Why Evolution Is True (37 page)

Conversely, genes that were once useful may, due to cultural change, now have destructive effects. Our love of sweets and fats, for example, may well have been adaptive in our ancestors, for whom such treats were a valuable but rare source of energy.
⁵⁴
But these once rare foods are now readily available, and so our genetic heritage brings us tooth decay, obesity, and heart problems. Too, our tendency to lay on fat from rich food may also have been adaptive during times when variation in local food abundance produced a feast-or-famine situation, giving a selective advantage to those who were able to store up calories for lean times.

Does this mean that we’re really de-evolving? To some degree, yes, but we’re probably also becoming more adapted to modern environments that create new types of selection. We should remember that so long as people die before they’ve stopped reproducing, and so long as some people leave more offspring than others, there is an opportunity for natural selection to improve us. And if there’s genetic variation that affects our ability to survive and leave children, it will promote evolutionary change. That is certainly happening now. Although pre-reproductive mortality is low in some Western populations, it’s high in many other places, especially Africa, where child mortality can exceed 25 percent. And that mortality is often caused by infectious diseases such as cholera, typhoid fever, and tuberculosis. Other diseases, like malaria and AIDS, continue to kill many children and adults of reproductive age.

The sources of mortality are there, and so are the genes that alleviate them. Variant alleles of some enzymes, for example hemoglobin (notably the sickle-cell allele), confer resistance to malaria. And there is one mutant gene—an allele called CCR5-Δ32—that provides its carriers with strong protection against infection with the AIDS virus. We can predict that if AIDS continues as a significant source of mortality, the frequency of this allele will rise in affected populations. That’s evolution, as surely as is antibiotic resistance in bacteria. And there are undoubtedly other sources of mortality that we don’t fully understand: toxins, pollution, stress, and the like. If we’ve learned anything from breeding experiments, it is that nearly every species has genetic variation to respond to nearly any form of selection. Slowly, inexorably, and invisibly, our genome adapts to many new sources of mortality. But not every source. Conditions that have both genetic and environmental causes, including obesity, diabetes, and heart disease, may not respond to selection because the mortality they produce occurs mostly after their victims have stopped reproducing. Survival of the fittest is accompanied by survival of the fattest.

But people don’t care that much about disease resistance, important as it is. They want to know whether humans are getting stronger, smarter, or prettier. That, of course, depends on whether these traits are associated with differential reproduction, and this we just don’t know. Nor does it much matter. In our rapidly changing culture, social improvements enhance our abilities far more than any changes in our genes—unless, that is, we decide to tinker with our evolution through genetic manipulations like preselecting favorable sperm and eggs.

The lesson from the human fossil record, then, combined with more recent discoveries in human genetics, confirms that we are evolved mammals—proud and accomplished ones, to be sure, but mammals built by the same processes that transformed every form of life over the past few billion years. Like all species, we are not an end product of evolution, but a work in progress, though our own genetic progress may be slow. And though we have come a long way from ancestral apes, the marks of our heritage still betray us. Gilbert and Sullivan joked that we are just depilated monkeys; Darwin was not as funny but far more lyrical—and truthful:

Chapter 9

Evolution Redux

After sleeping through a hundred million centuries
we have finally opened our eyes on a sumptuous planet,
sparkling with color, bountiful with life. Within decades we
must close our eyes again. Isn’t it a noble, an enlightened way
of spending our brief time in the sun, to work at understanding
the universe and how we have come to wake up in it?
This is how I answer when I am asked
—
as I am surprisingly
often
—
why I bother to get up in the mornings.

-Richard Dawkins

 

 

 

A
few years ago, a group of businessmen in a ritzy suburb of Chicago asked me to speak on the topic of evolution versus intelligent design. To their credit, they were intellectually curious enough to want to learn more about the supposed “controversy.” I laid out the evidence for evolution and then explained why intelligent design was a religious rather than a scientific explanation of life. After the talk, a member of the audience approached me and said, “I found your evidence for evolution very convincing—but I still don’t believe it.”

This statement encapsulates a deep and widespread ambiguity that many feel about evolutionary biology. The evidence is convincing, but they’re not convinced. How can that be? Other areas of science aren’t plagued by such problems. We don’t doubt the existence of electrons or black holes, despite the fact that these phenomena are much further removed from everyday experience than is evolution. After all, you can see fossils in any natural history museum, and we read constantly about how bacteria and viruses are evolving resistance to drugs. So what’s the problem with evolution?

What’s not a problem is the lack of evidence. Since you’ve read this far, I hope you’re convinced that evolution is far more than a scientific theory: it is a scientific fact. We’ve looked at evidence from many areas—the fossil record, biogeography, embryology, vestigial structures, suboptimal design, and so on—all of that evidence showing, without a scintilla of doubt, that organisms have evolved. And it’s not just small “microevolutionary” changes, either: we’ve seen new species form, both in real time and in the fossil record, and we’ve found transitional forms between major groups, such as whales and land animals. We’ve observed natural selection in action, and have every reason to think that it can produce complex organisms and features.

