Does Natural Selection Explain Skin Color?
Now let’s turn to the question of how our visible geographic differences originated. One argument is that they are the result of natural selection, a mechanism that drives evolution, the pattern of change in life forms over time, as new species develop from earlier ones. Natural selection simply means that genetic traits that help a plant or animal survive get passed on to that organism’s offspring.
New traits arise, or existing traits change, when changes occur in genes because of random mutations. Genetic changes are just as likely to harm an individual plant or animal, or to have no effect on it, as they are to help it. But if the mutation does help the organism—if it leads to a slightly longer bill, say, that lets a bird pick more beetles out of tree bark—that organism is likely to live longer than others of its species, which means that it can have more offspring. Those offspring will inherit its genetic program, including the new mutation. They will have the survival advantage that the mutation brought, and their offspring will spread it more widely. In time, the mutation becomes established in a population, resulting in a new subspecies or even a whole new species.
Natural selection accounts for many differences between species, such as the fact that lions have paws with claws while we have grasping fingers. Natural selection also explains some racial or geographic variation within species. For example, Arctic weasels that live in areas covered by winter snow change color from brown in summer to white in winter, while weasels that live farther south stay brown all year. That racial difference improves the weasels’ survival. White weasels would be glaringly easy to see against a brown background, but against snow they are camouflaged and less obvious to their prey.
Natural selection surely explains some geographic variation in humans. For example, many Africans but no Swedes have the sickle cell hemoglobin gene, because that gene protects against malaria, a tropical disease that occurs in Africa but not in Sweden. Other localized human traits that must have evolved through natural selection include the big chests of Indians in the Andes Mountains of South America (good for getting oxygen from thin air at high altitudes) and the compact shapes of Eskimos (good for saving heat in a cold climate).
Can natural selection explain the racial differences that we think of first—skin color and eye color? If so, we might expect that the same trait, such as blue eyes, would reappear in parts of the world with similar climates, and that scientists would agree on what the trait is good for.
Skin color should be the easiest trait to understand. Our skins run the spectrum from various shades of black, brown, copper, and yellowish to pink with or without freckles. The usual story to explain this variation by natural selection goes like this: People from sunny Africa have black skin. So do people from other sunny areas, such as southern India and New Guinea. Skin gets paler as you move north or south away from the equator, until you reach northern Europe, where you find the palest skin of all. Obviously, dark skin evolved in people who were exposed to a lot of sunlight, because dark skin protects against sunburn and skin cancer. Doesn’t that make sense?
In some parts of Australia, more than three-quarters of Aboriginal children have light blond hair, although it sometimes turns brown as they grow up. The other population in which blond hair is common is pale-skinned northern Europeans.
Unfortunately, it’s not so simple. To begin with, sunburn and skin cancer cause very few deaths compared with infectious diseases. This means that sunburn and skin cancer would not be powerful pressures for natural selection. At least eight other theories have been suggested to explain why natural selection could have favored dark skin in the tropics and pale skin in the north. Among these theories: dark skin provides camouflage in the jungle, and pale skin is less sensitive to frostbite.
But the biggest objection to any of these theories is that the link between dark skin and sunny climates is not perfect. Native peoples evolved dark skin in some areas with relatively little sunlight, such as the forested Australian island of Tasmania, while in sunny areas of tropical Southeast Asia, skin color is only medium. No American Indians have black skin, even in the sunniest regions. In the Solomon Islands of the Pacific, jet-black people and lighter-skinned people live on islands that share the same climate conditions.
Anthropologists argue that some light-skinned tropical people migrated to their sunny regions too recently to have evolved dark skin. The ancestors of American Indians may have reached the Americas just eleven thousand years ago—perhaps not long enough to evolve black skin in the hot American tropics. To support the climate theory of skin color, anthropologists also point out that Scandinavians, who have pale skin, live in the cold, dark, foggy North. The problem is that Scandinavians have been in northern Europe for only four or five thousand years—even less time than American Indians have been in the Amazon. Either Scandinavians acquired their pale skin long ago in some other place, or they evolved it in less than half the time that American Indians have lived in the Amazon without evolving black skin.
If the link between skin color and climate seems weak, there appears to be no link at all between climate and the color of hair or eyes. Blond hair is common in cold, wet, Scandinavia and also among Aborigines of the hot, dry center of Australia. What do these two areas have in common, and how could blond hair help both Swedes and Aborigines to survive?
Sexual Selection and Physical Appearances
When Darwin considered the problem of human geographic variations, he decided that natural selection had nothing to do with it. He came up with a theory he preferred: sexual selection.
