Thumbs, Toes, and Tears

by Chip Walter

What distinguishes us from other animals? There are plenty of answers to this question, although none of them are quite definitive. One answer, toolmaking, is compromised by the fact that chimpanzees and corvids have been observed in the wild using tools. Narrowing the idea to “culturally transmitted use of tools” doesn’t get around that objection: bonobos have been observed learning new tools from other bonobos. How about language? That’s certainly unique to humans, isn’t it? Well, yes, if you confine yourself to grammatical language. But lots of animals communicate vocally: whales and birds sing, cows moo, monkeys chatter, eagles call, elephants trumpet, dolphins squeak and so on. Detailed studies of animal communications have demonstrated surprising sophistication in the vocalizations of many animals. So although there’s no question that human vocalizations are immensely more complex, they are not fundamentally unique in the animal world.

It’s futile trying to find the one major trait that differentiates
homo sapiens from other species -- every trait we can point to is in some fashion analogous to some trait of some other species. A more effective approach is to think in terms of a suite of traits. Humans are equipped with a variety of characteristics that, while individually not absolutely, positively unique, are nevertheless unique when considered as a set. Chip Walter set out to define and document these traits in this book. He presents ten such traits:

1. Big toes

These may not seem especially important, but in fact they’re what make it possible for us to walk smoothly. The big toe does for human locomotion what the propellor and rudder for a boat: it’s the crucial point of contact where impelling directional force is delivered from the foot to the ground. It is a remarkably strong appendage, capable of pushing with enormous force. While the ball of the foot bears most of the thrust in running, in walking the big toe pushes in the exact direction required to maintain a smooth gait despite irregularities in the ground surface. Without the big toes, humans would be reduced to walking in the slow and clumsy manner that chimps use.

2. Premature birth

Human babies pop out of the womb long before they’re ready to survive on their own; they require months of additional rearing before they reach the level of development attained by, say, gorilla babies at birth. Homo sapiens has shortened pregnancies by splitting fetal development between internal and external rearing. In this sense, we are behaviorally something of a blend between marsupials and placental mammals. The reason for this oddity is the size of the human brain: at full size, it is far too large to get through the birth canal. If human females were to widen their birth canals to accommodate larger heads (thereby permitting longer pregnancies), their pelvises would have to widen to a degree that would make the female gait impossibly ungainly. So the only solution is to pop the kid out of the oven while his brain is still about one-third of its final size, and then take close care of the kid while his brain grows to full size.

3. Thumbs

This is one trait most people know. Yes, we have opposable thumbs -- but so do Old World monkeys, the great apes, gibbons, an African rat, opossums, koalas, a South American frog, and a few dinosaurs. We’re not very special when it comes to opposable thumbs. However, when combined with big toes (which permit us to walk and thereby frees up the hands), those opposable thumbs become a lot more useful. This permitted us to start feeling up Mother Nature: manipulating things, poking at them, turning them around in our hands, pulling at them, ripping them apart, and all manner of other curious proddings. In the process, we learned a great deal about the natural world, then started manipulating that world (manus is the Latin word for hand). 

4. Language

Yes, although we don’t have language all to ourselves, we still have what is far and away the most expressive language system in the animal kingdom. Yes, language gives us vast power to share ideas, to develop and refine culture, to accumulate knowledge and technology, and to write limericks. But you already knew that.

5. The descended pharynx

This should have preceded the chapter on language, because the pharynx is the crucial organ that enables rich language. The pharynx sits just above the larynx; it joins the airway with the esophagus (the tube that goes to the stomach). This merging of the two passages truly is unique in the animal kingdom, because it’s so stupid. It permits food to go down the air tube and choke us to death. It is also thought to play a role in crib death (also known as Sudden Infant Death Syndrome), because so many of these deaths occur at the age then the pharynx, which at birth is high in the vocal tract, descends to a lower position that simultaneously permits choking and speech. The hypothesis is that the brain must learn at that point to control the musculature around the pharynx more precisely. If the brain doesn’t master the technique soon enough, the infant loses airflow and quietly asphyxiates. With so much risk attendant to its lowered position, one may wonder why humans have such dangerously low pharynxes. The answer is that a lowered pharynx permits much richer control of vocalization. Because of the pharynx, we can precisely control the flow of breath so as to increase or decrease the amount of pneumatic force behind each sound we make.