We’ve also seen that evolutionary biology makes testable predictions, though not of course in the sense of predicting how a particular species will evolve, for that depends on a myriad of uncertain factors such as which mutations crop up and how environments may change. But we
can
predict where fossils will be found (take Darwin’s prediction that human ancestors would be found in Africa), we can predict when common ancestors would appear (for example, the discovery of the “fishapod”
Tiktaalik
in 370-million-year-old rocks, described in chapter 2), and we can predict what those ancestors should look like before we find them (one is the remarkable “missing link” between ants and wasps, also shown in chapter 2). Scientists predicted that they would find fossils of marsupials in Antarctica—and they did. And we can predict that if we find an animal species in which males are brightly colored and females are not, that species will have a polygynous mating system.

Every day, hundreds of observations and experiments pour into the hopper of the scientific literature. Many of them don’t have much to do with evolution—they’re observations about details of physiology, biochemistry, development, and so on—but many of them do. And every fact that has something to do with evolution confirms its truth. Every fossil that we find, every DNA molecule that we sequence, every organ system that we dissect supports the idea that species evolved from common ancestors. Despite innumerable possible observations that could prove evolution untrue, we don’t have a single one. We don’t find mammals in Precambrian rocks, humans in the same layers as dinosaurs, or any other fossils out of evolutionary order. DNA sequencing supports the evolutionary relationships of species originally deduced from the fossil record. And, as natural selection predicts, we find no species with adaptations that benefit only a different species. We do find dead genes and vestigial organs, incomprehensible under the idea of special creation. Despite a million chances to be wrong, evolution always comes up right. That is as close as we can get to a scientific truth.

Now, when we say that “evolution is true,” what we mean is that the major tenets of Darwinism have been verified. Organisms evolved, they did so gradually, lineages split into different species from common ancestors, and natural selection is the major engine of adaptation. No serious biologist doubts these propositions. But this doesn’t mean that Darwinism is scientifically exhausted, with nothing left to understand. Far from it. Evolutionary biology is teeming with questions and controversies. How exactly does sexual selection work? Do females select males with good genes? How much of a role does genetic drift (as opposed to natural or sexual selection) play in the evolution of DNA sequences or the features of organisms? Which fossil hominins are on the direct line to
Homo sapiens?
What caused the Cambrian “explosion” of life, in which many new types of animals appeared within only a few million years?

Critics of evolution seize upon these controversies, arguing that they show that something is wrong with the theory of evolution itself. But this is specious. There is no dissent among serious biologists about the major claims of evolutionary theory—only about the details of how evolution occurred, and about the relative roles of various evolutionary mechanisms. Far from discrediting evolution, the “controversies” are in fact the sign of a vibrant, thriving field. What moves science forward is ignorance, debate, and the testing of alternative theories with observations and experiments. A science without controversy is a science without progress.

At this point I could simply say, “I’ve given the evidence, and it shows that evolution is true. Q.E.D.” But I’d be remiss if I did that, because, like the businessman I encountered after my lecture, many people require more than just evidence before they’ll accept evolution. To these folks, evolution raises such profound questions of purpose, morality, and meaning that they just can’t accept it no matter how much evidence they see. It’s not that we evolved from apes that bothers them so much; it’s the emotional
consequences
of facing that fact. And unless we address those concerns, we won’t progress in making evolution a universally acknowledged truth. As the American philosopher Michael Ruse noted, “Nobody lies awake worrying about gaps in the fossil record. Many people lie awake worrying about abortion and drugs and the decline of the family and gay marriage and all of the other things that are opposed to so-called ‘moral values.’”

Nancy Pearcey, a conservative American philosopher and advocate of intelligent design, expressed this common fear:

Pearcey argues (and many American creationists agree) that all the perceived evils of evolution come from two worldviews that are part of science: naturalism and materialism. Naturalism is the view that the only way to understand our universe is through the scientific method. Materialism is the idea that the only reality is the physical matter of the universe, and that everything else, including thoughts, will, and emotions, comes from physical laws acting on that matter. The message of evolution, and all of science, is one of naturalistic materialism. Darwinism tells us that, like all species, human beings arose from the working of blind, purposeless forces over eons of time. As far as we can determine, the same forces that gave rise to ferns, mushrooms, lizards, and squirrels also produced us. Now, science cannot completely exclude the possibility of supernatural explanation. It is possible— though very unlikely—that our whole world is controlled by elves. But supernatural explanations like these are simply never needed: we manage to understand the natural world just fine using reason and materialism. Furthermore, supernatural explanations always mean the end of inquiry: that’s the way God wants it, end of story. Science, on the other hand, is never satisfied: our studies of the universe will continue until humans go extinct.

But Pearcey’s notion that these lessons of evolution will inevitably spill over into the study of ethics, history, and “family life” is unnecessarily alarmist. How can you derive meaning, purpose, or ethics from evolution? You can’t. Evolution is simply a theory about the process and patterns of life’s diversification, not a grand philosophical scheme about the meaning of life. It can’t tell us what to do, or how we should behave. And this is the big problem for many believers, who want to find in the story of our origins a reason for our existence, and a sense of how to behave.

Most of us do need meaning, purpose, and moral guidance in our lives. How do we find them if we accept that evolution is the real story of our origin? That question is outside the domain of science. But evolution can still shed some light on whether our morality is constrained by our genetics. If our bodies are the product of evolution, what about our behavior? Do we carry the psychological baggage of our millions of years on the African savanna? If so, how far can we overcome it?