Darwin had noticed that many animals have features with no obvious survival value, such as the long, colorful tails of male peacocks and the dark, shaggy manes of male lions. These features do help animals get mates, however, either by attracting individuals of the opposite sex or by intimidating rivals. Male peacocks and lions that are especially successful at attracting females and scaring off rivals will leave more descendants than other males. Their genes will get passed on, and those genes will spread through a population because of sexual selection, or mating preferences, not because of natural selection. The same argument applies to female traits as well.
For sexual selection to work, evolution must produce two changes at the same time. One sex must evolve a trait, and the other sex must evolve a liking for that trait. Male peacocks could hardly afford to flash their fancy tails if the sight revolted females and drove them away. As long as one sex has it and the other sex likes it, sexual selection could lead to just about any trait, if that trait doesn’t interfere too much with survival.
Could human variations such as skin color be the result of sexual preferences that just happen to vary from place to place? Darwin believed the answer was yes. He noted that people in different parts of the world define beauty in terms of what is familiar to them. Individuals on the Pacific island of Fiji, or among the Bushmen of southern Africa, or in Iceland grow up learning the local standards of beauty. These standards tend to live on in each population because individuals who match them closely have the greatest success in finding mates and passing on their genes to offspring.
Darwin died before his theory of sexual selection could be tested against studies of how people actually choose their mates. As we saw in the last chapter, plenty of such studies have now taken place. They show that, on the whole, people tend to marry individuals who resemble them in many features, including skin, hair, and eye color. Our beauty standards are based on people we see around us in childhood, especially our parents and siblings, whom we see the most—and who resemble us most closely, since we share their genes.
WHITE, BLUE—AND PINK?
WE APPEAR TO CHOOSE OUR MATES BASED on standards of beauty that we imprint on, or form an attachment to, early in our lives. For a strong test of the imprinting theory of mate choice, we would have to do some experiments. And while it is not practical or possible to do such experiments with people, we can do them with animals.
One study involved snow geese. These birds occur in the wild in either of two colors, called the white phase and the blue phase. Canadian researchers wanted to know if snow geese are born with an inherited preference for white or blue mates, or if they learn which phase to prefer as they grow up. They hatched goose eggs in an incubator, then placed the goslings, or infant geese, with foster parents. When these infants grew up, they preferred mates of the same color as their foster parents. But goslings that grew up in a mixed flock of both blue and white birds showed no preference for one color of mate over another.
As a final touch, the biologists dyed some of the white parent geese pink, a color that does not occur in nature! Goslings raised by these parents came to prefer pink-dyed geese as mates. This showed that geese do not inherit their color preference. They learn it in childhood, by imprinting on their parents, siblings, and playmates.
Traits, Tastes, and Mating Choices
How do I think people in different parts of the world evolved their differences? Our insides are invisible to us, so they were molded only by natural selection. This is why tropical Africans, but not Swedes, got the antimalaria defense of the sickle cell hemoglobin gene. Many visible features on our outside also got molded by natural selection. But just as in animals, sexual selection had a big effect on molding the visible traits by which we are attracted to our mates.
For us humans, those traits include especially the skin, hair, and eyes. in each part of the world, those traits developed in lockstep with our imprinted preferences, the tastes and ideals of beauty that form when we are young. The traits reinforced the preferences, and the preferences reinforced the traits. The result was different color packages in different parts of the world.
Which particular human population ended up with a given eye or hair color may have been partly an accident of what biologists call the “founder effect.” This means that if a few individuals colonize an empty land, and their descendants multiply and fill the land, the genes of those few founding individuals may still dominate in the population generations later.
I don’t mean to say that climate has nothing to do with skin color. Tropical peoples tend on average to have darker skin than those who live farther from the equator (although there are many exceptions). This is probably due to natural selection, even though we are not sure exactly how. i am saying that sexual selection has been strong enough to make any link between skin color and sun exposure quite imperfect.
If you doubt the idea that traits and preferences can evolve together to different end points, consider fashion. in the 1950s, right after World War ii, women favored men with crew cuts and clean-shaven faces. Since then we’ve seen a parade of men’s fashions, including beards, long hair, earrings, and purple-dyed Mohawks. A young man who dared to flaunt any of those fashions in the 1950s would have revolted the girls and had zero mating success. in more recent times, however, those looks have gained appeal within some populations, where they are preferred by females. This is not because short hair was especially suited to the climate of the 1950s, while Mohawks aid survival today. it’s because men’s appearances and women’s tastes evolved together, far faster than any evolutionary change, since no gene mutations were needed. The same thing happens with female fashions.
The visible geographic variability that sexual selection has produced in humans is impressive. i don’t know of any other wild animal species in which the eye color of different populations can be green, blue, gray, brown, or black, while skin color varies geographically from pale to black and hair is either red, yellow, brown, or black. There may be no limits to the colors with which sexual selection can adorn us, except the time required by evolution. Twenty thousand years from now there might be women with naturally green hair and red eyes—and men who think they are gorgeous.
