Introduction
The Good Dinosaur

The trailer for the Pixar movie The Good Dinosaur begins with an asteroid belt packed full of oversized boulders. One asteroid shoots through the rock pile, slamming into another, which ricochets into a third, sending it zooming off into space, straight toward a distant object. As the object gets larger, its identity becomes obvious: a blue planet with patches of green and wisps of white. “Millions of years ago, an asteroid six miles wide destroyed every dinosaur on Earth,” the narrator intones. We see the asteroid entering Earth’s atmosphere, turning orange, sizzling.

You know what comes next: the impact in the Gulf of Mexico, earthquakes around the world, forests in the Northern Hemisphere spontaneously bursting into flames, the sky blackened for months by soot. The dinosaurs, and many other creatures, wiped out. A sad day, indeed. This Pixar offering, apparently, is darker than most of their movies, a tragedy, ending with the demise of the great reptiles.

Or maybe it isn’t.

“But what if,” the trailer asks, and then shows the asteroid streaking through the Cretaceous sky. Grazing behemoths—sauropods, duck-billed dinosaurs—look up momentarily, then go back to filling their cavernous bellies with leafy food. The asteroid flies by, a near miss instead of a fatal impact. Life goes on. The dinosaurs’ salad days continue.

I know the answer to the question “What if?” The dinosaurs were at the peak of their reign sixty-six million years ago. They had dominated the world for more than one hundred million years. Sans asteroid, the dinos would have continued their global rule: T. rex, Triceratops , Velociraptor , Ankylosaurus —they all would have survived. New dinosaurs would have evolved, replacing the old ones. The ever-changing dinosaurian parade would have marched on. In all likelihood, the dinosaurs would still be walking the Earth today.

And who wouldn’t be here today? We wouldn’t, that’s who. Even though we mammals evolved about 225 million years ago, almost exactly at the same time as the dinosaurs, for the first 160 million years of our existence, we didn’t amount to much. The dinosaurs saw to that. Our furry forebears were an insignificant afterthought in the global biosphere, generally much smaller than the smallest dinosaur, active at night to avoid their reptilian overlords, scurrying in the underbrush, eating whatever scraps they could find. If you think of an opossum, you have a good idea of the looks and lifestyle of our Cretaceous kin, though most were probably even smaller.

It wasn’t until the asteroid wiped out the dinosaurs that Team Mammal got its evolutionary opportunity—and we certainly took advantage of it, quickly proliferating to fill the empty ecosphere, transforming the last sixty-six million years into the Age of Mammals. But we owe all of that to the asteroid.

We—scientists and laypersons alike—once thought that the rise of mammals was inevitable, that we mammals are inherently superior to those reptilian brutes, thanks to our big brains and our internal combustion engines generating body heat. It took some time, so the idea went, but we eventually supplanted the dinosaurs, perhaps by eating their eggs into extinction or otherwise showing them who’s who.

We now know this is nonsense. Mammals had bit parts in the Mesozoic evolutionary play. The dinos were doing just fine on that lovely day in 66 million BC, their dominance in no manner challenged by the vermin underfoot. Without the asteroid, life would have continued on its merry way, with reptilian intrigue and machinations, new species evolving, others going extinct, as they had for millions of years. There’s little reason to think that we mammals would have emerged from the shadows to become major players in the ecosystem. The dinosaurs were already there, filling the ecological niches, using the resources—it was only after they were gone that we had our evolutionary turn.

No asteroid, no mass extinction, no mammal evolutionary flowering, no you and me. So, these first few moments of the movie trailer had me excited. Pixar had made a movie all about dinosaurs and how the world would have turned out differently if the asteroid had sailed on by. Forty-five seconds into the preview, I knew the movie was going to be a winner.

The trailer continued with a T. rex chasing a herd of plant eaters, causing them to stampede, a pell-mell rush of enormous herbivores, long-necked brontosaurs ^[1] and three-horned Triceratops , a typical day in the Mesozoic. But then I did a double take—some of those beasts looked more like hairy, big-horned bison than ceratopsians. And the next scene shows a brontosaur bounding along with something on its head—a human child!

