At about six o'clock in the morning, long before light broke on a cold November day in 2014, I pushed through the Beijing station and fought my way onto a crowded train. I was headed for Jinzhou, a Chicago-sized city in the northeastern fringes of China. I tried to steal back some sleep as we crawled past concrete factories and hazy cornfields, but I was too excited to nod off. Something rumored to be incredible was waiting for me at my destination—a mysterious fossil that a farmer had stumbled on while harvesting his crops.

Four hours later I stepped onto the platform in Jinzhou, trailing behind my colleague Junchang Lü, a famous dinosaur hunter at the Chinese Academy of Geological Sciences in Beijing who had asked for my help in studying the fossil. A small band of local dignitaries greeted us and whisked us away to the city's museum, a rickety building on the outskirts of town. With the seriousness of a high-level political summit, our party proceeded down a long hallway and into a side room where a slab of rock perched on a small table. It was then that I found myself face-to-face with one of the most beautiful fossils I had ever seen: a skeleton about the size of a donkey, its chocolate-brown bones contrasting with the surrounding gray limestone.

Clearly a dinosaur, the creature had steak knife teeth, pointy claws and a long tail that left no doubt that it was a close cousin of Jurassic Park's villainous Velociraptor. Yet the Chinese specimen differed from such ordinary dinosaurs in important ways. Its bones were light and hollow, its legs long and skinny like a heron's, and its body covered with assorted types of feathers, including big quill pens on the arms, stacked over one another to form wings. This dinosaur bore a striking resemblance to a bird.

About a year later Lü and I described this skeleton as a new species, which we called Zhenyuanlong. It is the latest of many feathered dinosaurs found in China's Liaoning Province over the past two decades—a remarkable series of fossils that illustrate, like a flip book, how the monstrous dinosaurs of yore transformed into the birds of today.

The implications of these fossils are momentous. Ever since Charles Darwin, scientists have wondered how evolution produces radically new groups of animals. Does it happen rapidly, the accident of some freak mutation that can turn a land-bound creature into a master of the skies? Or are these new groups forged more slowly, as organisms adapt to changing environments over millions of years? Zhenyuanlong and the other fossils from Liaoning and elsewhere are starting to provide an answer.

Transitional Fossils

Birds have a host of features that set them apart from all other modern animals. In addition to traits that enable them to fly, they possess high metabolisms that allow them to grow incredibly quickly and large brains that endow them with high intelligence and keen senses. Birds are so distinctive, in fact, that researchers have long puzzled over their origins.

In the 1860s English biologist Thomas Henry Huxley—one of Darwin's closest friends and most vociferous supporters—began to figure out the mystery of where birds came from. Just a few years after Darwin published On the Origin of Species in 1859, quarry workers in Bavaria split open a limestone slab with the 150-million-year-old skeleton of a Frankenstein creature inside. It had sharp claws and a long tail like a reptile but feathers and wings like a bird. Huxley realized that the beast, dubbed “Archaeopteryx,” bore an uncanny resemblance to small flesh-eating dinosaurs such as Compsognathus that were also starting to come to light at around the same time. So he proposed a radical idea: birds descended from dinosaurs. Others disagreed, and the debate went back and forth for the next 100 years.

The question was ultimately settled, as these things usually are, by the discovery of new fossils. In the mid-1960s Yale University paleontologist John Ostrom unearthed the astonishingly birdlike dinosaur Deinonychus in western North America. It had long arms that looked almost like wings and a lithe build indicative of an active, energetic animal. Maybe, Ostrom surmised, Deinonychus even had feathers. After all, if birds derived from dinosaurs—which by now many paleontologists were beginning to accept—feathers must have developed somewhere along that evolutionary lineage. But Ostrom could not be sure, because all he had were the creature's bones. Sadly, soft bits like feathers rarely survive the ravages of death, decay and burial to become fossilized.

Ostrom waited. He kept looking for the holy grail that would prove beyond any doubt the connection between birds and dinosaurs: dinosaur skeletons preserved in the type of exquisite detail needed to document feathers. Then, in 1996, as his career was drawing to a close, Ostrom was at the annual meeting of the Society of Vertebrate Paleontology in New York City when Philip Currie, now at the University of Alberta, approached him. Currie, who had also been studying birdlike dinosaurs, had recently returned from a trip to China, where he caught wind of an extraordinary fossil. He pulled out a photograph and showed it to Ostrom. There it was, a small dinosaur surrounded by a halo of feathery fluff, immaculately preserved because volcanic ash had quickly buried it, Pompeii-style. Ostrom began to cry. Somebody had finally found his feathered dinosaur.

