Do we have dinosaurs living amongst us? Consider this! The sparrow that just landed on your window ledge, or the crow cawing from the tree outside, can trace their common lineage back millions of years, when a group of meat-eating dinosaurs called theropods evolved into bird-like creatures over generations, creating the strain of the first ancestors of our feathered friends. Theropods are the same family of dinosaurs to which Tyrannosaurus Rex belonged. Though birds are known to have evolved from smaller theropods, their ancestry dates to a much older phase, with the oldest bird fossils being mapped to 150 million years ago.
Birds are among the most conspicuously recognisable fauna inhabiting our planet. They are a diverse group, comprising more than 10,000 extant species, occupying a range of ecological niches, from forests, deserts, caves, the Savannah, water bodies, the Poles, to even cities. They are distinct in their anatomical features, ranging in size from the tiny bee hummingbird (~2 grams) to the mighty ostrich (~140,000 grams).
Biologically classified as Aves, birds are nearly perfect evolved beings. For instance, penguins have wings that have evolved into flippers and their bodies have taken a cylindrical shape, allowing them to dart through seawater, as they hunt for fish or outswim predators – such as orcas and leopard seals. Likewise, other flightless birds, such as the kiwi, which has a small body closer to the ground and a long narrow beak to help it forage for insects, have developed specific body structures suited to their geographical habitats. Another principal reason for these terrestrial birds to lose their ability to fly is a lack of natural predators.
However, most birds do fly, with their feathered bodies optimised for flight. They have specific body features such as sharp beaks and talons among carnivorous raptors, the ability to turn their necks 135 degrees and uniquely-positioned eyes among nocturnal owls, a strong beak allowing woodpeckers to peck at 1300 Gs, and webbed feet allowing ducks to wade on lake surfaces. Wings of albatrosses and terns allow them to glide as they seasonally migrate thousands of miles in search of food and nesting grounds. Overall, the supercharged growth rates and metabolism in birds stand out amongst living organisms, while their large brains, keen senses, and the supreme ability to vocalise, use tools, build elaborate nests, and develop strong bonds (a lot of birds mate for life and care for offspring), make them some of the smartest animals in nature.
How did birds arrive at this diversity and evolutionary success? Over the decades, a wealth of new fossil discoveries, molecular phylogenetic analyses of living birds, and quantitative macroevolutionary study have revolutionised our understanding of the origins of birds. This new information reveals the aforesaid theory, that birds evolved from dinosaurs and share a deep natural history with them. Their signature body plan evolved over 100 million years of steady evolution, alongside their dinosaurian forefathers, before several modern groups of birds diversified after the non-avian dinosaurs went extinct 65 mya.
According to most experts, birds first appeared in the Jurassic period, with the earliest specimens derived from a clade of theropods named Paraves. For over a century, the small theropod dinosaur 'Archaeopteryx lithographica' from the Late Jurassic period was considered to be the earliest bird. Four distinct bird lineages survived the Cretaceous-Paleogene extinction of 65 mya – ostriches and relatives (Palaeognathae), waterfowl (Anseriformes), ground-living fowl (Galliformes), and modern birds (Neoaves). It is also widely accepted phylogenetically that Aves are usually defined as all descendants of the most recent common ancestor of a specific modern bird species (such as house sparrow), and either Archaeopteryx, or some prehistoric species closer to Neornithes. If the latter classification is used, the larger group is termed Avialae. Currently, the relationship between dinosaurs, Archaeopteryx, and modern birds is still hotly debated.
Enigma of the Archaeopteryx
In 1861, the first fossil of an Archaeopteryx was discovered in Germany. Like modern average-sized birds, the fossil revealed that this creature grew to a size of 1 ft 8 inches, like a raven, and was found to possess feathers along its arms and tail, but unlike living birds, had teeth and a long bony tail. Also, many of the bones in Archaeopteryx's forelimbs, shoulder girdles, pelvis, and feet weren't fused and reduced, similar to birds. The skeletal remains also showed the presence of jaws with sharp teeth and three fingers with claws – hyperextensible second toes (killing claw).
Separately, it had close ties with small Mesozoic dinosaurs. In this regard, Archaeopteryx shared links with Maniraptora, a group of theropods such as dromaeosauridae (Velociraptor and Deinonychus) and oviraptoridae (Oviraptor, Utahraptor, etc). These factors led to the long-standing theory of this hybrid marvel being the missing evolutionary piece of the bird-dinosaur puzzle. “Archaeopteryx seemed to emerge fully fledged with the characteristics of modern birds,” says Michael Benton, a palaeontologist at the University of Bristol, as quoted by Scientific American. Interestingly, Archaeopteryx even predates most known dinosaur species, which appeared in the Triassic period. This fact makes birds an animal group in constant evolutionary transition, even from before the days of the dinosaurs.
How did most large dinosaurs go extinct during the mass extinction of 65 mya, while several smaller ones survived and evolved into birds, in the perfect example of Survival of the Fittest, confirming Darwin's theories? Careful observations in recent times have allowed scientists to propound the "hopeful monster theory", which states that the perfection that birds attempted to achieve over millions of years required major evolutionary leaps, with large-scale genetic changes that are qualitatively different from routine modifications within a species. Substantial alterations on a short timescale, led to a sudden transformation from a 300-pound theropod to an Iberomesornis (prehistoric sparrow-size bird).
Even from the days of the Archaeopteryx, it is learnt that unique features such as the growth of feathers proved advantageous. Discoveries reveal that birds simply adapted these pre-existing features to a novel use to survive.
Also, the adoption of a more baby-like skull shape into adulthood – likely played an essential role in the final push to bird-hood. Not only are birds smaller than their dinosaur ancestors, but they resemble dinosaur embryos. Such adaptations paved the way for modern birds’ distinguishing features, like their ability to fly and their agile beaks, a significant leap in evolution through small steps.
Wings that won
In the 1990s, an array of new dinosaur fossils were unearthed in China, which doubted the theories of that time. Feathers, once thought unique to birds, must have evolved in dinosaurs themselves, towards the Early Jurassic period, long before birds had even developed. It showed avian features evolved over time and continue to evolve, however retaining the common character of ‘Dinosauria’.
Modern palaeontologists such as Steve Brusatte from the University of Edinburgh, are digging deeper into this evolutionary conundrum. Brusatte examined fossils of Coelurosaurs, a theropod sub-group that produced the Archaeopteryx, and tracked changes in skeletal properties to discover that there were no great jumps distinguishing birds from other Coelurosaurs. Also, it is suggested that modern evolutionary leaps did not produce birds. Rather, birds produced modern leaps in evolution.
Another theory that explains the success of modern birds from their less-successful ancestors, is miniaturisation. Some Coelurosaurs started shrinking as far back as 200 mya – 50 million years before Archaeopteryx. "The distinct, prolonged phase of miniaturisation along the bird stem would have facilitated the evolution of many novelties associated with small body size, such as reorientation of body mass, increased aerial ability, and paedomorphic skulls with reduced snouts but enlarged eyes and brains," mentions an article published in Science magazine. The small size meant lesser body weight, requiring lesser food, easier to protect against rough weather and surviving large predators, and a longer lifespan.
Overtime, birds lost their teeth, and developed strong beaks allowing them to eat more nuts, fruit, small insects, and seeds, while the growth of wings, webbed feet, and flippers made them the most versatile group in the animal kingdom. Some of them developed strong eyesight for a panoramic view of the Earth below. So, the next time you hear a koel cooing outside your window, remember it's humming a million-year-old song.