If the asteroid was a near miss, what are mammals doing there? This is a Pixar movie, after all, so one expects a few liberties to be taken (dinosaurs speaking English, for example), but is there any scientific evidence supporting the juxtaposition of Brontosaurus , bison, and baby? If the dinosaurs hadn’t been wiped out, might mammals have diversified anyway, producing bison and—more importantly—us? Dinosaurs had kept mammals in their place—that place being tiny and in the underbrush—for millions of years. Is it possible that somehow, after all that time, mammals could have cut loose evolutionarily and prospered, even while the rule of the big reptiles continued?

There is one possibility, at least according to British paleontologist Simon Conway Morris. Dinosaurs, being reptiles, liked it hot. Their low metabolic rates did not produce much internal heat. When it was warm outside, that wasn’t a problem—they could get their heat from the ambient environment, supplementing it when necessary by sitting in the Sun. The dinosaur dynasty was enabled by a long stretch of global warming, a time when much of the world was tropical, a good time to be a reptile.

But Conway Morris points out that the climate finally began to change about thirty-four million years ago. The world got cooler. Eventually, the ice ages came, glaciers expanded, much of the world became chilly. There’s a reason you don’t find reptiles in the far north and south today—it’s too cold for them. Conway Morris suggests that even with dinosaurs still extant, this global cooldown would have sprung the mammals, kick-starting their evolutionary radiation. Dinosaurs would have had to retreat to the tropical equator, leaving the higher and mid-latitudes free, giving mammals their evolutionary chance at last.

Let’s humor Conway Morris for the moment and assume his scenario is correct. Mammals start diversifying, occupying ecological niches long filled by dinosaurs, becoming bigger, more diverse. Maybe this Ice Age–enabled evolutionary diversification would have led to an Age of Mammals equally as magnificent and multifarious as the one the asteroid spawned.

But would it have been the same Age of Mammals? Would there be elephants and rhinos and tigers and aardvarks? Or would this alternative world have produced a very different ensemble of animals—species completely unrecognizable to us, dividing up the world’s resources and filling its ecological niches, but in different ways than the creatures around us today? Or, to place the question closer to home, would we have evolved? Would there be humans to produce babies to sit atop Pixar’s Brontosaurus ?

Conway Morris responds with an emphatic “yes.” To him and other scientists in his camp, evolution is deterministic, predictable, following the same course time after time. The reason, they argue, is that there are only so many ways to make a living in the world. To each problem posed by the environment, a single, optimal solution exists, leading natural selection to produce the same evolutionary outcomes over and over.

As evidence, they point to convergent evolution, the phenomenon that species independently evolve similar features. If there are limited ways to adapt to a given environmental circumstance, then we would expect species occupying similar environments to convergently evolve the same adaptations, and that’s exactly what happens. There’s a reason that dolphins and sharks look so much alike—they evolved the same body shape to move rapidly through the water in pursuit of prey. The eyes of octopuses and humans are nearly indistinguishable because the ancestors of both evolved very similar organs to detect and focus light. The list of evolutionary convergences goes on and on, as we shall soon see. Conway Morris and his colleagues see it as ubiquitous and inevitable, allowing us to predict how evolution would have unfolded, what a late-blooming mammal radiation might have looked like. Conway Morris concludes that “ the rise of active, agile, and arboreal ape-like mammals, and ultimately a hominid-like form, would have been postponed, not cancelled... without the end-Cretaceous asteroid impact... the appearance of the hominids would have been delayed by approximately thirty million years.” Pixar, in other words, was on solid ground in commingling babies and brontos.

But let’s take this argument one step further. Even if the mammals forever stayed in the shadows, could a species like us have evolved from some other ancestral lineage? If convergence is so ineluctable, the push to particular solutions so unrelenting, there’s no reason to think the rise of mammals was a necessary prerequisite. A big-brained, bipedal, highly social species with forward-facing eyes and forelimbs capable of manipulating objects could have evolved from some other ancestor. But if not from mammals, then descended from what?

Answering that question requires no more than switching from The Good Dinosaur to the bad dinosaur. Specifically, to Velociraptor , the villain of Jurassic Park (and, in an unexpected case of redemption twenty years later, the hero of Jurassic World ) . Talk about smarts! These wily reptiles worked as a team, outwitted the hardened safari hunter, and even figured out how to open doors with their three-fingered hands. And they were visually oriented and bipedal. Beginning to sound familiar?