Feathered dinosaur Zhenyuanlong from Jinzhou, China, is one of many recently discovered fossils that document how birds arose from their terrestrial ancestors to conquer the skies. Source: From “A Large, Short-Armed, Winged Dromaesaurid (Dinosauria: Theropoda) from The Early Cretaceous of China and Its Implications for Feather Evolution,” by Junchang Lü and Stephen L. Brusatte, in Scientific Reports, Vol. 5, Article No. 11775; July 16, 2015

The fossil that Currie showed Ostrom, later named Sinosauropteryx, opened the floodgates of discovery. Scientists sprinted to the Liaoning region of China where it was found, like prospectors in a gold rush, although it was really the local farmers who knew where to look. Today, two decades after the discovery of Sinosauropteryx, fossil hunters have recovered more than 20 species of feathered dinosaurs from Liaoning. They run the gamut from nine-meter-long primitive cousins of Tyrannosaurus rex coated in hairlike fuzz, to dog-sized herbivores with simple, porcupine-style quills, to crow-sized gliders with full-on wings. They are among the most celebrated fossils in the world.

The feathered dinosaurs of Liaoning clinched it: birds really did evolve from dinosaurs. But that statement is perhaps a little misleading because it suggests that the two groups are totally different things. In truth, birds are dinosaurs—they are one of the many subgroups that can trace their heritage back to the common ancestor of dinosaurs and therefore every bit as dinosaurian as Triceratops or Brontosaurus. You can think of it this way: birds are dinosaurs in the same way that bats are an aberrant type of mammal that can fly.

The Liaoning fossils have also helped untangle the genealogy of birds, revealing where they perch on the dinosaur family tree. Birds are a type of theropod—the same group to which ferocious meat eaters typified by behemoths such as T. rex, Allosaurus and Spinosaurus belong. But the very closest relatives of birds are a subset of much smaller, nimbler, brainier theropods: the raptors, which include Velociraptor, Ostrom's Deinonychus and the oh-so-birdlike Zhenyuanlong that Lü and I described in Jinzhou. Somewhere within this flock of feathery species lies the line between nonbird and bird.

There are now so many feathered dinosaurs from Liaoning and elsewhere that, taken together, they provide the best glimpse at a major evolutionary transition in the fossil record. I and other scientists are applying a wealth of cutting-edge techniques to these fossils—computed tomographic scans to visualize anatomy, computational analyses for building family trees, computer models of how these animals moved, and advanced statistical techniques to track how evolution produces new species and body plans. Recent insights from these investigations are allowing us to piece together the story of how a dinosaur turned into a bird—keystone evidence for solving that age-old conundrum of how major new groups come to be.

Accidental Liftoff

The origin of feathers is central to the enigma of bird evolution. Feathers are to birds what slicked-back hair and sideburns were to Elvis. A calling card. One glance at the outstretched wings of an eagle or the gaudy tail of a peacock, and you know exactly what you are looking at. It must be a bird because unlike mammals, or reptiles, or any other groups of living animals, only birds have feathers. And what a thing to have. Feathers are nature's Swiss Army knives, multipurpose tools that can enable flight, impress mates or rivals, and retain warmth and brood eggs while an animal sits on a nest. Indeed, they have so many uses it has been hard to figure out which purpose they first evolved to serve.

Sinosauropteryx and the other Liaoning fossils make one thing certain: feathers did not suddenly spring forth with the first birds but originally debuted far earlier, in their distant dinosaurian ancestors. The common ancestor of all dinosaurs may have even been a feathered species. These earliest feathers looked very different from the quill pens of modern birds, however. The plumage of Sinosauropteryx, along with many other dinosaurs, looked more like fluff, made up of thousands of hairlike filaments. No way could these dinosaurs fly—their feathers were too simple to catch the wind, and they did not even have wings. The first feathers must have therefore evolved for something else, probably to keep these small dinosaurs warm.

For most dinosaurs, a coat of bristly feathers was enough. But one subgroup—the maniraptoran theropods—went for a makeover. The hairlike strands grew longer and then started to branch, first into a few simple tufts and then later into a much more orderly system of barbs projecting sideways from a central shaft. Thus, the quill pen was born. Lined up and layered across one another on the arms, these more complex feathers then joined into wings. Some of the Liaoning dinosaurs, such as the raven-sized Microraptor described by Xu Xing of Beijing's Institute of Vertebrate Paleontology and Paleoanthropology, also had wings on the legs and tail, an arrangement unknown in any modern bird.

