Thumbs, Toes, and Tears: And Other Traits That Make Us Human by Chip Walter is an evolutionary story about what sets human beings apart from other animals. Walter specifically identifies six traits of humans that can be found in no other species: Big toes, opposable thumbs, the pharynx, laughter, tears, and kissing.

The big toe is important in that helped humans walk upright which in turn modified our anatomy leading to larger brains and skulls. At the same time the body adapting to walking and running means the birth canal is too small to allow easy egress of our big heads. So human infants are born far earlier in their development than other mammals and that is why humans are so helpless in their early years. In fact the long period of ongoing learning and development is uniquely human.

The opposable thumb is well-respected as the difference between humans and other animals, allowing us to create and manipulate tools. Our hands are also central to our ability to communicate and the genesis of language.

The pharynx is most fascinating in that it physically allows us to speak. I never knew that it does this by intersecting the airways and the esophagus, which does not happen in oher mammals. The price to pay for being the animal that speaks is that we are also the animal that can choke. Being able to speak gives rise to consciousness and the prefontal cortex of the brain.

Laughter allows us to connect emotionally to others and is rooted both in our primal urges and our ability to walk upright.

Crying is the most mysterious as humans are the only beings that have tears that well up and drip out of the eyes as a means of expressing emotion (as opposed to merely keeping the eyes clean). One theory is that crying actually brings us back into a state of equilibrium. Crying is also a means of body language to communicate emotions to other humans.

Finally, kissing is a learned cultural trait that allows us to communicate our love and affections in a way that feels really good.

Walter writes all of this in a lively style that makes it a fun and engrossing read. In many ways it is a gentler version of Desmond Morris' The Naked Ape which Walter cites several times. It's a work of popular science, and it hems & haws with a number of "probablys" and "we don't know for sures", but I think it teaches an illuminating lesson in human evolution and unique traits.

Interesting Quotes:

If we were born as fully formed and physically mature as the babies of contemporary great apes, human gestation would last not nine months, but twenty-one! This means we are born a fully year premature. We may define "full term" as nine months in the womb, but by ape standards we are fetuses that have arrived twelve months earlier than we should have. - p.34-35

We tend to think that the forces of evolution are terrifically at rooting out all wastefulness to make the brain thoroughly optimized for smooth, clean operation. But the truth is that evolution feels it way toward success, tinkering and puttering until it stumbles across marvelously inventive solutions to the problems that need for survival presents, and then shambles on. Our brain, amazing as it is, is not an efficient machine, but a maddeningly complicated organ that stubbornly resists analysis. - p. 107

Quite possibly we cry not because we are getting agitated and upset, but because it is a way for our nervous system to bring us back into equilibrium.

One study reveals, for example, that if the nerves central to the sympathetic system are paralyzed, patients cry more. But when important parasympathetic nerves are damaged, they cry less. If crying was driven by the sympathetic nervous system, it would be the other way around. In other words, we don't cry because we are upset, which is the way it feels, but because we are trying to get over being upset. That may be the real reason why feel better after we have a good cry. p. 173

Our simian cousins, gifted and intelligent as they are, don't have the capacity for the powerful marriage of thoughts and emotions. They can feel rage, frustration, or loss, but they do not reflect on them. The random emergence of genes that connected the emotional and intellectual parts of our brains to lacrimal glands that sit above our eyes gave us a new way to express those elusive feelings. And in the bargain we gained an emotional stamp we can put on our cries for help that no other creature possess. - p. 179

( )