With a few exceptions, Jurassic Park ’s portrayal of Velociraptor was reasonably accurate. ^[2] Of course, we don’t know how smart they were, but they did have large brains and some paleontologists have speculated that they may have been social, living in groups and coordinating their predatory attacks like lions or wolves. If you were looking for a jumping-off point for the evolution of a hominid-like animal, Velociraptor would seem like a good place to start.

And that’s just where Canadian paleontologist Dale Russell began in the early 1980s. He studied a close relative of Velociraptor , another small theropod dinosaur named Troodon that also lived at the end of the Cretaceous period. Troodon had the largest brain relative to its body weight of any dinosaur, a brain comparable in size to that of an armadillo or a guinea fowl. In other words, these reptiles were no geniuses, but they weren’t completely clueless, either. Russell noted that over the course of hundreds of millions of years, animals have steadily evolved bigger brains. The fact that the largest dinosaur brain occurred in a species that lived at the end of their tenure suggested that dinosaurs, too, were following this evolutionary trend of increasing brain size through time. What would have happened, Russell asked, if the asteroid hadn’t wiped them out? How would Troodon ’s descendants have evolved if natural selection pushed them toward ever larger brains?

Russell went through a chain of logic to speculate what a modern-day descendant of Troodon would have looked like: larger brains require larger braincases; bigger braincases usually are associated with a shortening of the facial region; heavier heads are more easily balanced by placement directly on top of the body; this in turn favors an upright posture, which means that a tail is no longer needed as a counterweight to the no longer forward-leaning front half of the body. A few more assumptions about the best leg and ankle structure for walking upright and, voilà, what was termed inelegantly the “dinosauroid,” a green, scaly creature with an uncanny resemblance to a human, right down to the butt cheeks and fingernails.

Remember, Russell did not set out to ask how a dinosaur could evolve into a humanoid. Rather, his goal was to think about how selection for increased brain size would lead to other anatomical changes. The end result of this project led to envisioning a creature strikingly similar to us, a reptilian humanoid.

Russell’s evolutionary projection, though conjured years in advance, is consistent with Conway Morris’ ideas that the evolution of hominid-like life-forms is inevitable. So consistent, in fact, that Conway Morris even appeared in a BBC documentary, sipping coffee at a café next to a dinosauroid reading a newspaper.

So Pixar had a couple of plot options. If the Cretaceous asteroid had, indeed, missed Earth, then according to Conway Morris and others, humans or something like us would have evolved one way or another. The only question was whether they would have been hairy, the result of delayed mammalian evolutionary diversification, or scaly, an outcome of natural selection on increased dinosaur brain size.

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IT’S FUN TO THINK COUNTERFACTUALLY, to wonder what might have transpired if history had unfolded differently. But questions about the inevitability of humanoid evolution transcend speculation about Earth’s history.

We now know there are a lot of planets in the universe that potentially could harbor life as we know it. These “habitable exoplanets” are neither too hot nor too cold and have liquid water on the surface. A recent study indicated that billions of such planets may exist in the Milky Way galaxy alone. The nearest may be only four light-years away.

Suppose life has evolved on some of these planets. What would it look like? Would the life-forms resemble those here? And what about intelligent life-forms, as smart as us, or even much smarter? How much, if at all, would they be like humans?

Quite a lot, if we believe what we see in the movies, and some well-renowned scientists agree. “ If we ever succeed in communicating with conceptualizing beings in outer space,” wrote the late biologist Robert Bieri, “they won’t be spheres, pyramids, cubes, or pancakes. In all probability, they will look an awful lot like us.” David Grinspoon, doyen of the emerging interdisciplinary field of astrobiology, goes a step further: “when they [aliens] do finally land on the White House lawn, whatever walks or slithers down the gangplank may look strangely familiar.” Not surprisingly, Conway Morris agrees, suggesting that “the constraints of evolution and the ubiquity of convergence make the emergence of something like ourselves a near-inevitability.” But before exploring the scientific basis for these scientists’ extraterrestrial predictions, let’s return to Planet Earth.

