The early birds set the scene | Mint – Mint

Archaeopteryx’s avian features arose in 10 mn years, an instant in evolutionary terms
Intriguing factoid to kick off 2023: Biologically and in an evolutionary sense, alligators are more closely related to birds than to lizards.
Since I learned that, I’ve been more interested than usual in the barbets and drongos on the tree outside. In the occasional lizard that appears high on our walls. No alligators in the neighborhood, but even so, it’s a wonder to ruminate about that factoid with these real-life exemplars as fuel. It’s a wonder to grasp the reason for this closeness: that birds and alligators have both descended, again in the evolutionary sense, from dinosaurs. So some features in alligators seem bird-like, and vice-versa.
But step back for a moment. Probably like you, I’ve always been intrigued by that particular evolutionary line—dinosaurs to birds. Of course, it has been many million years, plenty of time for evolution to work its mysterious magic. But still: how can the barbet calling outside as I write possibly have, somewhere high up in its evolutionary tree, a Tyrannosaurus rex? Those particular dinosaurs—the two-legged family called theropods—were originally large, heavy, had teeth and relatively tiny brains. What moulded them into birds? What moulded them into about 10,000 living species of such exquisite diversity?
To answer that, palaeontologists relied for years on the fossils of Archaeopteryx. This is an in-between animal that had the tail and teeth of dinosaurs, but also had wings with feathers, meaning it could fly. Yet, there was something puzzling about the evolutionary step Archaeopteryx represented: Its avian characteristics had appeared in a mere 10 million years. In evolutionary terms, that’s a mere instant. In effect, it’s as if Archaeopteryx appeared on the planet fully equipped with those characteristics. Explaining this would need some new theory and, of course, evidence. For, “fully equipped” is not how evolution typically happens.
But in the 1990s, a whole trove of fossils of dinosaurs were found in China. Unlike Archaeopteryx, these did not have wings, but they did have feathers. This has led to some argument among palaeontologists about which species was—or were, it need not have been just one—the most “basal”, or earliest, bird. Feathered dinosaurs like Anchiornis, xioatingia, Aurornis and Epidexipteryx (which had a tail like a ribbon) joined Archaeopteryx as contenders, and there are more. But that debate only drives home the point that birds—or early bird-like creatures—didn’t suddenly appear on our Earth, requiring today’s scientists to find new hypotheses for their presence.
So fine, we now know there were several primitive bird-like species roaming the planet. To some degree or another, they had features we associate with birds today: feathers, wings of sorts, the wishbone that’s 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 instant that produced birds. But the rate of evolution along the theropod lines that led to birds was faster than on other lines.
At the time, all those millions of years ago, the various feathered claimants to the basal bird pedestal were not significantly different from other theropods or, indeed, other dinosaurs. In fact, some theropods went on to become giant dinosaurs. But in some others, starting with the feathered claimants, certain skeletal characteristics in particular changed, and quickly. The heads became smaller. The overall body sizes shrank, until flight became a viable option. There is a study that suggests they were growing smaller 160 times faster—again speaking in evolutionary terms —than their theropod cousins were growing into giants.
In effect, what these animals had was a simple bird “body plan”. Besides feathers and wings, they had relatively small and agile bodies, even the strength-to-weight and wing-size-to-weight ratios that allow such bodies to fly. This was essential for the theropods to evolve into birds. Besides, the “long-term miniaturization” of these theropods had another significant benefit. In the asteroid strike of about 65 million years ago, that killed off the non-avian dinosaurs and pretty much all large animals, most birds—or actually, those early “birds”—survived. They were not driven suddenly to extinction.
But the miniaturization also suggests that at least at the time, it was advantageous to be smaller. Possibly, these early birds were unearthing different ways to live, maybe new sources of shelter and food.
Like trees, for example.
There are two particular characteristics that offer clues to how this theropod evolution proceeded. Alligator embryos resemble chickens; and baby dinosaurs’ skulls—or at any rate, their fossils—resemble birds.
These suggested to biologists a specific evolutionary process called paedomorphosis. That is, the animal retains juvenile—even embryonic—features as it becomes an adult. That is part of what happened as dinosaurs evolved into birds. Biologists focused on the shape of the skull of these animals as they evolved. They found that the dinosaur face shrank considerably, while the eyes, beak and brain grew (relative to the face size). The effect is that birds look like dinosaur embryos and infants. The effect also is that birds have larger brains, relative to body size, than dinosaurs did—precisely because embryos and babies have larger heads relative to their bodies than adults do. As the English palaeontologist Michael Benton put it, “a great way to improve brain size is to retain child size into adulthood”.
There are more fascinating facets in this process. But there’s also what happened once evolution had produced this basic body plan, these early birds. Because of their small size, these were mobile, adaptable, hardy creatures that could fly. In a real sense, this was a moment in time when they were perfectly poised to explode, evolutionarily speaking. 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 great spurt of evolution gave us the birds of today. Instead, it was birds that drove that spurt.
In the end, and as I listen to the barbet and gasp at the drongo, that may be the most captivating factoid of them all.
Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun.
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