Why did these dinosaurs convert their fuzz into wings? The intuitive answer is flight: the maniraptorans were turning their bodies into airplanes, and the wings evolved to become the airfoils that generate lift. But a closer look at the fossil evidence suggests otherwise. Although some of the small winged critters such as Microraptor could probably glide, as has been demonstrated by wind-tunnel experiments and computer simulations led by Gareth Dyke of the University of Debrecen in Hungary, others such as Zhenyuanlong from Jinzhou had hefty, short-armed bodies that were confined to the ground. Moreover, none of these winged dinosaurs had the huge chest muscles necessary to power flight, and few had the asymmetrical quill pens (with a shorter and stiffer leading vane compared with the trailing vane) that are optimized to withstand the severe forces of surging through an airstream.

The latest findings suggest that wings instead evolved to serve another, less widely recognized function: display. One line of evidence comes from work pioneered by Jakob Vinther of the University of Bristol in England, who uses high-powered microscopes to identify the pigment-bearing structures, called melanosomes, in fossil dinosaur feathers. It turns out that the feathers of nonflying, winged dinosaurs were a rainbow of colors. Some were even iridescent, like the plumage of today's crows. These shiny-sheened accoutrements would have been perfect for attracting mates or intimidating rivals.

The apparent splendor of these dinosaur feathers has spawned a radical new hypothesis for the origin of wings: they first evolved as advertisements—billboards projecting from the arms and legs and tail. Then these suave-winged dinosaurs suddenly found themselves with big, broad surfaces that also, by the laws of physics, had an aerodynamic function. In other words, flight evolved by accident. And it may have evolved many times in parallel, as different maniraptorans found themselves generating lift from their wings as they leaped from the ground, scurried up trees or jumped between branches. Ultimately members of one of these maniraptoran lineages got small, developed big chest muscles and hyperelongated arms, and lost their long tails, becoming the birds of today.

Piecemeal Evolution

The evolution of feathers and wings is emblematic of a much bigger pattern. The Liaoning dinosaurs demonstrate that many other supposedly singular features of birds first evolved millions of years before birds themselves and for reasons totally unrelated to flight.

Long, straight legs and feet with three skinny main toes—hallmarks of the modern bird silhouette—first appeared more than 230 million years ago in the most primitive dinosaurs. Their emergence seems to be part of an overall reshaping of dinosaur bodies into upright-walking, fast-running machines that could outpace and outhunt their rivals. These hind-limb features are some of the defining characteristics of all dinosaurs, the very things that helped them rule the world for so long. Some of these dinosaurs—the earliest members of the theropod dynasty—then fused their left and right collarbones into a new structure, the wishbone. It was a seemingly minor change, which stabilized the shoulder girdle and allowed these stealthy, dog-sized predators to better absorb the shock forces of grabbing prey. Birds later co-opted the wishbone to serve as a spring that stores energy when they flap their wings.

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Credit: Portia Sloan Rollings (animals) and Jen Christiansen (cladogram)

The distinctive hollow bones and rapid growth of birds, both of which are important for flight, also have deep dinosaurian roots. Many dinosaurs had bones hollowed out by air sacs, a telltale sign that they had ultraefficient “flow-through” lungs that take in oxygen during not only inhalation but also exhalation. In birds, this type of lung delivers the juice needed to maintain their high-energy way of life, in addition to lightening the skeleton for flight. The microscopic structure of dinosaur bones, meanwhile, indicates that these animals had growth rates and physiologies intermediate between slow-maturing, cold-blooded reptiles and the fast-growing, warm-blooded birds of today. Thus, researchers now know that a flow-through lung and fast growth emerged more than 100 million years before birds took wing, when the first fast-running, long-legged dinosaurs were carving out a new livelihood as energetic dynamos—so different from the sluggish amphibians, lizards and crocodiles they were battling against.

The pint-sized proportions of birds—infinitely daintier than T. rex and company—also stem from a time before birds themselves. Mike Lee of Flinders University in Australia and Roger Benson of the University of Oxford have independently determined that small body size evolved through a gradual trend of reduction that began with maniraptorans and lasted more than 50 million years. Exactly what drove this trend is unclear, but one possibility is that the ever shrinking physiques of these feathery dinosaurs gave them entry to new ecological niches—trees, brush, perhaps even underground caves or burrows that were inaccessible to giants such as Brachiosaurus and Stegosaurus.

Neurological and behavioral attributes of living birds can be traced back to the dinosaurs, too. Much of the key evidence for the deep history of these traits comes from the Gobi Desert in Mongolia, where for the past quarter of a century a joint team from the American Museum of Natural History (AMNH) in New York City and the Mongolian Academy of Sciences has been collecting fossils. Under the leadership of Mark Norell and Mike Novacek of the AMNH, the annual summer expeditions have compiled a bounty of specimens from the Late Cretaceous period, between 84 million and 66 million years ago, that provide unprecedentedly detailed insights into the lives of dinosaurs and early birds. Among their finds is a trove of well-preserved skulls belonging to Velociraptor and other feathered maniraptorans. CT scanning of these specimens, conducted by Amy Balanoff of Stony Brook University, has revealed that these species had a big brain and that the forward-most part of the organ was expanded. A large forebrain is what makes birds so intelligent and acts as their in-flight computer, allowing them to control the complicated business of flying and to navigate the complex 3-D world of the air. Scientists do not yet know why these dinosaurs evolved such keen intelligence, but the fossils clearly show that the ancestors of birds got smart before they took to the skies.