Spanish soldier and military governor Juan Ponce de Leon explored Florida in 1513. Later, some historians claimed that he was searching for the Fountain of Youth, a mythical spring whose waters cured disease and granted eternal life. Some scientists still hope to find the secret of endless—or at least longer—life.
CHAPTER 5
WHY DO WE GROW OLD AND DIE?
IS THERE A BELOVED GRANDPARENT IN YOUR life? Or maybe a respected teacher? You can probably think of at least one older person who has enriched your life.
We enter life surrounded by people who are older than we are: our parents, grandparents, aunts and uncles, and maybe older brothers and sisters. They become our protectors, guides, families, and friends. We cherish our relationships with these loved ones. it is hard to accept the fact that we will lose them someday, but life naturally draws to an end as old age is followed by death. it is a fate that all of us will eventually share.
Like members of every other species on earth, individuals of our species, Homo sapiens, have a life expectancy that is a feature of our life cycle. Life expectancy is the term scientists use to describe the average period that a member of any species can expect to live. Many factors influence life expectancy. For humans, a key factor is where you live. People born in different countries may have different life expectancies, based on things such as the quality of the food, water, and medical care available to them. The life expectancy for adults in the United States is now seventy-six years for men and almost eighty- one years for women. Few of us, though, will survive to one hundred.
Why is it so easy to live almost 80 years, so hard to live 100, and almost impossible to survive to 120? Why do humans with access to the best medical care, and animals kept in a cage with plenty of food and no predators, inevitably grow weak or sick and die? Death is one of the most obvious features of our life cycle, but there’s nothing obvious about what causes it.
Slow Aging
We age more slowly than our closest relatives. Not a single ape of any species has been recorded as achieving the current life expectancy of American humans. Only exceptional apes reach their fifties. Some of our slow aging may have developed fairly recently in our evolutionary history, around the time of the Great Leap Forward, sixty thousand years ago. Few Neanderthals survived past the age of forty. Among the Cro-Magnons who replaced them, quite a few lived into their sixties.
Slow aging is vital to the human life cycle, which depends on shared information. As language evolved, we became able to pass on far more information than before. Today we can pass it in written or recorded form, but writing is a fairly recent development in our history. For tens of thousands of years before writing, old people were our libraries. They served as keepers of a group’s shared information and experience, just as they continue to do in tribal societies today. Under hunter-gatherer conditions, the knowledge possessed by even one seventy- year-old could mean the difference between starvation and survival for a whole clan.
Our ability to survive to a ripe old age had something to do with our advances in culture and technology. it’s easier to defend yourself against a lion with a spear than with a hand-held stone, easier still with a high-powered rifle. But advances in culture and technology would not have been enough to give us longer lives unless our bodies had also been redesigned to last longer. As we’ll see in this chapter, our biology became remolded to match the increased life expectancy made possible by our cultural and technological advances.
Aging is studied by two groups of scientists who take very different approaches. Physiologists explore the body and its structures, searching for the mechanisms within our cells that bring about aging. Evolutionary biologists try to understand how natural selection could ever permit aging to occur. I think that aging can’t be understood unless we seek both explanations. I expect that the evolutionary explanation (why we age) will help us find the physiological explanation (what specific features and processes in our bodies cause us to age).
Repair and Replace
Physiologists tend to think that something about our bodies and their systems makes aging unavoidable. One theory is that aging occurs because our immune systems find it harder and ha
rder to tell the difference between our own cells and foreign cells from outside our bodies. This is a fatal defect in our immune systems. Could natural selection have created an immune system without that flaw? To answer that question, we need to look at how our bodies maintain themselves.
Aging can be viewed simply as damage or deterioration that doesn’t get repaired. We are unconsciously but constantly repairing ourselves at every level, from molecules to tissues to whole organs. In the same way, we spend money to repair our cars. Our bodies’ self-repair mechanisms, like car repairs, fall into two categories: damage control and regular replacement.
For a car, damage control means things such as repairing a flat tire or replacing a smashed fender. For us, the most visible example of damage control is wound healing, which repairs damage to our skin. Some animals can achieve more spectacular damage control. Lizards regrow tails they have lost, starfish regrow severed limbs, and sea cucumbers can even regrow their intestines. At the invisible, molecular level, we have enzymes that recognize and fix damaged sites in our genetic material, DNA.
The other type of repair is regular replacement, also familiar to any car owner. We periodically change the oil, air filter, and other parts without waiting for the car to break down first. In the animal world, teeth are replaced on a scheduled basis. Humans go through two sets in their lifetime, elephants six sets, and sharks an indefinite number of sets. Lobsters and insects are among the creatures that regularly replace their external skeletons, or hard shells, by shedding them and growing new ones. Hair growth is another example of regular replacement. Hair keeps steadily growing, no matter how short we cut it.
The Third Chimpanzee for Young People Page 6