6. Consciousness

On this I disagree with the author and most of the world: I consider the concept of consciousness to be nothing more than a modern-day expression of the concept of a soul, except that consciousness is not immortal. The notion of consciousness is far too amorphous for my taste. I believe that we have developed something unique in the animal kingdom, but there’s nothing magical about it. Start with the idea of theory of mind – the ability to put yourself in another’s shoes and see the world from their point of view, and then use that knowledge to anticipate their behavior and adjust your own behavior accordingly. Theory of mind is not unique to homo sapiens; a number of species possess it. There’s a standard test for theory of mind: anesthetize a creature and paint a spot onto its forehead. When it wakes up, put it in front of a mirror. The animal will see itself in the mirror, but does it know that the image is really itself? If it touches the spot on its forehead, then it possesses theory of mind. That’s the acid test. I believe that consciousness is something like “theory of mind squared”: the application of theory of mind to oneself as if one were outside oneself. This is what permits us to realize our own mortality, for example. Indeed, a great many interesting behaviors arise when one applies theory of mind to oneself. That’s what most people call consciousness – but it really is just an explainable behavior.

7. Social reasoning

This chapter is a bit muddled, as the author mixes together a number of factors, but the central theme appears to be the more complex social relationships that humans develop. Lots of species demonstrate social behavior, but human social behavior is more complicated, largely because of the heavy reliance on theory of mind. We get into layers of anticipation; we can even estimate how deeply people can anticipate by considering our language’s ability to understand nested relationships. Consider this sentence: “Margy rebuked Joan for her stupidity in inviting Tom and Joe to the party; after all, Tom hates Joe.” This sentence describes Margy’s assessment of Joan’s assessment of Tom’s assessment of Joe – and it’s not difficult to understand, even though it takes theory of mind three levels deep. We think that most people can handle four layers of anticipatory depth, and with effort can figure out five levels.

8. Laughter

Only people laugh. Perhaps it’s merely because nobody else has anything to laugh about (after all, what’s funny about living in a world controlled by humans?) But more likely it’s a consequence of our unique mentation. Mr. Walker explains it as a sudden release of tension, but contradicts himself with the joke he offers at the beginning of the chapter:

Two cannibals are sitting beside a large fire after eating the best meal they’ve had in ages.
“Your wife sure makes a good roast,” says one cannibal.
“Yeah.” replies the other. “I’m really going to miss her.”

The joke lies not in a release of tension but from the opposite: a sudden jerk from normality to evil. A better explanation is provided by a variation on the mechanism explained in detail here. A joke sets up a mental pattern in our minds looking rather like one in the diagrams – but the punchline renders that pattern inappropriate by removing just one element of the pattern and adding a completely different element. Thus, the joke teaches us that one pattern in our mind can be hooked up with other elements in two completely different ways. It’s an educational experience, a snippet of surprising truth, and our minds are addicted to learning – all it takes is a little whiff of learning to give us a high. That’s why we laugh.

9. Tears

Crying is an odd behavior. Why in the world would great sadness would be manifested by hyperactivity in the glands that produce lubricant for our eyes? There’s no physiological or evolutionary connection between sadness and tears. Since no other species behaves this way, it must be a recent evolutionary development. It does appear to be a beneficial trait: crying always evokes sympathy, which in turn often yields benefits. Moreover, it appears to be most strongly manifested in females. That makes sense: since women can’t use physical force people to get what they want, the alternative of emotional manipulation needs to be amplified. Men cry, but with lesser frequency than women, and it’s certainly the case that the sight of a woman crying hits the male psyche quite hard. Yet how the specific manifestation appeared is not obvious. It would seem that crying was one of those weird mutations that just popped up and spread through the gene pool because it was advantageous.

10. Kissing

This behavior is not instinctive; there are cultures that do not engage in kissing. However, it seems that people quickly take to the practice the minute they see it. The lips are richly blanketed with sensory nerves for detecting the precise texture of food; those nerves have been recruited, it seems, for the additional purpose of providing sensory pleasure in a kiss. Additionally, humans are powerfully but unconsciously affected by pheromones and other odors emanating from members of the opposite sex. Bringing the mouths and noses close together amplifies the experience. Dogs sniff each other’s anuses; we sniff each other’s mouths.

This book is an easy read; the author is a good clean writer with enough flair to make the journey pleasant. I would not call either the book or the author “great”; but if you recall just how scathingly critical I can be, the “B” grade I award it should impress you. This truly is an above-average book.