Size is not everything, however. Natural selection has also shaped some human genes that appear to control particular kinds of thought. Take language. As I wrote in 2001, our capacity to learn a language shows some signs of being a hard-wired instinct. That suggested that genes shaped language, and yet at the time scientists didn’t know of a single language-linked gene. Now they have one. It was discovered in a family in London who suffered a hereditary difficulty with both speech and grammar. In 2002 British scientists announced that the family members who had language problems all carried a mutant form of a gene they called FOXP2. Brain scans later revealed that people who carry mutant forms of FOXP2 have less activity in a language-related region of their brain called Broca’s area.
Scientists then compared human FOXP2 to the version carried by other mammals. Obviously, FOXP2 does not produce a capacity for language in other species. But in a 2005 experiment with mice, scientists showed that it influences animal communication. Mouse pups with only one working copy made far fewer cries to their mother. Those without any working copies made no cries at all.
A comparison of silent and non-silent substitutions revealed that FOXP2 has undergone intense natural selection in humans. Scientists were even able to estimate when that natural selection took place: less than two hundred thousand years ago. That just so happens to be around the time our species first emerged. These results hint that fullblown language was a late-arriving skill and has only evolved relatively recently in the hominid lineage.
But natural selection did not stop there. Several new studies have identified genes that have evolved over just the past fifty thousand years. One of these studies, published in March 2006 by scientists at the University of Chicago, proved particularly exciting. They looked for signs of natural selection in just the past few thousand years. The scientists based their search on the way genes get split apart from one generation to the next. Each person carries two pairs of each chromosome. As eggs and sperm form, each chromosome may swap a chunk of genes with its partner. One of the chunks inherited by a child may carry a gene that offers a major reproductive advantage. Over the course of generations, that gene will spread rapidly through the population—along with the surrounding genes in its chunk of chromosome.
The scientists looked for versions of genes that consistently turned up lodged in the same chunk of surrounding DNA. They discovered seven hundred regions of the human genome that contained these fast-spreading genes. The genes influence traits ranging from skin color to digestion. Taste and smell genes have evolved rapidly as well. Many of these genes, which are estimated to have evolved over the past six thousand to ten thousand years, may have been favored as humans began to shift to eating domesticated plants and animals. A few of the still-evolving genes are active in the brain. Could the rise of civilization and rich human culture have driven the evolution of these genes? Check back in five years—at the rate things are going, scientists may have some answers.
These past five years have seen great advances in evolutionary biology, but they’ve also seen the loss of some of its most important thinkers. In 2004, the English biologist John Maynard Smith died at age eighty-four. Maynard Smith realized that he could make sense of evolution by borrowing concepts from mathematics and economics. One of his most fruitful imports was game theory, the study of how different strategies lead players to victory or defeat. Maynard Smith made organisms into players, and their behaviors into strategies. It then became possible to analyze how different strategies would thrive thanks to natural selection or be driven to extinction.
Scientists have found that in many cases several different behaviors can coexist. A male elephant seal may try to gain reproductive success, for example, either by challenging a big male or skulking at the edge of the big male’s harm, mating secretly with a few females. Scientists have found lots of these so-called evolutionarily stable strategies. Evolutionary stable strategies may have a lot to tell us about human behavior as well. Genes have a role in personality, intelligence, and behavior, and there’s obviously a lot of variation in all these factors. It’s possible that these genes have, over millions of years, reached an evolutionarily stable state with one another. And these games may also be a model for how something as peculiar as cooperation evolved in our own species.
In Evolution: The Triumph of an Idea, I describe how a young ornithologist named Ernst Mayr explored Pacific islands in the 1920s and in the process laid the groundwork for the modern understanding of species and how they are born. Mayr died in 2005 at the age of one hundred. He spent the last few decades of his life watching his ideas inspire generations of new biologists, but he enjoyed watching them pushing beyond his own ideas. “The new research has one most encouraging message for the active evolutionist,” he wrote in an essay shortly before his death. “It is that evolutionary biology is an endless frontier and there is still plenty to be discovered. I only regret that I won’t be present to enjoy these future developments.”
Sadly, Stephen Jay Gould did not enjoy the longevity of Maynard Smith or Mayr, dying at age sixty in 2002. When his introduction graced my book the year before his death, I had no idea that he would leave us so soon. I was honored then, and I’m even more honored now that now that Evolution: The Triumph of an Idea remains associated with him. Gould achieved greatness both as a scientist and as a writer. He pressed biologists to think about evolution in new ways, whether they were looking at the fossil record or at embryos. And few writers of the past one hundred fifty years could compete with him in bringing the glory of evolutionary biology to the public at large. It is to these three great scientists—and to future evolutionary biologists—that I rededicate this book.
INTRODUCTION
A famous legend (perhaps even true) from the early days of Darwinism provides a good organizing theme for understanding the centrality and importance of evolution both in science and for human life in general. A prominent English lady, the wife of a lord or a bishop (yes, they may marry in the Church of England), exclaimed to her husband when she grasped the scary novelty of evolution: “Oh my dear, let us hope that what Mr. Darwin says is not true. But if it is true, let us hope that it will not become generally known!”
Scientists invoke this familiar story to laugh at the recalcitrant stodginess of old belief and breeding—especially the risible image of the upper classes keeping a revolutionary fact of nature in the Pandora’s box of their own private learning. Thus, the lady of this anecdote enters history as a quintessential patrician fool. Let me suggest, however, if only to organize the outline of this introduction, that we reconceptualize her as a prophet. For what Mr. Darwin said is clearly true, and it has also not become generally known (or, at least in the United States, albeit uniquely in the Western world, even generally acknowledged). We need to understand the reasons for this exceedingly curious situation.