The tiny town of Chicxulub Puerto in south-eastern Mexico houses only a few thousand residents. A casual glance at the sleepy settlement suggests a peaceful community, one amongst many dotting the vast Yucatan peninsula. 

Yet, buried just about a kilometre beneath Chicxulub’s surface and written in lines of quartz and melted rock lies the evidence of its uniquely violent history. The ground where Chicxulub Puerto stands today was once ground zero for one of the most catastrophic events in the history of life on earth.

On a day 65.5 million years ago, a piece of extra-terrestrial rock as big as a mountain crashed into earth with the explosive power of over a hundred trillion tons of TNT (see Fig. 1).

Releasing more than a billion times the energy of the atomic bomb dropped on Hiroshima, the rock’s impact created an almost perfectly circular crater, nearly 180 km in diameter, right atop the centre of which Chicxulub stands today (see Fig. 2).

It resulted in a massive heat pulse, vapourising all living forms for miles around, followed by earthquakes and massive tsunamis that radiated out in all directions. 

The debris and dust thrown out into the atmosphere blocked out sunlight for over a year, resulting in a nuclear winter where plants failed to grow, and predator and prey alike starved to death (see Box 1).

All of this occurred in the background of an earth that was already ravaged by severe volcanism and global climate change. The Cretaceous-Tertiary (K‑T) mass extinction event, as it is called today, ended up wiping out nearly two-thirds of all the species inhabiting the planet at the time (some estimates go as high as 80% of all life on earth).

Amongst these was a massively successful group of reptiles, of which more than a thousand species of every size had once roamed the jungles. The age of the dinosaurs had come to an end (see Box 2).

However, not every dinosaur perished in the K‑T extinction. One unlikely group survived, and they persist to this date nesting on our windowsills, stealing bits of food from roadside stalls, and waking us up in the morning with their cacophonous warbles.

Birds are the last living dinosaurs on our planet, and this is their story.

The ancient wings of Archaeopteryx

It was not always known that birds were related to dinosaurs. Sometime in the mid-1870s, a German farmer named Jakob Niemeyer discovered a strange fossil in the Blumenberg quarry near the town of Eichstätt in Germany. In need of money to buy a cow, Niemeyer decided to sell the fossil to a local innkeeper. 

After changing hands a few more times, the fossil was finally bought by the Natural History Museum of Berlin in 1881, for the princely sum of 20,000 gold marks. This ​‘Berlin specimen’, as it came to be called, contains an excellently preserved skeleton and skull of a creature with spread wings, a bony tail, and three claws at the end of each ​‘hand’ (see Fig. 3).

What Niemeyer had discovered, and sold for want of a cow, remains one of the most perfect and complete specimens of Archaeopteryx till date. With a name derived from Greek ​‘archaīos’ meaning ancient, and ​‘ptéryx’ meaning wing, it was our first clue to the mysterious origins of modern birds.

Archaeopteryx was first described in 1861, by the German palaeontologist Christian Erick Hermann von Meyer, using a single fossilised feather. The first skeleton was unearthed in the same year from a quarry near Langenaltheim in Germany. 

In an uncanny parallel with Niemeyer, the unknown discoverer of this specimen gave the fossil as payment to a local doctor, who sold it to the Natural History Museum in London. So, what makes this prehistoric bird, that lived almost 160 million years ago, unique?

Archaeopteryx had a near-perfect mix of reptile-like and bird-like features. About the size of a big crow, it had a long tail and jaws with sharp teeth like a reptile, but also sported two large feather-covered wings and three- fingered hands like modern birds (see Fig. 4).

New theories and old theories

Thomas Huxley, an English biologist and a great proponent of Darwin’s theory (to the extent that many people called him ​‘Darwin’s bulldog’), was the first to note the similarities between the skeletons of birds and theropods. Theropods are a class of dinosaurs characterised by hollow bones, and three toes on each arm and leg (see Fig. 5).

Based on this observation, he suggested that birds might have evolved from theropod dinosaurs (see Box 3).

However, this theory soon fell out of favour, particularly with the publication of an influential book, in 1926, by the Danish anatomist Gerhard Heilman. Through a detailed comparative analysis of the skeletal features of birds and other closely related groups, Heilman concluded that birds are more closely related to theropod dinosaurs than any other group of animals. 

However, certain anatomical differences, particularly the presence of a fused collarbone in birds but absent in dinosaurs, make it impossible for the former to have evolved from the latter.

Instead, he suggested that birds and dinosaurs may have evolved from a common ancestor called thecodont. So popular was this theory (and this book) that, over the next half a century, the ​‘thecodont hypothesis’ came to be accepted by almost every expert in the field.

It was only in the late 1970s when a convergence of evidence from multiple sources resurrected the dinosaur –> bird theory. Along with anatomical and fossil-based studies, the new science of cladistics (in which organisms are classified into groups based on how recently they shared a common ancestor) played a big role in this resurrection. Today, the dinosaurian origins of birds, and hence their status as ​‘living dinosaurs’ is widely accepted. ¹

What was the evidence that cinched the deal? Fossils from many sources, particularly from the Jehol Biota of China, showed that early birds and certain groups of theropod dinosaurs shared many features. 

These include characteristics that we assume as being peculiar to modern birds, and that differentiate them from almost all other groups of animals living today. Examples of such features include feathers, wishbones, egg-brooding, two-legged walking, and even flight (see Box 4).

Let’s take feathers, for instance. Bird feathers are complex structures that help in the aerodynamics of flight. They also help perform a number of other functions, such as maintaining body heat, egg-brooding, and attracting mates. 

Some fossils of early theropod dinosaurs show hair-like filaments that can be called protofeathers. Later theropods appeared to have feathers whose structures are much more similar to those of birds. 