The bird body plan was therefore not so much a fixed blueprint but more of a Lego set that was assembled brick by brick over evolutionary time. The transition between dinosaur and bird did not happen in one fell swoop but through tens of millions of years of gradual evolution.

A Seamless Transition

The transition from dinosaur to bird was so gradual, in fact, that there is no clear distinction between “nonbirds” and “birds” on the family tree, as I demonstrated in 2014 using statistics. My study stemmed from my Ph.D. project, under Norell's tutelage. In addition to his 25-year quest in the Gobi, Norell has been working with successive waves of graduate students over the past two decades to build ever larger family trees of dinosaurs. He and I, along with our colleagues Graeme Lloyd of the University of Leeds in England and Steve Wang of Swarthmore College, compiled a data set of more than 850 skeletal features of some 150 theropods spanning the dinosaur-to-bird transition. We then used multivariate statistics to plot each species in a so-called morphospace—basically a map that clusters species together based on the percentage of features they share. Two species that are very similar anatomically plot close together, like Chicago and Indianapolis on a road map, whereas two species with vastly different skeletons sit far apart, like Chicago and Phoenix. If birds evolved from dinosaurs via a series of rapid, dramatic mutations that quickly produced a totally different type of animal, then the two groups should plot onto distinctly different parts of the map. Instead the morphospace we produced was a mess: birds were interspersed among a bigger cloud of dinosaurs. There was no clear separation between them, indicating that the transition was so slow as to be imperceptible.

Birds, therefore, are just another type of dinosaur. If I had been standing around in Jinzhou some 125 million years ago, when Zhenyuanlong was alive and flapping its wings in vain as it tried to outrun the ash cloud that would eventually suffocate it, I probably would have simply regarded it as some kind of large bird. I would have considered dinosaurs and birds to be the same general thing. That it is technically categorized as a dinosaur and not a bird has to do with scientific convention and tradition: paleontologists have long defined birds as anything that stems from the most recent common ancestor of Huxley's Archaeopteryx and modern birds—basically small animals with full-on wings that could fly. Because dromaeosaurids such as Zhenyuanlong are a few branches outside of that part of the family tree, they are not considered to be birds by definition.

Yet we should not sell birds short. They may be dinosaurs, not a class apart on their own, but they are special. They carved out a completely new way of life, and today they thrive as upward of 10,000 species that exhibit a spectacular diversity of forms, from hummingbirds to ostriches. What is more, birds were able to hold on while all the other dinosaurs died out 66 million years ago.

It is remarkable to think of all the random twists of fate that worked over tens of millions of years to produce this indomitable group of animals. Their ancestors did not know they were becoming more birdlike. Nor could any of us, if we were around as witnesses, have predicted that many of the features that developed to help these dinosaurs keep warm or attract mates would eventually be repurposed as integral components of a flight system.

Evolution has no foresight; it acts only on what is available in the moment, shaped by the never-ending but always changing pressures of environment and competition. There was no moment when a dinosaur became a bird, no big bang when a T. rex turned into a chicken. It was a journey. And the more scientists learn about other major evolutionary transitions—fish evolving into tetrapods with limbs and digits, land mammals turning into whales, tree-swinging primates becoming upright-walking humans—the more we see a consistent theme in how this kind of transformation works: it is a marathon, not a sprint, and there is no finish line.

One more facet of the bird-origins saga bears mention here. The statistical study my colleagues and I carried out may explain how birds persevered through the cataclysmic extinction event that claimed the other dinosaurs. As part of that work, we used our big data set to measure evolutionary rates: how quickly birds and their dinosaur cousins were changing features of their skeleton, which is a sign of evolutionary vitality. And the results surprised us. Those earliest-emerging birds that lived alongside their dinosaur forebears were evolving at supercharged rates—faster than Velociraptor, Zhenyuanlong and other nonbird species. It seems that once a small, flight-capable dinosaur had been assembled, once that Lego kit was complete, incredible evolutionary potential was unlocked. These airborne dinosaurs now had access to new ecological niches and opportunities. And whereas their brethren were unable to cope with the apocalyptic impact of the six-mile-wide asteroid that slammed into Earth at the end of the Cretaceous, birds flew right through the destruction—and had a new world to conquer on the other side.