early birds set the scene

Ever since I learned this, I’ve been more interested than usual in the barbet and drongos on the trees outside. Sometimes in lizards that appear high up on our walls. There are no alligators in the neighborhood, but still, it’s surprising to think about that fact with these real-life examples as fuel. It is surprising to understand the reason for this closeness: that both birds and alligators, again in the evolutionary sense, descended from dinosaurs. So alligators have some bird-like features, and vice versa.

But step back for a moment. Like you probably, I’ve always been intrigued by that particular evolutionary line—from dinosaurs to birds. Of course, several million years have passed, plenty of time for evolution to work its mysterious magic. But still: As I write, the barbet calling out could possibly be somewhere higher up in its evolutionary tree, Tyrannosaurus rex? Those particular dinosaurs—the two-legged family called theropods—were originally large, heavy, toothy, and relatively small-brained. What made them into birds? What molded them into such an exquisite variety of nearly 10,000 living species?

To answer it, paleontologists have relied on the fossils of Archeopteryx for years. It’s a beech animal that had a dinosaur’s tail and teeth, but also wings with wings, meaning it could fly. Still, there was something intriguing about the evolutionary step represented by Archeopteryx: its avian features appeared in a mere 10 million years. In evolutionary terms, this is just a moment. In fact, it seems likely that Archeopteryx appeared on the planet fully equipped with those features. To explain this would require some new theory and of course evidence. For, “fully equipped” is not how development usually goes.

But in the 1990s, a whole trove of dinosaur fossils was found in China. Unlike Archeopteryx, these did not have feathers, but they did have wings. This has led to some argument among paleontologists as to which species was – or was, it was not just one – the most “basal”, or earliest, bird. Feathered dinosaurs such as Anchiornis, xioatingia, Aurornis and Epidexipteryx (whose tails were like a ribbon) joined Archeopteryx as contenders, and there are more. But this debate only drives home the point that birds—or early bird-like creatures—did not suddenly appear on our Earth, requiring today’s scientists to find new hypotheses for their appearance.

Well then, we now know that there were many primitive bird-like species roaming the planet. To some extent, they had features we associate with birds today: feathers, the type of feathers, wishbones that are found in bird breasts, and they walked on two legs. The question remains: how did they evolve into birds?

Recent research suggests something remarkable. Granted it wasn’t an evolutionary moment that produced birds. But the rate of evolution along the theropod lines that led to birds was faster than other lines.

At the time, millions of years ago, the various feathered contenders for the basal bird pedestal weren’t much different from other theropods or, indeed, other dinosaurs. In fact, some theropods became giant dinosaurs. But in some others, starting with the feathered contenders, certain skeletal features in particular changed, and quickly. The heads got smaller. The overall body size shrunk, until flight became a viable option. There’s a study that suggests they were getting smaller 160 times faster – again speaking in evolutionary terms – than their theropod cousins ​​were growing into giants.

In fact, these animals had a simple bird “body plan”. In addition to wings and wings, they had relatively small and agile bodies, even with the strength-to-weight and wing-size-to-weight ratios that allow such bodies to fly. This was necessary for theropods to evolve into birds. In addition, the “long-term miniaturization” of these theropods had another important advantage. In the asteroid strike about 65 million years ago that killed off the non-avian dinosaurs and nearly all large animals, most birds—or really, those early “birds”—survived. They were not suddenly driven to extinction.

But miniaturization also suggests that at least at the time, being small was beneficial. Presumably, these early birds were exploring different ways of living, perhaps new sources of shelter and food.

Like trees, for example.

There are two special features that give clues to how this theropod evolution proceeded. The embryos of alligators are similar to those of chickens; And the skulls of baby dinosaurs—or at any rate, their fossils—are similar to birds.

These suggest to biologists a specific evolutionary process known as padomorphosis. That is, the animal retains juvenile—even embryonic—characteristics as it becomes an adult. That’s part of what happened after dinosaurs evolved into birds. Biologists focused on the shape of the skulls of these animals as they evolved. They found that the dinosaur’s face had shrunk significantly, while the eyes, beak, and brain (relative to the size of the face) had grown. The effect is that the birds look like embryos and babies of dinosaurs. It also has the effect that birds have larger brains relative to their body size than dinosaurs—precisely because the heads of embryos and infants are larger relative to their bodies than those of adults. As the English paleontologist Michael Benton said, “an excellent way to improve brain size is to maintain child size into adulthood”.

There are other fascinating aspects to this process as well. But there’s also what happened once evolution produced this basic body plan, these early birds. Because of their small size, they were mobile, adaptable, hardy creatures that could fly. In a real sense, it was a moment when they were all set to explode developmentally. And that’s just what happened: a “spectacular radiation”, a “runaway diversification” of bird species (Evolution: A Rapid Flight Towards Birds, Daniel Ksepka, Current Biology, 3 November 2014). Suddenly, there were 10,000.

So, it wasn’t so much that a huge spurt of evolution gave us the birds of today. Instead, it was the birds that powered that boom.

Finally, and as I listen to barbets and gasp at drongos, this might be the most delectable factoid of them all.

Dilip D’Souza, once a computer scientist, now lives in Mumbai and writes for his dinner. His Twitter handle is @DeathEndsFun.