Interestingly, as many fossil studies have shown, many non-bird dinosaurs were also covered in feathers.

Feathered wings may have helped some of these, like microraptors (four-winged dinosaurs covered with feathers that are virtually indistinguishable from the feathers of modern birds), glide expertly (see Fig. 6).

In other (clearly non-flying) dinosaurs, feathers may have served a completely different function. All of these point towards the idea that feathers probably evolved for a function other than flight — perhaps these colourful plumages were used to attract mates, or scare predators and competitors. Only later were they co-opted into helping the animal fly, and became specialised for the purpose.

Apart from feathers, birds have extraordinary sensory capabilities. These are helped by a large forebrain, which aids them in the rapid sensory processing and decision-making required for flight. 

Scientists have found evidence that some dinosaurian ancestors of birds also had largish forebrains, suggesting that their brains had already started achieving a state that would allow them to eventually evolve flight (in a sense, becoming ​“flight-ready”). Birds also have a rapid metabolism, which supplies the massive amount of energy required for flight.

While reptiles are cold-blooded, birds are not; and evidence suggests that dinosaurs may have fallen somewhere between the two in the spectrum (see Box 5).

The first true birds appear in the fossil record around 150 million years ago, about 85 million years before the Chicxulub collision (see Box 6).

Scientists often have difficulty distinguishing the fossils of these early birds from non- bird dinosaurs, showing how similar the two branches could sometimes be (see Box 7).

Box 7. Teachers toolkit — understanding evolutionary distance:

1. Create a randomised set of about twenty to thirty anatomical features (e.g. long tail, five fingers, blue feathers etc.)
2. Print four copies of each of these features on small index cards.
3. Divide students into 5 – 10 small groups, and have each group draw five cards from the pack.
4. Encourage students to use their cards and the features printed on them to create an imaginary animal and name it whatever they like.
5. Invite students to compare the features of the different imaginary animals to figure out how closely related they are to each other.
6. Get students to create an evolutionary tree to connect their animals.

The early forefathers of birds coexisted alongside their dinosaur brothers for nearly 100 million years, with small gradually accumulating changes distinguishing them from each other. 

The closest relatives of birds at the time were the small-bodied, feathered, large-brained dromaeosaurids and troodontids —theropod dinosaurs that resembled birds almost more than they resembled other dinosaurs (see Fig. 7).

Surviving apocalypse

All of this gives rise to the question — how did birds survive the mega- extinction event that marked the end of the age of the dinosaurs? Well, our first clue lies in the fact that not all birds did. 

Most bird species did go extinct in the aftermath of the Chicxulub impact, and only a tiny fraction survived. We can only speculate on what marked this handful of species for survival.

A recent hypothesis suggests that the surviving branches of the bird-family consisted of birds that had evolved hard beaks, specialised to feed on grains or seeds, to replace jaws and teeth.²

While much of plant foliage, fruits, and small prey would have been destroyed in the aftermath of the impact, seeds are much hardier. Their ability to feed on seeds may have been the one factor that tipped the scale in favour of these birds, protecting them from death by starvation. 

However, some scientists have disputed this hypothesis based on the existence of some toothed and beakless ancestors of birds that were also known to have fed on seeds. In addition, this hypothesis doesn’t explain why certain species of beaked birds (especially small ones) appeared more likely to survive than others.

Another hypothesis contends that it is the habitat that these early birds were adapted to live in that made the biggest difference to their survival.³

There is evidence of massive global deforestation occurring in the aftermath of the Chicxulub impact — the initial shockwave is itself estimated to have flattened all trees within a 1500 km radius. For a long time afterwards, dust and minerals released by the impact would have blocked out photosynthesis and resulted in frequent acid rains.

When we consider these factors, it seems likely that tree-dwelling creatures would have swiftly run out of luck. The only birds to survive this period were much like today’s chicken and other fowl — ground-dwelling, and small in size.

Parting thoughts

While it is still unclear how birds managed to survive the Chicxulub impact, today they are one of the most successful groups of animals on the planet. 

With more than 10,000 different species, birds have colonised almost every habitat on earth. Their powerful flight muscles allow them to soar for thousands of kilometres and, sometimes, at a height of over 9000 m above sea level.

So the next time you look out of the window and observe a crow scowling at you, remember that its ancestors once ruled the earth and, somewhere in its genes, the same majesty lives on.

Key takeaways

• A few families survived the catastrophic asteroid impact that led to the extinction of most dinosaurs.

• Birds are the natural descendants of theropods — a branch of one of these surviving dinosaur families.

• The survival of theropods may be linked to their ability to eat seed-like food (that was more abundant in the aftermath of the impact).

• Theropod ancestors of birds may also have survived the asteroid impact because they were small in size, and ground-dwelling rather than tree-living (trees were decimated by the asteroid impact).

• Many of the features we associate with birds today, like flight or feathers, were already present in their dinosaurian ancestors.

References:

1. Brusatte, S. L., O’Connor, J. K., & Jarvis, E. D. (2015, October 5). The origin and diversification of birds. Current Biology. Cell Press.
2. Larson, D. W., Brown, C. M., & Evans, D. C. (2016). Dental disparity and ecological stability in bird-like dinosaurs prior to the end-cretaceous mass extinction. Current Biology, 26(10), 1325 – 1333.
3. Field, D. J., Bercovici, A., Berv, J. S., Dunn, R., Fastovsky, D. E., Lyson, T. R., & Gauthier, J. A. (2018). Early Evolution of Modern Birds Structured by Global Forest Collapse at the End-Cretaceous Mass Extinction. Current Biology, 28(11), 1825 – 1831.e2.

About the author:

Shreya Ghosh is a science writer and editor for IndiaBioscience. 

She can be contacted at shreya@​indiabioscience.​org.