Undeniable (31 page)

In 2005, my staff and I were in the laboratory of Hans Keirstead (then at UC Irvine), where laboratory rats are sedated and purposefully injured. Their backs are broken, and when they awaken from anesthesia, they are partially paralyzed. This is the kind of medical research that many of us find troubling. But read on, because the results are astonishing. Sometime after the fracturing impact, the spines of these rats are injected with human stem cells. These have been grown from fewer than ten strains of stem cells that date back decades, when stem cell research was regulated differently. These stem cell lines have been continued and nurtured in exceptional, carefully controlled laboratories. Along with the human stem cells, the rats are given the same type of drugs used to prevent organ recipients' bodies from rejecting the donor's organ.

In a few days, the rats are able to move their back legs. In a few weeks, they are walking pretty well, and regain control of their bowels. That is, they were incontinent after the spinal injury (even for rats, it can be trouble). Upon close examination, the nerves in the spines of the rats have grown back, to a large extent. They are regenerating their own spines in their own backs. It is amazing. The ramifications for human medicine are far-reaching. Or they could be.

Here's the thing: The technology to extract stem cells from a fertilized and growing human egg is exactly the same technology, at least right now, as the means by which we can implant somatic cell DNA into an egg to start the process of cloning. It is a line of reasoning that we should all follow and form an opinion on, because as the techniques become more refined in animal research, the possibility of performing humane work becomes feasible. Does it become imperative? How do we draw a clear line that allows therapeutic work but prevents full-blown human cloning? That's for all of us taxpayers and voters to ponder.

Because a blastocyst would be involved, researchers are looking to isolate and grow stem cells not from an egg (which could, in principle, develop into a viable fetus) but from a cell taken from another part of the human body. Perhaps one day soon, regeneration of one's own organ cells or nerve cells will be possible and as common as a hip joint replacement. When you consider the extraordinary potential cost savings in having a patient's body do all the work compared with the costs of high-tech prosthetics, it may be an ethical imperative to use a patient's own stem cells for this sort of improvement in his or her quality of life.

As you think further about this research and the potential ramifications of the use of human stem cells in medicine, keep in mind that research on rats and other animals is possible because we are all so much alike. This is direct evidence of evolution. We share so much with our placental mammalian cousins because we all had a common ancestor around 70 million years ago. Our understanding of medicine, blood types, the central nervous system, and where we ultimately came from are all direct results of our understanding of evolution.

If we did not have a common ancestor, if we did not share DNA sequences, if we had not all descended from ancestral living things, all of life science, all that we see living in nature would be far, far more mysterious and hard to understand. That the essential discovery of evolution was made barely a century and a half ago indicates for me how primitive we all must still be. We are only now starting to use our knowledge of living things, gained through the study of natural selection, to become more compassionate toward our fellow humans and better stewards of Earth. We have a long, long way to go. It's exciting to contemplate.

 

32

OUR SKIN COLORS

As an elementary-school kid in the 1960s, I was very aware of race and racism. Back then, Washington, D.C., was in many ways a racist Southern town. That awful sentiment was just part of the scene. I could hear it without listening very hard. It was right there in background conversations in busy restaurants, along with the sound of clinking glassware. I could also see what was happening. Newspaper headlines described famous people who were assassinated for clearly racial reasons. At the same time, the civil rights movement achieved sweeping changes in laws and perceptions. It all had a deep effect on me. Today, it's easy for me to see that racism starts on the surface. Nowhere are the forces of evolution that have shaped our societies more apparent than in the color of our skin. The surprising thing is that skin color does not mean what a great many of us think it does.

I've traveled around the world a little bit (largely because I often attend the International Astronautical Congress, which is held in a different city every year). If nothing else, I've learned that people are a great deal more alike than they are different. In evolutionary terms or fact, we are all almost identical. We each share 99.9 percent of the same DNA. I can prove it to you. Better yet, you can prove it to yourself. What do you think would happen if a man from Scandinavia married and enjoyed sexual encounters with a woman from East Africa? They might easily have a child. That kid would be a human. This union is not going to produce anything else but a human.

There is only one species of
Homo sapiens
. We all share common ancestors. This may be where myths like Adam and Eve living together in a garden come from. If you just sit and think about it, and realize that we are all extraordinarily alike, you might just conclude that there must have been an original pair of humans that led to you, me, and everybody we'll ever see. The author or authors of the Book of Genesis may have reached the same conclusion logically, i.e. by just thinking about it. Humans, all of us, must have had a common ancestor. Otherwise, how could we all be of one species, able to reproduce so effectively? There are more than 7 billion of us strutting, sexing, and texting in the world today.

Despite that line of reasoning, people of different tribes or geographic regions have been warring with each other, distrusting each other, and forbidding marriages with each other for millennia. In many instances, skin color has been a trigger for these conflicts. It raises a fascinating evolutionary question: If we are all one species, why do our colors differ so dramatically? Is skin color connected with deeper differences between various groups of humans? Or you could turn it around and ask, are races real?

Here's the short answer: No. Skin color is a tiny, recent, and transient feature of human genetics. One of
The Eyes of Nye
television shows was about this issue. I stood in a field with a few dozen cattle. I pointed out that the colors of those particular animals varied widely. There were black, white, and brown cows of various shades, and they exhibited no discrimination. They roamed and grazed showing no preference for any one variation. Racism did not seem to be an issue with them; they were all of one species. So are we. Race, as it is commonly understood, is an illusion. But don't take my word on it. Let's see what two centuries of evolutionary research have to say.

The first place to look for answers is on the skin of our closest primate relatives. As our understanding of DNA has increased, we have come to understand that we share around 98.8 percent of our gene sequence with chimpanzees. This is striking evidence for chimps and chumps to have a common ancestor. Chimpanzees have very light-colored skin; you can see for yourself where the skin is exposed on their cute cheeks and jowls. So, we might expect humans to have about the same color of skin as our very close genetic cousins. But with few exceptions, we don't.

