The
Eyes
Protozoa have dot like eyes that sense the mere presence of light.
We have two eyes, just like every other sighted vertebrate except the four-eyed
fish. There is literally no animal except in myth (the Cyclops) that has only
one eye. Even the primitive worms that dwell under rocks have two paired eyes.
Two eyes, of course, gives creatures parallax to judge distance – the two eyes
have slightly different angles that the brain recombines noting the differences.
There are three parts to an eye – the white, iris, and the pupil. Why white – it contrasts with the darker parts and highlights eye movements. Detecting gaze direction is a vital ability and our brains have special wiring for it. Gaze direction awareness, scientists suggest, clues us into who is looking at who and we are able to build a social “attention structure”. Gaze also implies one next movements, and thus signals purpose or desire.
The brain has special areas devoted to face recognition in 3 distinct areas: to interpret identity, expression and eye/head movements. The later orientation system decodes social attention and reacts to head direction, since it suggest intent. In monkeys, which have a less complex brain apparatus for eye/head orientation, some neurons fire more to a full face, others to a three-quarters view, still others to a profile. These cells react to gaze as well – some reacting to eye contact while others to a downcast head. So we have an enormous sensitivity to aim of the eyes – it is wired in us.
The stare is special. Our mental gaze-radar detects it quickly. Being stared out increases arousal, raising the heartbeat and altering the galvanic skin response. This kind of automatic reaction forces us to deal with the situation. Hence beggars stare in solicitation. When we can’t halt the stare, we tend to flee. In one study, researchers on a street corner gazed steadily at the drivers who had stopped at the red light. The driver noticed the stare in seconds and when the light changed green, these drivers drove off significantly faster than the drivers who were not stared upon.
Sensitivity to stares is genetically ancient – it goes back to reptiles and insects. Snakes will fake death longer when stared at. This defensive technique does little good unless it occurs exactly as a predator begins it stare and not a millisecond later. Lizards too freeze longer in the presence of a person gazing at them. Such evidence suggests staring is the oldest and most stable facial expression on earth.
Eyebrows. Joggers well know that they keep perspiration from falling in your eyes. Given all the sweet glands in forehead to keep your brain cool, this proved very important to our ancient hunters on the savanna, for it is hard to hunt with perspiration in your eyes. Besides sweat deflection and eye adornment, eyebrows play a huge role in face expression and communication.
A note a blinks. They recall our aquatic origin. In water, animals never dry out; on land the sun takes away the very substance of the eye. Early amphibians slipped in and out of the water often. When eyes became in contacted with air for longer periods of time, land creatures developed eyelids and tear ducts to keep them wet. Each blink is a little dunk in the primeval sea.
Eyelids block the light and the outside world when we want to sleep, but at a mere millimeter deep, the thinnest skin on the body, it is slightly translucent, so sunrise or sudden light wakes us up.
The cornea must be transparent so we can see through it, and hence it has no blood vessels. Tears bring oxygen to it and keep it alive. They also contain chemicals that kill bacteria and proteins that smooth the eye surface and trap debris. So blinking is like breathing for the eyes, we do it 15 times a minute. We will get more into some of the studies of when and why we blink in social situation in a later class.
We are the only primates that cry. In apes tears simply keep the eyeballs moist. Human tears come in two kinds, emotional and “irritant”, which differ in chemical composition. Biochemists found that emotional tears have 21% more protein and suggest that they carry away chemicals that the body produces in stress. That’s why we fell better after a good cry.
The Nose – As I mentioned in the first lecture, when our muzzle receded into flat face we now have, the nose then jutted out and there are several unresolved theories why. Remember our jutting noses are zoologically bizarre -- among living primates, only the proboscis monkey has a protruding nose. Theories of interest:
- An odor canopy.
- For vision, animals compare the visual world against there muzzle or snout. Without one we needed a protruding nose.
- The aquatic ape theory. “We” went back into water and changed several traits include lose of smell and a protruding nose to swim and breath easier are some of them.
Mouth – The mouth is the oldest part of the face, the gateway for food, drink and at times air. It is primal and even the one-celled paramecium has a mouth. The mouth was the first facial organ to form. Before swallowing food we test odor, taste, texture, temperature, shape and irritability, and we have a gag reflex to stop it at the last minute. Our mouth sets us apart from other creatures in many ways but mainly in it narrowness. Human mouths are usually no wider than the span between pupils. Even chimps have huge mouths, running hairline to hairline and a horse and alligator’s faces are almost all mouth. Why did ours shrink?
Muzzles need wraparound mouths. Also since we no longer used our teeth as weapons, the mouth need not be as wide and a smaller mouth is safer from contaminants. And of course – expression. The face can control small lips more deftly and for language certain sounds like “oh” and “w” require the lips to form an O
The lips are transition zones between dry skin of the face and the moist mucous membrane of the inner mouth. Lips are border guards. They excel at distinguishing foreign objects, and can detect a single hair in our food. But our lips are so advanced because they cue us to speech. In a study people recognized 23% of sentences with a noisy background, but 65% when they could see the speakers face.
Ears – Fish lack true ears. Rather they have a lateral line system, which detect quivers in water. But sound waves in air are too week for a lateral line system so creatures developed both an eardrum and the hammer-anvil stirrup system, which cleverly magnify air waves. This system still has a fluid inside it to vibrate. So in a sense we still hear underwater.
Hair – Only mammals have hair. Hair was thinned out only in our recent evolution. We are the only mammals with constantly growing hair. In most mammals hair grows to a certain length, then halts and remain in place for a long time before falling out. Not in humans; when a strand stops growing, it soon falls out and a new shaft replaces it. We are born with our facial structure but anyone can reshape the hair. Rather ironic then that humans can recognize hair shape, say to describe a criminal, better than any other feature of the head.>
Skin – Until recently we were all black. Best new estimates are that Homo Sapiens left Africa about 100,000 years ago. In Eurasia, white and oriental forked about 40,000 years ago.
Why is African skin black and why did emigrant skin change color. It reduces death from exposure to too much or too little sun. Black skin with its melanocytes shields us from too much. The suns rays cause steroids beneath the skin to churn out vitamin D. So skin color change occurred in more northern environs to get more sun.
There is more evolutionary information I have gathered on face recognition, expression and beauty, but I will discuss these in parallel with our learning the face proportions, muscles and expressions
One last note on laughter. What is the point of laughter? We tend to view laughter as a frivolity, but all facial displays are costly, they consume energy and can attract predators and laughter is noisy – so why laughter.
A tentative theory of the evolution of laughter starts with tickling. In children tickling seems like play-attack – it helps them develop defensive reflexes. From tickling, laughter generalized to play-fighting. Primates, children and young adults often laugh when hitting or wrestling each other in fun. Their laughter marks the combat as a game and protects against damage from misunderstandings. And from tickling and physical play, the theory suggests, laughter became a response to social play, where it also encourages practice. Banter, for instance, is a play-attack and sharpens our ability to react to genuine insults.