조류의 진화
(1) 시조새 이전의 진화 : 조류는 파충류로부터 진화되었다고 알려져 있다. 오랜 지질시대를 거치면서 파충류의 일부가 조류로 진화된 것으로 추정되는 증거들이 발견되었다. 그 가장 오래된 증거는 파충류의 조상형인 조룡류(祖龍類, Archosauria；총칭하여 공룡 ＜Dinosauria＞이라고 한다)에 속하는 조치목(槽齒目；Thecodontia)에서 볼 수 있다. 이들은 이미 두 다리로 달리며 1억 9000만 년 전에 살았다. 이어서 나타난 위악목(僞鰐目；Pseudosuchia)은 새처럼 속이 빈 가벼운 뼈를 가지고 두 다리로 다녔다. 몸길이 약 3.6m인 오르니토수쿠스(Ornithosuchus)가 대표적이다. 이들로부터 그 다음의 용반목(龍盤目, Saurischia)이 진화된 것은 1억 6000만 년 전부터이고, 역시 두 다리로 다녔던 오르니톨레스테스(Ornitholestes)는 북아메리카의 쥐라기에서 백악기(1억 3000만년 전∼2000만년 전)에 걸쳐 살았던, 몸길이 2m 이하인 종이다. 또 아메리카, 아시아에서는 타조와 흡사한 2.5m의 초식성 오르니토미무스(Ornithomimus)가 백악기의 6000만년 전에 살았다. 다시 말하면 쥐라기에서 백악기에 걸쳐서 이미 파충류 중에는 이처럼 뼈 속이 비는 이각성(二脚性)이 나타나는 것으로 알려져 있으며, 이는 점점 새에 가까워지고 있음을 의미한다. 그러나 이렇게 다양한 진화는 더 이상 발전하지 못하고, 그 가운데 어느 한 계통이 시조새로 진화했을 것으로 추정 된다. 결국 큰 틀에서 중간 화석으로 판단되는 시조새는 발견되었지만 비늘이 깃털로 변하는 과정이나, 언제 온혈성이 되었는지, 또 어떻게 날 수 있게 되었는지, 물새나 타조와 같은 큰 새도 과연 시조새에서 진화된 것인지, 혹은 별도로 타조형 조상이 있었던 것인지 등은 풀리지 않는 의문으로 남아있다.
(2) 시조새 : 조류는 파충류에서 진화되었다고 알려져 있다. 1억 3800만년전인 쥐라기 지층이 발견된 독일의 바이에른의 졸른호펜지방의 석회암층에서 출토된 3개의 시조새(아르케오프테릭스 Archaeopteryx)가 가장 오래된 화석으로 알려져 있다. 이 화석을 살펴보면 파충류와 조류의 형질을 반반씩 갖고 있다. 이 동물은 파충류와 비슷한 모습으로 크기는 대략 까마귀 정도로 작았으며, 두 발 보행을 하는 작은 공룡처럼 보인다. 그리고 두개골은 오늘날의 조류와는 달리 잘 발달된 이빨을 가지고 있으며, 목은 길고 가늘며 매우 유연했을 것으로 추정된다. 척추는 비교적 단순한 모양이며 끝에 길고 잘 발달된 꼬리를 가지고 있다. 그러나 꼬리에는 양쪽으로 깃털이 달려있었다. 또한, 뒷다리 끝부분에는 발톱이 3개 있었고, 날개 모양을 하고 있는 앞다리는 날개의 모양을 가지고 있었지만 완전한 모양은 아니었다. 그 이유는 중간에 발가락이 있기 때문이다. 가슴뼈는 조류에서 흔히 볼 수 있는 용골돌기나 흉골의 발달이 미약한 점으로 미루어 지속적인 비행을 할 수는 없었을 것으로 추정된다. 이런 형태적 특징을 근거로 일부 학자들은 시조새를 깃털이 잘 발달된 조류의 일종으로 분류하고 있다.