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MORE SPECIFICALLY, to southeast Africa. Darkness comes quickly in the Zambian woodlands. I’m a herpetologist—a lizard guy—so tracking nocturnal lions is not my day job, but I’ve come to Zambia for a little R & R prior to fieldwork in South Africa. Amazingly, lions can become accustomed to the presence of vehicles and will allow you to shadow them as they go on the prowl, and that’s just what we’re doing.

Off to the right there’s a movement, something not too large approaching, unaware that it’s on a collision course with a pride of lions. As it shuffles closer, its identity becomes clear—a crested porcupine, the sixty-pound rodent covered head to tail with pointy spines, some a foot and a half long. Its spines, of course, are for defense, for situations just like this, but they’re not always effective. Lions have a counterstrategy, slipping a paw underneath the porcupine’s body to flip it over, exposing the vulnerable belly. You can imagine the rest.

There’s a Seinfeld episode in which Jerry’s watching a nature documentary on antelopes, and the lions attack, and Jerry’s yelling, “Run, antelope, run! Use your speed. Get away!” And the next night, he’s watching another nature flick, this time focused on lions, and they go for an antelope, and he’s shouting, “Get the antelope; eat him; bite his head! Trap him; don’t let him use his speed!” But even though tonight we’ve been following the lions, I’m rooting for the porcupine. Leave him alone and go after something your own size!

But, of course, they don’t. One of the lionesses wanders over to the porcupine. He turns his backside to her, erects his spines, sort of like a cat arching its back and bristling its hair, and then he starts shaking the tail spines against each other, clackity-clack, clackity-clack.

And, amazingly, it works. After a moment, the lioness turns away and rejoins the pride, and the porcupine wanders off into the night.

At the end of the evening, I replayed the events in my head, mulling over my previous porcupine encounters. As well as Africa and Asia, porkies also occur throughout most of the New World. I’ve only seen the North American porcupine in the wild once, in a tree of all places—thirty feet up as I glided by on a ski lift. In the rainforests of Costa Rica, however, I’ve seen prehensile-tailed porcupines a number of times, again mostly in trees.

Certainly, there are differences among these species. The most obvious is size: the crested porcupine is twice the weight of its North American counterpart and thirty times that of the diminutive Rothschild’s porcupine from Panama. The quills correspondingly vary in length—fourteen inches in the crested, four inches in the North American, shorter yet in the Rothschild’s. Some species have red noses, others brown; prehensile-tailed porcupines have no quills on their tails. Yet, the differences pale in comparison to the similarities: not only possession of quills, but also a similar stocky body with short legs, small eyes, spiky hairdo. Given these similarities, I never questioned my assumption that porcupines were one happy evolutionary family, all descended from the same ancestral spiny ur-porcupine.

Imagine my surprise, then, when I learned that I had it all wrong. Despite their shared prickliness, New and Old World porcupines do not share a common evolutionary heritage. Rather than owing their pointy good looks to descent from a common, bristly ancestor, the two lineages have independently evolved their quills from different, unquilled rodent species. They are the result of convergent evolution.

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I’M NOT THE FIRST PERSON in history to be fooled by convergence. In fact, I’m in pretty good company. Charles Darwin himself was bamboozled on his famous visit to the Galápagos Islands. There he discovered the small birds that now bear his name, the Darwin’s finches. But Darwin did not realize that these bird species were all closely related to each other, descendants of a single ancestral finch that colonized the islands sometime in the past. Rather, he thought the species represented four groups with which he was familiar from home: true finches, grosbeaks, blackbirds, and wrens.

It was only when Darwin returned to London and turned his specimens over to the noted ornithologist John Gould that he learned his mistake. The species were not representatives of a diverse set of familiar types after all, but instead members of a single group of birds unique to the Galápagos—Darwin had been hoodwinked by convergent evolution. This revelation fit in with other findings from Darwin’s voyage, all pointing in one direction, toward the “transmutability” of species. By the time that he revised his best-selling Voyage of the Beagle in 1845, the finch story intimated what was to come a decade later: “Seeing this gradation and diversity of structure in one small, intimately related group of birds, one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends.”

