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When Sean Carroll was a graduate student at Tufts School of Medicine in Boston, he found himself seduced by spectacular new studies of the humble fruit fly. That work, which eventually won a Nobel Prize for its principals, showed that modifying a single gene during a fly’s embryonic development could transform the insect’s body plan: Instead of becoming an antenna, a body extension could develop into a leg. Carroll continued to study these genes and, some years later, found that they were not restricted to fruit flies; they turned out to be part of a master tool kit that sculpts the body structures of all animals, ranging from humans to nematode worms.
The discovery of this small set of universal body-building genes gave Carroll and others a fresh way to explore the inner workings of evolution. By observing how the genes changed during the course of embryonic development, scientists could track the emergence of a novel physical trait, the first step toward the creation of a new species. For the first time, researchers had direct access to the machinery of evolution and could actually watch it in the act. A new science, known as evolutionary developmental biology, or evo devo, was born.
One of the great triumphs of modern evolutionary science, evo devo addresses many of the key questions that were unanswerable when Charles Darwin published On the Origin of Species in 1859, and Carroll has become a leader in this nascent field. Now a professor of molecular biology and genetics at the University of Wisconsin, he continues to decode the genes that control life’s physical forms and to explore how mutations in those genes drive evolutionary change. These days, Carroll also devotes increasing energy to telling the public about his field’s remarkable discoveries through a series of books—Endless Forms Most Beautiful, The Making of the Fittest, and the brand-new Remarkable Creatures. He spoke with DISCOVER senior editor Pamela Weintraub about what his work has taught him about Darwin, the nature of evolution, and how life really works.
It has been 150 years since Charles Darwin proposed his theory of evolution in On the Origin of Species, yet in some ways the concept of evolution seems more controversial than ever today. Why do you think that is?
It is a cultural issue, not a scientific one. On the science side our confidence grows yearly because we see independent lines of evidence converge. What we’ve learned from the fossil record is confirmed by the DNA record and confirmed again by embryology. But people have been raised to disbelieve evolution and to hold other ideas more precious than this knowledge. At the same time, we routinely rely on DNA to convict and exonerate criminals. We rely on DNA science for things like paternity. We rely on DNA science in the clinic to weigh our disease risks or maybe even to look at prognoses for things like cancer. DNA science surrounds us, but in this one realm we seem unwilling to accept its facts. Juries are willing to put people to death based upon the variations in DNA, but they’re not willing to understand the mechanism that creates that variation and shapes what makes humans different from other things. It’s a blindness. I think this is a phase that we’ll eventually get through. Other countries have come to peace with DNA. I don’t know how many decades or centuries it’s going to take us.
In your new book, Remarkable Creatures, you relate how Darwin arrived at his theory of evolution. Can you connect the dots?
As a college student Darwin collected beetles. He was looking for more opportunities to collect when there came this opportunity to be a naturalist on the British ship the HMS Beagle. It was seductive. He could go to faraway places—visit the tropics, places of incredible richness of life relative to cold, damp, gray England. It was difficult to persuade his father to allow him to go—he was just 22—but he got the chance. Two stops in this five-year journey were pivotal. The first came early in the voyage when he arrived on the coast of Argentina and unearthed fossils of many species, including some unknown to science—for instance, fossils of giant, extinct sloths that had been enormous compared with the living sloths he saw in the South American forests. So it planted the seed in his mind that life had changed.
Then Darwin got to the Galápagos Islands. He went from island to island collecting birds—mockingbirds and then finches—and realized that even when the birds appeared to be similar, on each island they were slightly different. After he left the Galápagos, on his way home to England, the lightbulb went on. He realized that if these birds lived on such similar islands but were slightly different from one another, there could be just one explanation: They had started out as a single species, but over time and with separation they had drifted apart and changed.
This insight was widely regarded as heresy, but why?
The prevailing idea was called special creation: that every species was created by a supernatural power and put in place on the earth for a specified role in a specified time by a completely mysterious process. It wasn’t open to natural science. Instead, Darwin said no, species are changeable, and the introduction of new species is a completely natural process that follows natural laws just the way physics does. A fundamental aspect of human existence has been to ask how we got here. Evolution is the big answer to that big question. Obviously there are alternative answers that have prevailed for a very long time, but evolution has replaced a supernatural explanation of human origins with a naturalistic one.
Beneath diverse exteriors, all animals share a set of body-building genes. If I had five minutes with Darwin, I would start right there. It would blow his mind.
Is that why Darwin waited more than 20 years to publish his theory of evolution?
Darwin was a pretty insecure 22-year-old when he boarded the ship. As these thoughts started to occur to him by the time he was about 27, he was just getting his feet under himself back in England. He realized what these thoughts meant, but he was just being accepted into scientific circles, just getting a lot of attaboys. Why risk that? This was not a time to challenge the establishment. You have to look at Darwin the human being to understand why he would not spill the beans.
What piqued your own interest in evolution?
As a kid, I was fascinated by zebras and giraffes and leopards. I kept snakes, and I loved their color patterns. As I got older I asked deeper questions—mainly, how are pattern and form generated? One of the most spectacular pageants on earth involves a complex creature developing from a single fertilized egg. Anyone who’s a parent is still amazed that it works. When I was a graduate student, we could watch this happen, but we didn’t understand the mechanics. What was going on inside that would put limbs in the right place, put eyes in the right place, carve the circulatory system and the backbone? It was an irresistible mystery, made even more irresistible with the realization that what makes a snake different from a lizard, what makes a zebra different from a giraffe, are changes within that developmental process. Understanding development was a passport to two fundamental questions: How does a complex creature form from an egg, and how have different types of creatures evolved?
