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The evolution of flight

Flight is still a matter of hot debate. Insects did it first of course, followed by the flying reptiles, and then birds. Finally, within the past 50 million years, came mammals (bats).

the first birds bird brains

This is a personal view, but I think that oxygen levels will certainly have had a lot to do with it. There is nothing magic about our atmosphere’s current 20% oxygen. The amount of oxygen around could, and certainly did, vary widely down the ages (more).

More oxygen in the atmosphere would make it much easier for not-yet-properly-adapted creatures to get airborne. It both enables them to get more oxygen to their muscles and therefore to work them harder. And it makes the air denser (heavier) which helps not-very-good wings to do the job.

Insects mastered flight during the Carboniferous period, when you would certainly expect oxygen levels to be high. And they were. Huge dragonflies appeared twice. Both during oxygen-rich periods.

We know nothing about how or when reptiles acquired flight. But the first birds certainly appeared at a high-oxygen time.

Birds could have discovered flight ‘top-down’, gliding down from trees, or ‘bottom-up’ jumping off the ground. Apparently there were no dinosaurs up trees, so if you believe the former theory then you have to believe that birds are descended from reptiles – the lizards and other small creatures that were already up there. Note that we are not talking about the flying reptiles, or pterosaurs, here. They had been around as long as the dinosaurs had. We are talking about a new group of reptiles that may (or may not) have rediscovered flight and evolved into modern birds. I don’t think many scientists believe this.

The rest have to believe that it was the dinosaurs that learned to fly by jumping up from the ground. Remember that they had already had feathers about their persons for tens of millions of years. There are plenty of Web sites that will show you modern birds – flightless or nearly flightless – using their wings to help them run faster, climb steeper gradients or even to jump up. Remember also that there was more oxygen around at this time.

The first birds

We’ve discussed elsewhere, the anatomical features that make birds so dinosaur-like. Many scientists insist that they are not just descended from dinosaurs, they actually are dinosaurs.

The first known bird was Archaeopteryx, which appeared during the late Jurassic, say 150 million years ago. These pictures are of a model in Oxford University Museum. Actually I have problems with the concept of Archaeopteryx being a bird. To me, birds are good flyers, and Archaeopteryx wasn’t. It still had too many dino-like features and not enough bird-like ones. Its niche will have been the zone among the trees – safe from the flying reptiles but out of reach of the land animals. To me, Archaeopteryx was a flying dinosaur. As we’ve said, the late Jurassic was a time of high oxygen levels (more), which must have helped the pioneers of dinosaur flight no end.

But movement was afoot. Another ‘bird’ from the same time, Confuciusornis, had already lost its heavy jawbones and teeth. By 115 million years ago (say 30 million years after Archaeopteryx) a much more bird-like creature appeared. It was about the size of a goldfinch. It was found in Spain, and called Eoalulavis hoyasi. The fossil was found squashed between two layers of rock. It was so exquisitely preserved that you can see the feathers. The researchers could even tell what the specimen had for lunch that day.

This picture comes from Eastern Kentucky University. It shows the skeleton as found and a reconstruction. Note however that the tail has been artfully hidden (not by me). I found a skeletal reconstruction on the Internet which suggests that the tail was still quite long and bony.

But E. hoyasi had very bird-like wings, and something even more exciting. This is the little spike on its wing marked ‘A’ in the picture. It is not a claw, like Archaeopteryx or Confuciusornis sported. This is an ‘alula’ or bastard wing. Now ‘bastard wing’ must be one of the most ill-named features in the history of anatomy. The alula is in fact an aerodynamic flap. It’s the magic fitment that enables all modern birds to fly slowly and to manoeuvre in tight places. (Modern aircraft have ‘leading edge’ flaps to do the same thing.) And E hoyasi was the first to sport it. It also had ‘perching’ claws – rather useful if you’re a small bird preferring to live in the trees. So everything about E. hoyasi except its tail is very much ‘modern bird’.

Apparently the fossil record of early bird-kind is pretty meagre. Their hollow bones normally rot away very quickly. But most if not all the essential features of modern birds, including their feathered aerodynamic tails, have been found from the ‘early Cretaceous’. This must be around the same time as E. hoyasi or very soon afterwards.

A birdwatcher from somewhat later, 65 Million years ago, will have seen a wide range of birds looking remarkably similar to those of today. (This comes from, I think, the Berkeley Website. And I assume that we are talking about before the K-T crash.)

Bird brains

In addition to losing their heavy jawbones, teeth and long bony tails, I reckon that the pressure will also have been on for birds to keep their brains small and light. A flying machine really does not need a heavy dead weight right out at the front on a stalk.

And yet birds may have had to increase their intelligence fast. Some scientists have developed sophisticated theories about the relationship between intelligence and brain size (more). I don’t see how these theories could possibly apply to birds. Some modern birds are seriously bright, but I’ve not read that their brains are any bigger. Be prepared for it to turn out that birds’ brains are considerably more efficient than ours.