The broader implication of the story—that the finches had diversified on the Galápagos to mirror species using a variety of habitats elsewhere—was also not lost on Darwin. Although he didn’t allude to convergent evolution in the Voyage , he clearly articulated the idea fourteen years later in the Origin : “in nearly the same way as two men have sometimes hit on the very same invention, so natural selection... has sometimes modified in very nearly the same manner two parts in two organic beings, which owe but little of their structure in common to inheritance from the same ancestor.”

Darwin was not the only early naturalist so fooled by convergence. When Captain Cook landed in Botany Bay in 1770 on his first South Pacific voyage, the naturalist on the expedition, Joseph Banks, sent specimens and drawings of Australian birds back to England. This began a flood of material dispatched to the motherland by colonists and explorers over the next half century, revealing the existence of many new species.

The key figure in making sense of this profusion of new species was John Gould. At around the same time that he was consulting with Darwin on the finches, Gould decided to take up a comprehensive description of Australian birds. Quickly realizing that he needed to go to Australia to do the job right, he picked up and relocated Down Under, spending three years there and eventually producing a mammoth seven-volume series of paintings and descriptions.

But Gould, so right about Darwin’s finches, turned out to be equally wrong about the evolutionary affinities of Australia’s avifauna. Many Aussie birds are very similar in appearance and habit to species in Europe, such as wrens, warblers, babblers, flycatchers, robins, nuthatches, and others. As a result, Gould assigned the newly discovered Australian birds to the familiar Northern Hemisphere families.

Gould’s error is understandable. Over the course of the next century and a half, many very knowledgeable ornithologists were equally deceived and treated these birds as colonial outposts, the result of a wave of invasions of Australia by many types of birds.

However, genetic studies starting in the 1980s showed that, in fact, most of the species are part of a large Australian bird radiation that evolved in situ. In other words, these Australian birds are closely related to each other; they are not members of many different Northern Hemisphere families, but convergent with them.

The discovery of unexpected cases of convergent evolution continues to this day. Indeed, with the flood of genetic data now available for so many different species, our understanding of evolutionary relationships is advancing by leaps and bounds, producing a much firmer grasp on the evolutionary tree of life. One consequence is that we are increasingly finding new cases in which we had been misled by anatomical similarity, only now realizing that it results not from descent from a similar common ancestor, but from independent derivation.

How can we explain this rampant convergent evolution? There’s a commonsense explanation, the one Darwin proposed. If species live in similar environments and face similar challenges to their survival and reproduction, then natural selection will lead to the evolution of similar traits: the existence of large seeds is a resource for birds, requiring big beaks to crack them open, and so similar, big-beaked birds evolve in numerous seedy locations; threatened by big cats, oversized rodents repeatedly evolve a spiny defense, as effective against lions in Africa as it is against pumas in the Americas.

In the last two decades, some biologists have extended this view to the cosmos. Here on Earth, species face the same challenges around the world and through time, and they evolve the same solutions. These scientists argue that the same physical challenges that occur here will also be faced by life-forms on similar planets and will lead to the same biological solutions. George McGhee, a paleontologist from Rutgers University, argues that there’s only one way to build a fast-swimming aquatic organism, and that’s why dolphins, sharks, tunas, and ichthyosaurs (extinct marine reptiles from the Age of Dinosaurs) all look alike.

Taking this a step further, he argues that “if any large, fast-swimming organisms exist in the oceans of Jupiter’s moon Europa, swimming under the perpetual ice that covers their world, I predict with confidence that they will have streamlined, fusiform bodies... very similar to a porpoise, an ichthyosaur, a swordfish, or a shark.” Conway Morris agrees, saying, “Certainly it’s not the case that every Earth-like planet will have life let alone humanoids. But if you want a sophisticated plant it will look awfully like a flower. If you want a fly there’s only a few ways you can do that. If you want to swim, like a shark, there’s only a few ways you can do that. If you want to invent warm-bloodedness, like birds and mammals, there’s only a few ways to do that.”

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NOT EVERYONE AGREES with this viewpoint. Let’s go back to the movies to see why.