(3) 시조새 이후의 진화 : 시조새 이후에는 조류의 화석이 매우 드물게 발견되고 있다. 그 이유는 공중을 날아다니는 조류는 현실적으로 화석이 되기 어렵기 때문이다. 그러나 백악기의 지층에서 발견된 헤스페로르니스는 물에서 살았던 새로 추정되고 있는데 날개는 없지만 발로 헤엄친 것으로 추정되며 아직 이빨은 있었다. 그 밖의 화석에는 이렇다 할 자료가 남아 있지 않지만, 시노사우롭테릭스, 안키오르니스와 같은 깃털공룡이 발견되면서 깃털의 발달과 조류의 등장에 대한 혁명적인 단서가 되었다.
(4) 백악기 화석조
① 갈로르니스 Gallornis: 깃털의 화석만이 알려졌다. 1억 3500만 년 전의 대형 섭금류(涉禽類).
② 에날리오르니스 Enaliornis: 1억 2000만 년 전의 아비형 물새.
③ 헤스페로르니스 Hesperornis: 8800만 년 전의 아비형 물새. 날개는 퇴화되어 있었다. 3종이 알려져 있는데 시조새를 제외하고는 유일하게 이를 가졌다.
④ 이크티오르니스 Ichthyornis: 중형의 바다새. 헤스페로르니스와 같은 시대에 서식하였으며 6종이 알려져 있고 이빨은 없다.
⑤ 아파토르니스 Apatornis: 이크티오르니스와 비슷한 형태이면서 동시대의 종.
⑥ 바프토르니스 Baptornis: 논병아리형의 잠수조. 아파토르니스와 같은 시대에 서식하였다.
(5) 신생대 제3기 화석조
① 가스토르니스 Gastornis: 두루미목에 속하는 대형종으로, 다리가 길고 부리가 날카로운 육식조이다. 에오세까지 있었다.
② 다이아토리마 Diatoryma: 두루미목, 가스토르니스와 닮은 거대조로 에오세까지 있었다.
③ 포르스라코스 Phorsrachos와 브론토르니스 Brontornis: 두루미목에 속하는 무비력(無飛力)인 거대조. 올리고세까지 있었다.
(6) 멸종된 조류 : 인류의 시대까지 서식하다가 멸종된 조류는 마다가스카르섬의 에피오르니스(Aepyornis), 뉴질랜드의 모아(moa；恐鳥, Dinornis), 인도양 마스카린제도의 무비력의 대형비둘기 도도(dodo)와 그 근사종 등이 있다.
조류와 파충류, 포유류의 차이
| 파충류 | 조류 | 포유류 | |
| 심장 구조 | 2심방 불완전2심실2심방 2심실(악어류) | 2심방 2심실 | 2심방 2심실 |
| 체온 변화 | 변온 | 정온 | 정온 |
| 출산 방식 | 난생난태생(살무사) | 난생 | 태생 |
| 척색 유무 | 척색 → 척추 | 척색 → 척추 | 척색 → 척추 |
| 골격(뼈) | 단단하고 속이 차있음 | 속이 비어있음 (날기에 유리) |
단단하고 속이 차있음 |
| 이 | 이 있음 | 이 없음, 부리 발달 | 이 있음 |
| 질소배설물 | 요산 | 요산 | 요소 |
| 몸 표면 | 각질의 표피(비늘) | 날개와 깃털(다리, 발은 비늘) | 털로 덮여있음 |
- Scientists have known for some time now that birds evolved from dinosaurs and are in fact a subgroup of dinosaurs. A rich fossil record of feathered dinosaurs discovered in China and elsewhere documents in detail the dramatic transformation of behemoth terrestrial dinosaurs into small, flight-capable birds.
- New techniques for analyzing fossils have enabled researchers to reconstruct how the distinctive bird body plan came together. The results indicate that the group's hallmark traits emerged piecemeal over tens of millions of years, for purposes other than those they serve today.
- The findings add to a growing body of evidence suggesting that major evolutionary transitions proceed gradually, not rapidly.
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.
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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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.