neural improvements were apes who were slightly better at various things: slightly defter at wielding sticks and stones; slightly cleverer at social scheming, wheeling and dealing; slightly more foresightful about the behaviors of game or the portents of weather and season; slightly better at remembering the distant past and seeing connections to the present.

Then sometime about a hundred and fifty thousand years ago there was an explosive development of certain key brain structures and functions whose fortuitous combinations resulted in the mental abilities that make us special in the sense that I am arguing for. We went through a mental phase transition. All the same old parts were there, but they started working together in new ways that were far more than the sum of their parts. This transition brought us things like full-fledged human language, artistic and religious sensibilities, and consciousness and self-awareness. Within the space of perhaps thirty thousand years we began to build our own shelters, stitch hides and furs into garments, create shell jewelry and rock paintings, and carve flutes out of bones. We were more or less finished with genetic evolution, but had embarked on a much (much!) faster-paced form of evolution that acted not on genes but on culture.

And just what structural brain improvements were the keys to all of this? I will be happy to explain. But before I do that, I should give you a survey of brain anatomy so you can best appreciate the answer.

A Brief Tour of Your Brain

The human brain is made up of about 100 billion nerve cells, or neurons (Figure Int.1). Neurons “talk” to each other through threadlike fibers that alternately resemble dense, twiggy thickets (dendrites) and long, sinuous transmission cables (axons). Each neuron makes from one thousand to ten thousand contacts with other neurons. These points of contact, called synapses, are where information gets shared between neurons. Each synapse can be excitatory or inhibitory, and at any given moment can be on or off. With all these permutations the number of possible brain states is staggeringly vast; in fact, it easily exceeds the number of elementary particles in the known universe.

Given this bewildering complexity, it’s hardly surprising that medical students find neuroanatomy tough going. There are almost a hundred structures to reckon with, most of them with arcane-sounding names. The fimbria. The fornix. The indusium griseum. The locus coeruleus. The nucleus motoris dissipatus formationis of Riley. The medulla oblongata. I must say, I love the way these Latin names roll off the tongue. Meh-dull-a oblong-gah-ta! My favorite is the substantia innominata, which literally means “substance without a name.” And the smallest muscle in the body, which is used to abduct the little toe, is the abductor ossis metatarsi digiti quinti minimi. I think it sounds like a poem. (With the first wave of the Harry Potter generation now coming up through medical school, perhaps soon we’ll finally start hearing these terms pronounced with more of the relish they deserve.)

Fortunately, underlying all this lyrical complexity there is a basic plan of organization that’s easy to understand. Neurons are connected into networks that can process information. The brain’s many dozens of structures are ultimately all purpose-built networks of neurons, and often have elegant internal organization. Each of these structures performs some set of discrete (though not always easy to decipher) cognitive or physiological functions. Each structure makes patterned connections with other brain structures, thus forming circuits. Circuits pass information back and forth and in repeating loops, and allow brain structures to work together to create sophisticated perceptions, thoughts, and behaviors.