In the climactic scene of the classic 1946 film It’s a Wonderful Life , George Bailey (played by Jimmy Stewart) despairs that his life has been a failure and wishes that he’d never been born. Clarence Odbody, George’s guardian angel, then shows him how life in Bedford Falls would have been radically different—and much for the worse—if George had never existed: his brother dead; his friends and family unhappy, homeless, and institutionalized; a boat full of soldiers sunk; the town a den of iniquity. George realizes that his life has been worthwhile and abandons his suicidal plans, then subsequently is redeemed when the townspeople come to his rescue in appreciation of all his good deeds.

The American Film Institute in 2006 named It’s a Wonderful Life the most inspirational movie of all time. Stephen Jay Gould, the famed paleontologist and evolutionary biologist, was among those inspired by it, but in a way different than most. To him, the movie was a parable for the evolutionary history of life, so much so that the title of his 1989 book, Wonderful Life , paid homage to the movie. In the book, Gould argued for the dominating importance of historical contingency in evolution. By contingency, he meant that the particular sequence of events critically determines the course of history: A leads to B, B to C, C to D, and so on. In a historically contingent world, if you alter A, you don’t get D. If George Bailey is never born, events in New Bedford unfold differently.

Gould argued that life is full of George Bailey events—some major, most minor—but any of which could send life in a different direction. Lightning strikes, falling trees, asteroid impacts, even the flip-of-a-coin determination of which genetic variant a mother passes on to her daughter—any of these could make a difference that would ramify through the eons. Like New Bedford without George Bailey, Gould wrote, “any replay [of the history of life] altered by an apparently insignificant jot or tittle at the outset, would have yielded an... outcome of entirely different form.”

This view has important implications for understanding the diversity of life we see around us. If evolution is dominated by contingency, then there can be no predictability, no Conway Morrisian determinism. The end result is so influenced by contingencies that there is no way one could predict at the beginning what would happen at the end. Start over again, and a completely different result might unfold. Hitting home where it matters the most, Gould concluded, “Replay the tape [of life] a million times... and I doubt that anything like Homo sapiens would ever evolve again.”

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GOULD’S ARGUMENT, elegantly and persuasively made, resonates with us all. Who hasn’t rued that “if I hadn’t done X, then Y wouldn’t have happened,” where X could be anything minor (mispronouncing a name) or major (having a drink too many) and Y is something you wish hadn’t taken place?

Still, sensible as the argument may be, what is the evidence? There’s only one history of life. How can we test the repeatability of evolution? Gould proposed a thought experiment to address such questions. Replay the tape of life, he suggested, go back to the same starting conditions and see if the same result ensues. Such “gedankenexperiments,” as the Germans call them, have a long pedigree in science and philosophy, and this one has been taken up by many and proven particularly fruitful.

Conway Morris and colleagues, of course, disagree with Gould’s basic premise—changing an earlier event need not substantially alter the downstream outcome. They argue that the ubiquity of convergent evolution demonstrates the impotence of contingency, that in many cases more or less the same outcome would ensue regardless of the specific historical sequence of events.

The issue of convergence and evolutionary determinism had not yet been raised when Gould wrote Wonderful Life . However, in an exchange with Conway Morris published nine years later, Gould’s response was simple: the importance of convergence is “overestimated,” he said, and pointed to Australia as state’s evidence number one.

Let’s again consider Captain Cook’s expedition to the Antipodes. Among the first animals they encountered was a kangaroo. Kangaroos are the major native plant eaters in Australia today. Functionally, they fill the same role as deer, bison, and myriad other herbivores in the rest of the world. And yet, as Gould (Stephen Jay, not John) noted, kangaroos haven’t converged upon these other types of herbivores—even a toddler can tell that a kangaroo and a deer are different sorts of animals.

And then there’s the koala, that lovable, bearish tree hugger that lives life in the slow lane, sleeping twenty hours a day as it detoxifies the eucalyptus leaves that comprise its diet (and that make its fur reek of menthol). Nothing like it exists anywhere else in the world, now or, according to the fossil record, ever.

But when we’re talking evolutionary one-offs, there’s only one king. Venomous ankle spurs, luxurious pelt, the ability to detect the electrical discharges of their prey’s muscles with electroreceptors on their snout. Powerful flat tail, webbed feet, lays eggs. Bill like a duck. The world’s greatest animal, the duck-billed platypus, a mishmash of parts borrowed from throughout the animal kingdom. An animal so confused that when the first specimens arrived in England at the end of the eighteenth century, shipped from Sydney across the Indian Ocean, scientists searched for hours in vain to locate the stitches by which crafty Chinese merchants must have assembled their hoax.