Anthropologists have been all over the world looking for fossils of our ancestors. And they've found them: dozens of closely related human skulls and nearly-human skulls (my old boss?) and other bones, which all indicate that humankind started out in East Africa. That's where we find the greatest genetic diversity of humans to this day, and that's where we find the oldest fossils and oldest evidence of human activity. If we started out related to an ancestor of chimpanzees, did we start out with skin about the same color? Fossil bones can't tell us. At least, if there is a way to extract information about the color of the skin that once protected those bones we haven't figured it out yet.

Since fossils don't provide the answer, scientists have turned around and tried to understand the adaptive function of skin color from an evolutionary point of view. Most obviously, our skin protects our insides from what comes at us from the outside—wind, rain, and thwipping tree branches. Less obviously, skin is an organ that produces a chemical we cannot live without, vitamin D. This vitamin got its D designation by being the fourth one to be identified. The main form is cholecalciferol; that's C
27
H
44
O. Researchers studied our beloved dogs to find it. The ability to synthesize vitamin D goes way back in evolution. Plankton at sea have been manufacturing it for 500 million years. Sea creatures use it to capture and make use of calcium in their environment. So do we.

One of the wonderful things about vitamin D is that you don't need to eat it; your body can make it. You and I just need a little exposure to ultraviolet light to give a form of cholesterol in the skin a jolt and convert it to vitamin D. But there is a catch: Too much ultraviolet light means trouble. It carries more energy than visible light and is able to break down or dissociate delicate biological molecules, especially your folic acid, your folates. Ultraviolet can burn your skin, for example. So to be successful, animals like us need a way to block a large fraction of the ultraviolet light that hits us while letting just enough through to maintain a proper level of vitamin D.

If you're a chimpanzee, you have solved this problem with one of the oldest tricks in the evolutionary book. You grow hair. Hair is made of keratin proteins that are not too different from those of distant relatives on the Tree of Life who use keratin to produce feathers and scales. As you no doubt realize, hair blocks light. Chimpanzee bodies are covered almost everywhere with thick, dark hair. Even if you make jokes about hairy guys, the hairiest among us don't come close. Chimps are protected from excessive UV rays by their hair. But if hair offers such good protection, why did we humans lose most of ours?

A likely answer emerges from the following thought experiment, which I hope none of us ever has to actually conduct. You may have heard about people who kept a chimpanzee as a pet. Everything is just fine while the chimp is young, but as he or she grows older, a chimpanzee proves to be surprisingly strong compared to a human. A chimpanzee can easily outwrestle you and pull your arm right out of its socket. But if push comes to shove and you get into a violent disagreement with a chimpanzee, there's one thing to do. You can outrun him or her.

Humans are champion long-distance runners. As soon as a person and a chimp start running they both get hot. Chimps quickly overheat; humans do not, because they are much better at shedding body heat. You see where I'm going. According to one leading theory, ancestral humans lost their hair over successive generations of hunter, gatherer, scavenger offspring because less hair meant cooler, more effective long-distance running. That ability let our ancestors outmaneuver and outrun prey. Try wearing a couple of extra jackets—or better yet, fur coats—on a hot humid day and run a mile. Now, take those jackets off and try it again. You'll see what a difference a lack of fur makes. Overheating slows us endothermic animals down.

With the loss of hair our ancestors faced a new challenge, though, because it exposed them to more ultraviolet light. Ultraviolet helps make vitamin D, yes, but it also breaks down similarly crucial folic acid along with other ultraviolet sensitive compounds. You get your folates (like folic acid) from leafy green vegetables. Unlike the case with vitamin D, the human body cannot manufacture its own folates. Folates play a role in the growth of fetuses. Babies born to women who have been exposed to too much ultraviolet have defects in their spines and central nervous systems. They cannot survive. So those early humans who had a little extra sun blocking melanin in their skin did better in strong ultraviolet light environments than those with paler skin.

In general, the closer people live to the equator, the more ultraviolet exposure they receive and the darker their average skin color. Strong local weather conditions can also attenuate the ultraviolet levels. Take a look at the map of skin color of people native to different regions of Earth. Near the equator, people have darker skin. Where it's cloudy a lot, as it is in Britain, people have lighter skin. Where people live closer to outer space, as they do in Tibet, they are exposed to more ultraviolet and have darker skin. Skin color is basically a measure of the local ultraviolet levels, and it is controlled by relatively minor adaptive changes in the genome.

This fascinating line of reasoning was explained to me by Nina Jablonski, the scientist who did the fundamental research. (She was at the California Academy of Sciences at the time; now she's at Penn State.) She points out that between two people—any two people—the genetic differences are minuscule. Jablonski is a studious observer of those subtle differences. For her, the most stable physical distinctions are not in the color of our skin, but in the configuration of our bones, especially the shape of our heads. During our interview, she paused and leaned toward me with her hands open as though she were going to reach for a melon on a shelf. As she did she remarked, “Bill, you have a perfect European skull.” I had to stop her and explain that she couldn't have my skull right then, as I was still using it (I'm
still
very attached to it). But she got my attention for the rest of what she had to say about human migrations around the world.

With their success in Africa, our human ancestors went looking for new pastures and presumably new adventures to the north in what is now Iran and Iraq. Here, farther from the equator than East Africa is, any individuals who happened to be born with slightly lighter skin did a little better. They got just the right amount of vitamin D without breaking down their folic acid and other essential, ultraviolet-sensitive compounds. They survived a little longer than those who were born in this region but retained African, or very dark skin. Sure enough, modern Northern Africans have lighter skin than equatorial East Africans.