Free Last Ape Standing: The Seven-Million-Year Story of How and Why We Survived by Chip Walter Page A

hormones. But neurons are especially talented communicators. This is because whenever the biological dice fell in such a way that they came into existence, they began to evolve specialized connectors—dendrites and axons—that vastly improved their exchange of information compared to other cells in the body.
    Before brains came along, primitive protoneurons communicated by secreting hormones and electrical currents in no particular direction, mumbling their messages to the other cells and protoneurons in their vicinity, and not getting terribly quick results, at least compared with our current models. With the invention of dendrites and axons, however, they could form elegant, smart clusters that shared at high speed the information each of them held with the others nearby. (Planaria were among the first to accomplish this.)
    The emergence of high–speed, if exceedingly minute, communications cables meant that any creature fortunate enough to inherit them could more fully and rapidly sense the world it inhabited—light and dark, food, danger, pain and pleasure—then react to it all in a blink. Not only that, the cables could link different sectors of the brain the way highways connect cities. This meant the brain could not only improve contact with the world, but also stay in better touch with itself, not a trivial matter as brains grew larger. (This turns out to be important to consciousness, but we will visit that subject later.)
    Dendrites generally conduct signals coming into a brain cell while axons do the opposite. Dendrites (also known as dendrons) are so eager to make contact that they extend treelike in multiple directions and can place one neuron in touch with thousands of its neighbors. Axons aren’t nearly as obliging as dendrites, but can still make uncounted connections as they transmit signals outward when a neuron is stimulated and reaches what is known as a threshold point, a moment that is vitally important when it comes to thinking, feeling, and sensing. At that instant an electrical impulse bolts down the axon at 270 miles an hour. When it reaches the end of the axon, a tiny pouch of chemicals bursts, sending neurotransmitters across a synaptic gaplike party confetti, where they embrace the receptor sites of the next neuron like a long–lost relative and then pass along their message.
    Your brain is capable of making one quadrillion (that’s a 10 with fifteen zeros behind it) connections like these. Even as you read the words in front of you, impulses are flaring out and back at high speed, a three–dimensional, electrochemical storm tirelessly at work conjuring your thoughts, assessing your feelings, ensuring your body operates according to plan, and generating your personal version of reality. It’s a busy place.
    While neurons multiply at blistering rates before we are born, the business of building the brain continues even more earnestly after we enter the world. By strict decree, the twenty-five thousand genes—the “structural genome”—each of us inherits in fifty–fifty doses from our parents resolutely continue the construction of our own wetware, and its underlying neuronal infrastructure, complete with our specific talents and predispositions. Just as some of us may inherit stocky bodies and others long, slim ones, our parents can also issue brains that incline us to be gregarious or shy, a leader more than a follower, mathematically, musically, or verbally predisposed. This part of us is a genetic crapshoot, and we have no control over it.
    Nevertheless, more than other forms of life, even other primates, we can be thankful that we are not immutably linked to our genetic directives. In us they are editable, able to be altered by our personal experience and environment, a phenomenon that explains why each of us is not a clone of the other, not even in the case of identical twins, who carry precise copies of their sibling’s DNA. It is impossible to overemphasize the impact this