These examples have come from Down Under, but evolutionary one-offs occur everywhere. Giraffes, elephants, penguins, chameleons—these are all species exquisitely adapted to their specific ecological niches, with no evolutionary facsimile now or in the past (note that an “evolutionary one-off” is not necessarily a single species. For example, there are three living species of elephant, and many more that occurred in the past, like mastodons and mammoths. However, all elephant species are descended from a single ancestral elephant. That is why elephants can be considered evolutionarily unique—the proboscidean way of life only evolved a single time).

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CONVERGENT EVOLUTION is a scientific phenomenon, and you’d think that science should have been able to settle the question of its ubiquity by now. But the problem is that figuring out what happened in the past is not easy. We are taught in grade school about the scientific method, how observations lead to the formulation of a hypothesis that is then tested with a decisive experiment in the laboratory. That formulation in a very simplistic way captures the operation of mechanistically oriented sciences—that is, the sciences involved in understanding how something like a cell or an atom works. Think that a particular gene is important in producing a particular trait? Use molecular biology wizardry to disable the gene and see if the trait still develops.

But evolutionary biology is a historical science. Like astronomers and geologists, we evolutionary biologists try to figure out what happened in the past. And like historians, we are bedeviled by the asymmetry of time’s arrow—we can’t go back in time to see what happened. Moreover, evolution occurs notoriously slowly, seemingly making it impossible to watch it as it occurs.

Stephen Jay Gould laid out the experiment we’d like to do: replay evolution time and time again, and see how sensitive the outcome is to various experimental perturbations. But we call such ideas thought experiments for a reason—in the real world, they can’t be conducted. Or so we used to think.

It turns out that Darwin and a century of biologists following him were wrong in one key respect: evolution does not always plod along at a snail’s pace. When natural selection is strong—as occurs when conditions change—evolution can rip along at light speed (I’ll tell the story of how we came to realize that evolution is as much a hare as a tortoise in Chapter Four).

The reality of rapid evolution allows us to go beyond simply observing whether and how species respond. In a development that would have astonished Darwin, researchers are creating their own evolution experiments, altering conditions in a controlled and statistically designed way. Just like lab biologists, we can test evolutionary mechanisms, but out in nature, in real populations. Researchers are placing light-and dark-colored mice in half-acre-sized cages in the Nebraska sand dunes, moving guppies in Trinidad from stream pools with predators to those without, and switching walking stick insects from one habitat to another.

I’ve conducted some of these experiments myself, testing hypotheses about why small lizards in the Bahamas evolve longer or shorter legs. I know what you’re thinking, but my colleagues and I are willing to sacrifice for science. It’s a dirty job, hanging out on beautiful, windswept islands surrounded by ocean, but someone’s got to do it, and we’re the ones. I’ll go into much greater detail in Chapter Six, but for now suffice to say that if you go back to the Bahamas year after year and measure the legs of thousands of lizards with a portable x-ray machine, you’ll see that lizard populations can evolve rapidly. Moreover, if you experimentally alter the conditions the lizards experience, causing them to change their habitat use, populations on those islands will evolve quickly and in predictable directions.

Although evolution experiments in nature are still in their infancy, laboratory scientists have been conducting such work for decades. These studies trade the realism of nature for the hyper-precision of the lab, providing exquisite control over conditions experienced by the evolving populations. Moreover, the shorter life span of lab organisms, particularly microbes, means that these studies can be longer-term, encompassing more generations and creating more opportunity for evolution to occur. One laboratory experiment has been following microbial evolution for more than a quarter century, studying the extent to which twelve populations evolve in the same way.

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I OFTEN COMPARE EVOLUTIONARY BIOLOGY to a detective story, a whodunit. A crime has been committed—or in this case, something has evolved—and we want to know what happened. If we had a time machine, we could go back and watch for ourselves. If we could replay the tape, we’d just set things up like they were back then and start it again.

