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

And so it is that we can begin with a bizarre mystery that could have come straight from Edgar Allan Poe, apply Sherlock Holmes’s methods, diagnose and explain Mikhey’s symptoms, and, as a bonus, illuminate the possible evolution and biological function of a much treasured but deeply enigmatic aspect of the human mind.

CHAPTER 2

  Seeing and Knowing

“You see but you do not observe.”

—SHERLOCK HOLMES

THIS CHAPTER IS ABOUT VISION. OF COURSE, EYES AND VISION ARE not unique to humans—not by a long shot. In fact, the ability to see is so useful that eyes have evolved many separate times in the history of life. The eyes of the octopus are eerily similar to our own, despite the fact that our last common ancestor was a blind aquatic slug-or snail-like creature that lived well over half a billion years ago.¹ Eyes are not unique to us, but vision does not occur in the eye. It occurs in the brain. And there is no other creature on earth that sees objects quite the way we do. Some animals have much higher visual acuity than we do. You sometimes hear factoids like the fact that an eagle could read tiny newsprint from fifty feet away. But of course, eagles can’t read.

This book is about what makes humans special, and a recurring theme is that our unique mental traits must have evolved from preexisting brain structures. We begin our journey with visual perception, partly because more is known about its intricacies than about any other brain function and partly because the development of visual areas accelerated greatly in primate evolution, culminating in humans. Carnivores and herbivores probably have fewer than a dozen visual areas and no color vision. The same holds for our own ancestors, tiny nocturnal insectivores scurrying up tree branches, little realizing that their descendents would one day inherit—and possibly annihilate!—the earth. But humans have as many as thirty visual areas instead of a mere dozen. What are they doing, given that a sheep can get away with far fewer?

When our shrewlike ancestors became diurnal, evolving into prosimians and monkeys, they began to develop extrasophisticated visuomotor capacities for precisely grasping and manipulating branches, twigs, and leaves. Furthermore, the shift in diet from tiny nocturnal insects to red, yellow, and blue fruits, as well as to leaves whose nutritional value was color coded in various shades of green, brown, and yellow, propelled the emergence of a sophisticated system for color vision. This rewarding aspect of color perception may have subsequently been exploited by female primates to advertise their monthly sexual receptivity and ovulation with estrus—a conspicuous colorful swelling of the rumps to resemble ripe fruits. (This feature has been lost in human females, who have evolved to be continuously receptive sexually throughout the month—something I have yet to observe personally.) In a further twist, as our ape ancestors evolved toward adopting a full-time upright bipedal posture, the allure of swollen pink rumps may have been transferred to plump lips. One is tempted to suggest—tongue in cheek—that our predilection for oral sex may also be an evolutionary throwback to our ancestors’ days as frugivores (fruit eaters). It is an ironic thought that our enjoyment of a Monet or a Van Gogh or of Romeo’s savoring Juliet’s kiss may ultimately trace back to an ancient attraction to ripe fruits and rumps. (This is what makes evolutionary psychology so much fun: You can come up with an outlandishly satirical theory and get away with it.)

In addition to the extreme agility of our fingers, the human thumb developed a unique saddle joint allowing it to oppose the forefinger. This feature, which enables the so-called precision grip, may seem trivial, but it is useful for picking small fruits, nuts, and insects. It also turns out to be quite useful for threading needles, hafting hand axes, counting, or conveying Buddha’s peace gesture. The requirement for fine independent finger movements, opposable thumbs, and exquisitely precise eye-hand coordination—the evolution of which was set in motion early in the primate line—may have been the final source of selection pressure that led us to develop our plethora of sophisticated visual and visuomotor areas in the brain. Without all these areas, it is arguable whether you could blow a kiss, write, count, throw a dart, smoke a joint, or—if you are a monarch—wield a scepter.