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birds

An evolutionary tale

An evolutionary tale

It was a Sunday morning and my gumboot-wedded feet and I were strolling languidly through a landscape carpeted in snow. It stretched all the way up the valley and past the scoria cones where it hugged the majestic peaks of the interior with a delightful frigidity.

The air was so crisp and still; a tranquil remnant of the storm’s wake. The only inklings of sound were the distant murmurs of chattering king penguins and the intermittent cries of giant petrels. I thought about how strangely dinosaur-like these petrels sounded and began imagining them as the reptiles from which they descended. I came across a colony and stopped and stared at them like a Japanese tourist; my brain twitching as it tried to unravel the immediate mysteries surrounding the evolution of birds from dinosaurs. Why did dinosaurs suddenly develop feathers? What did they use them for if they couldn’t fly? And how did they even develop feathers in the first place? I thought about this long and hard and eventually did some reading.

I’m sure that for some of you

Archaeopteryx lithographica

will sound familiar and for others it won’t. Either way, welcome to the world of the most famous transitional fossil linking birds and reptiles. This feathered reptile was discovered in a limestone quarry in Germany in 1860. ‘Archaeopteryx’ means “ancient wing” and ‘lithographica’ comes from the Solnfern limestone in which it was found. Archaeopteryx lived around 145 million years ago and although most of its traits were reptilian, it possessed asymmetrical feathers (symbolic of aerodynamic flight) and an opposable big toe (probably used for perching). So it had both very bird-like and very reptilian-like features; what evolutionists call a “mosaic”. This fossil was revolutionary (or evolutionary) in the sense that it bridged the divide between fossils of fairly modern birds, which appear about 70 million years ago, and those of their theoretical ancestors, the theropods, which were agile, carnivorous dinosaurs that walked on two legs and lived around 200 million years ago. Have a look at the figure below (extracted from Why Evolution is True by Jerry Coyne) and think about the transitional similarities in the skeletal structures of

Compsognathus

,

Archaeopteryx

and the chicken. From the bottom up you can see the reptilian tail shrinking, the teeth disappearing, the claws fusing together, and the appearance of a large breastbone to anchor flight muscles.

After the discovery of

Archaeopteryx

, no other reptile-bird intermediates were found for many years, leaving a gaping hole between modern birds and their ancestors. Then, in the mid-1990s, a veritable parade of feathered theropods were discovered in the lake sediments of China. Two of these are shown below (extracted from Why Evolution is True by Jerry Coyne);

Sinornithosaurus millenii

, the “Chinese bird-lizard”, and

Microraptor gui

, the “four-winged dinosaur”.

Theropod dinosaurs didn’t just have primitive bird-like features, it seems; they even behaved in bird-like ways. One fossil shows a feathered female theropod who met her end while sitting on her nest of twenty-two eggs, showing brooding behaviour similar to that of birds. Another fossil shows a small feathered theropod named

Mei long

, Chinese for “soundly sleeping dragon”, sleeping with its head tucked under its folded, wing-like forearm – exactly as birds do today (see below; extracted from Why Evolution is True by Jerry Coyne).

Okay, so there’s all this evidence buried in the apple skin of our earth that indicates that once upon a time there existed transitional forms of reptiles and birds, but it still doesn’t tell us why or how the first dinosaurs developed feathers. The truth is nobody is sure. Some scientists have suggested that feathers derive from the same cells which give rise to reptilian scales, and that they developed in these cold-blooded reptiles to increase insulation and help maintain body temperature, but not everyone agrees. Whatever the case, they did develop, and that is a fact.

Another important fact to consider is that early carnivorous dinosaurs evolved longer forelimbs and hands, which probably helped them grab and handle prey. That kind of grabbing would favour the evolution of muscles that would quickly extend the front legs and pull them inward: exactly the motion used in the downstroke of true flight.

So, once feathers arrived on the scene and started having beneficial effects on the survival of those individuals who possessed them, it was all up to natural selection to turn them into the flying descendents they would become over the next hundred million years. Let me explain…

The year was 150 million BC and a certain theropod dinosaur was trying to outrun a predator. He didn’t, but some of his more feathered relatives did. They managed to escape because their partially feathered forearms acted as running aids, making them faster and more agile. Those theropods who were best adapted to escaping their predators went on to reproduce and pass their genes onto the next generation. In each generation there are likely to be genetic mutations; it’s just the way life is, you can’t always make a perfect copy. So in the next generation most of the theropods would be partially feathered, however, there would also be some with slightly fewer or shorter feathers and some with slightly more or longer feathers. This generation would be subject to the same predation pressures and natural selection would once again make sure the better adapted theropods would survive and go on to reproduce. And it goes on and on until one day BAM! there goes a flying theropod.

But hey, this is just one scenario and the fact that these changes were induced solely by predation pressure isn’t necessarily so. There may have been other evolutionary drivers such as competition for resources and there’s always the “tree down” theory. For instance there is evidence that some theropods lived in trees. Feathery forearms would have allowed them to glide from tree to tree (or tree to ground), helping them escape predators, find food more readily and cushion their falls. Whatever the case, feathered forearms were clearly an advantage and it was inevitable that as natural selection began to favour those who could fly farther instead of merely gliding, leaping or flying for short bursts, flight would soon become one of the most widespread and well used innovations of evolution. The incredible diversity of birds found today is testament to this fact.

Unfortunately, like many other groups, birds have suffered massively as a result of anthropogenic ways. Of the more than 10000 bird species that humans have co-existed with, 150 have become extinct, 189 are critically endangered, and over 1500 are listed as vulnerable or threatened. On the whole, the status of birds worldwide is getting worse and worse every day as more forests are burned, more wetlands are bulldozed, more pesticides are sprayed, more power lines and cell phone towers are erected, more invasive aliens and diseases are spread, and more longline fishing hooks are deployed. The situation is dire, but the important part is that more people are becoming aware and positive change is happening. Still, there are those who may argue that we don’t really need birds, so why waste time and money trying to save them? Well, then I must exclaim that these people clearly don’t understand or appreciate the significance of life, diversity and the overwhelmingly beautiful products of millions of years of gradual evolution.