But neither of these is possible (with one important exception I’ll get to in Chapter Nine). Instead, we’re left with a bunch of clues, and, like Sherlock Holmes, we have to figure it out as best we can. We can see the patterns of evolutionary history, the species that occur today and the fossils of what existed in the past, allowing us to assess the extent to which evolution has repeatedly produced the same outcome. And we can study the evolutionary process as it operates today. By conducting experiments, we can see how repeatable and predictable evolution is: If you start at the same point, will you always end up with the same outcome? And if you start at different points, but select in the same way, will you converge on the same result? So even though we can’t replay the tape, we can study evolutionary pattern and process. By putting the two together, scientists are now well on the way to understanding evolutionary repeatability.

This book, then, is about the extent to which life repeats itself, the result of species evolving similar adaptations in response to similar environmental circumstances. More loftily stated, it is about determinism, whether natural selection inevitably produces the same evolutionary outcomes or whether the particular events a lineage experiences—the contingencies of history—affect the end result.

At the same time, this is a book about how scientists study these topics, how tools from DNA sequencing to fieldwork in remote corners of the world are synthesized to understand the evolutionary origin of life around us. And it’s also about how science itself evolves, how new ideas are born and how research programs develop to test them. In particular, I’ll focus on the rise of experimental methods to studying evolution, an approach that was inconceivable for more than a century after Darwin’s time.

The book will be full of scientists and their research, in pristine lab and woolly nature, but the topic is of more than academic interest. Evolution is occurring all around us today and has consequences beyond informing our understanding of arcane debates. Most notable are the direct evolutionary battles between humans and our commensals. On the one hand, nature is fighting back against our efforts to control it. We consider some species to be pests because they have the audacity to use the resources we wish to reserve for ourselves. Weeds invading our fields, rats eating our grain, insects devastating our crops. We deploy an armory of chemical—and, increasingly, genetic—weapons to control them, but they quickly evolve ways around them.

Seven billion and counting, sometimes we are the resource being exploited. Malaria, HIV, hantavirus, influenza—to microorganisms, our bodies are like any other crop and they are evolving to take advantage of us. We, in turn, combat them as we do crop pests, with chemicals, and they rapidly evolve resistance.

This is where the debate between contingency and determinism becomes personal. If we can predict not only when rapid evolution will occur, but what form it will take, we will be able to derive general principles and thus be better positioned to respond effectively. But if each case of rapid evolution is contingent on the specific circumstances, then we’ll have to start from scratch each time we face a new weed, pest, or disease, figuring out how our evolutionary foe is adapting and what we can do about it.

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DEBATE ABOUT CONTINGENCY versus determinism affects us in another, more ethereal way. Humans are no less subject to convergent evolution than other species. Our ability to drink milk as adults, for example, is unique among animals; it was, of course, irrelevant until we domesticated livestock in the last few thousand years, and since then has evolved convergently in several pastoral societies around the world. Skin color, so important in the course of human history, is also the result of convergent evolution, as is the ability to survive at high elevations, and many other traits.

The human species itself, of course, is not convergent. We are one of the singletons, lacking an evolutionary duplicate. Does our understanding of evolutionary determinism have anything to say about how we evolved, or why? If we hadn’t come along, would some other lineage have taken our place, and would that species have ended up much like us, perhaps so much so that someone—something—else would have been writing this very book, albeit with scaly, three-fingered hands? And if not here, perhaps on the moons of Jupiter or xh3-9?

But once again, I get ahead of myself. Let’s return once more to Earth and see just how pervasive convergent evolution is on our own planet.

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[1] Dinosaur purists may note that the name Brontosaurus was long ago discarded, replaced for quirky scientific reasons with Apatosaurus . To those killjoy know-it-alls I respond, “Ha-ha! Thanks to new scientific discoveries, the name Brontosaurus was resurrected in 2015.”

[2] Although, in reality, the creature was based on the closely related dinosaur Deinonychus . One major difference between the movie and reality was that Velociraptor probably stood less than three feet tall. However, in an example of life imitating fiction, shortly after JP premiered, paleontologists described a larger cousin of Velociraptor , dubbed Utahraptor , which was about the size of the raptor in the movie. MJ6BHBdVILyRoh1Bw7Lp3fh7kF/wHqwymVYcbuEbZhLibPOEAnZEnwkMVcf4vBk2