Why have a face?

The Face: A Natural History
By Daniel McNeill

(Page 2 of 12)

The Early and the Odd

Why have a face?

We don't strictly need one. Many creatures, like sea urchins, starfish, clams, jellyfish, and protozoa, disdain it entirely. Others have partial faces. The microscopic rotifer has a pair of eye-spots on a rod in a feeding cup, an almost faceless face. The face of the sea anemone is all mouth, and of the octopus, two peering eyes. Snails have tiny mouths and eyes on stalks that wave over their heads.

Yet faces are amazingly common in the animal kingdom. Jaguars, salamanders, and hawks have them, as do all insects, fish, reptiles, birds, and mammals. Why them and not the jellyfish?

The answer lies in evolution, the treasure chest of meaning for anatomy. The faces of everyone-Marilyn Monroe, Ronald Reagan, William Shakespeare, Cher-began in the sea.

A true face bundles mouth and sense organs, and it may be older than shell or bone. Geneticists say multicelled life arose around 1.2 billion years ago, but hard fossils don't appear until much later, around 544 million years ago. Soft-bodied creatures like worms and the weird, feather-like Ediacarans scuttled about in this vast eon, but their remains are scarce. The first face likely coalesced toward the end of this period.

When shelled creatures suddenly did arrive, they were spectacular. It was the great Cambrian carnival of life, the zoological equivalent of the Paris art world in, say, 1910, and memorable faces adorned the seas. The tiny Opabinia, for instance, boasted a tentacle like an elephant's trunk and five mushroom-like eyes. It is so weird that, when first shown to an audience of paleontologists in 1972, they burst out in laughter.

But other creatures were more recognizable, like the slithery worms with eyes and mouth in front, and they show the origin of the face. It is the child of motion. When an animal swims regularly in one direction, the head becomes its leading edge. A forward mouth swallows food easily, through simple momentum; a mouth astern would recede from it. The head also contacts nonstop novelty, so the sense organs cluster up front, like the guidance system of a missile. There they reveal the future, an animal's fate in the space ahead.

Vertebrates like fish have four times more basic structure-mapping genes than invertebrates, and more intricate heads and bodies. And with fish, significant brains become common. The brain audits the facial senses. In theory it could go anywhere, like the central chip of a computer, but biological wiring is frail. So it too lodged up front, like a pilot before an instrument panel. And since the mass of neurons makes a choice killing point, fish evolved a hard covering for the brain, a skull or head-shield. Bone became the foundation of the face.

If you could take the stars in the Orion and shift them about at whim, the constellation would quickly vanish. Orion is more than Rigel, Betelgeuse, and its other stars. It is a pattern, and so is the face. In fact, we see the same array of mouth, nostrils, and eyes in creatures from eels to Einstein. It has been a marvel of hardiness, outlasting mountain ranges. That means evolution has crushed other designs.

What's made it so durable? Why, for instance, does the mouth always lie below the nostrils and eyes? And why don't we have eyes in the back of our heads, so we could see the whole world at once?

The face has a master sculptor: the quest for food. Hence the mouth dominates everywhere, in toads and foxes, caymans and wildebeest. It is the portal where an animal assimilates the world, begins to change it from nonself to self. Hazards abound here and care is paramount. So three checkpoint senses-taste, smell, and sight-lurk nearby to reject poisons and generally tell ambrosia from ash. The taste buds lie within the mouth, the nostrils sit just above, and the eyes perch a tier higher.

Why are the latter two above the mouth? As it happens, this placement yields many rewards. For vertebrates in general, it sets the eyes above falling food and out of the body's shadow. Fish especially need eyes oriented to sunlight, which fades even a few hundred feet down. The arrangement also lets land animals gobble morsels from the earth, sniff rising aromas, and view snout and ground at once, instead of snout and sky.

Some creatures boast extra senses on the face. Blind cavefish have ridges of tiny rods which detect the ripples in water from moving prey. The dimples of pit vipers like rattlesnakes can register shifts in heat as slight as 0.002° F. and help them strike rodents in dark burrows. And sharks have a bulb of flesh jutting out between their eyes and low-slung mouths. Long a puzzle to zoologists, this nosecone contains the ampullae of Lorenzini, which sense electrical pulses from living creatures. To a shark, all food is like a parolee with a radar bracelet.

Strong forces have fixed the face pattern, yet species can stretch it. Carnivores like cats and bears have frontal eyes like headlights to give them binocular vision, which sharpens their sense of 3D, makes them better hunters. But this pairing narrows the visual field, so they often compensate with swivel necks-an owl's can turn some 270 degrees-and eyes that rotate in their sockets. Like all higher primates, humans hew to the carnivore model and have necks and matched, movable eyes. We don't have eyes in the back of our heads because by turning our necks and eyes we can see the whole panorama anyway.

Prey, however, need a faster early-warning system. Many, like gazelles, have eyes on the sides of their heads, to scout a wider range and spot cheetahs earlier. Most fish employ this sentry strategy, and in fact their lenses can bulge through their pupils, giving them a near 360-degree view. Some bottom-feeders take a more drastic tack. Their faces split. The ray, for instance, has a mouth and two nostrils on the smooth, broad belly where it eats. But its eyes lie on top. Its enemies can only come from above, so eyes below would be a gift to barracudas.

Nature smiles on other deviations. Nostrils atop the skull seem pure Mandeville, but whales and dolphins have blowholes there to breathe more easily when they surface. A long, liana nose is bizarre, yet the elephant grew one because its head can't reach the ground. The great flanged face of the hammerhead shark is one of the oddest in the vertebrate world. It may have arisen to spread the nostrils farther apart, increasing the difference between odor levels in each, so these sharks could better track the origin of delectable scents.

Does any animal have eyes on the back of its head? The shrimp Rimicaris exoculata, which lives near seafloor vents in the Pacific, comes close. It has eyes on the rear of its shell. Oceanographers wondered why, and discovered a subtle, surprising glow from the vents themselves. The shrimp monitors this unexplained light to keep hot fluid from scalding it.

These are huge variations, as faces go. But here as everywhere, a small change can sometimes spur a destiny.

© 1998 by Daniel McNeill

About the Author

Daniel McNeill is a bestselling author and winner of the Los Angeles Times Book Prize for The Face. Mr. McNeill is the principal author of Fuzzy Logic, which won the 1992-93 Los Angeles Times Book Prize for Science and Technology. He has written numerous other books and articles on high technology, and his work has also appeared in fiction, travel, history, law, and education publications. He lives in Southern California.