Alan has always been a birder. In the 1950s, his father used to take him bird watching on Graveney Marshes in North Kent (what a great shame they are now to be more or less completely covered in solar panels). During his earning days, amongst other assignments, he was the environmental manager for a large consultancy in the oil and power industries. He has two qualifications, including one in ecology with a distinction, and one in evolutionary studies. He has his Osprey badge for twenty years volunteering for the RSPB doing bird and plant surveys, brush cutting, chainsaw work and banging in fence posts! He is a strong supporter of the RSPB and Wild Justice. He has a particular interest in the campaign to protect our birds of prey by banning driven grouse shooting.
Where Did Our Birds Come From?
Since about the mid-1990s, the great majority of palaeontologists have agreed that our birds today are direct descendants of the group of the dinosaurs. Having said this their evolutionary story is not as simple as all that.
Firstly, what were or are dinosaurs? They were groups of animals that evolved from a very ancient stock called archosaurs, ancient lizards, about 230 million years ago. The archosaurs gave rise to a number of reptile groups including, crocodiles, pterosaurs and dinosaurs. What distinguished the dinosaurs from the other archosaurs and made them so successful was that they evolved a pelvis that enabled them to walk upright so that their legs were directly under their body. This meant their legs took their body weight, as our legs take our body weight, rather than them having a sprawling action with arms and legs out to their sides like crocodiles and other reptiles.
There were two main groups of dinosaurs: the ornithischians which were almost all vegetarians like the Iguanodon, and which sometimes walked on two legs and sometimes on four. The other main group was the Saurischians which broke into two sub groups, the sauropods and the theropods. The sauropods were the huge long neck and long tailed animals that were vegetarian and walked on four legs. The theropods were the carnivorous dinosaurs, like tyrannosaurus rex and velociraptor. They were always bi-pedal. It is the theropods with which we are concerned when discussing bird evolution.
Charles Darwin postulated in his famous book The Origin of Species that missing links, especially in the form of fossils would probably be found which would have similarities between two separate groups of animals. Sure enough, not long after Darwin’s publication, a 125 million year old fossil was found in Germany which looked half bird and half theropod dinosaur. There were clear imprints in the very fine grained rock of its asymmetric feathers arranged just as the primary and secondary feathers of birds are today. The fossil also had a wish bone and a small breast bone. However, as well as these bird features it had theropod dinosaur features such as a long bony tail, rows of sharp teeth, claws on the ends of its arms or wings, three forward pointing toes and one backward pointing toe just as birds have today. The fossil was called Archaeopteryx, ancient wing. It was about as big as a magpie. It is thought it could fly but only for short distances.
However, for a long time after the discovery of this fossil, it was debated whether birds had evolved separately from the original archosaur stock as pterosaurs did, or whether they had evolved directly from theropod dinosaurs. There were good arguments on both sides of the case. Then, in the mid 1990s, some sensational fossils were discovered in China. These were clearly fossils of small theropod dinosaurs; slightly younger than archaeopteryx, and they too showed impressions of feathers. In other words, it showed that feathers, a much more complex item than hair, were not just confined to birds but were present in at least one major group of theropod dinosaurs. This group were called maniraptoran theropod dinosaurs. In common with fossil birds and todays birds, they possessed a half moon shaped bone in their wrist which enabled them to fold their wrist back on itself so that the hand could be held alongside the fore arm and quickly brought forward as soon as required, just as birds do when they want fly. These features clinched the argument for birds being descendants of maniraptoran theropod dinosaurs. The alternative argument of birds evolving along a separate line from archosaur base stock was finally set aside.
The numerous bird and theropod fossils found since archaeopteryx show that bird evolution moved forward in fits and starts. Sometimes fossils showed some birds had lost their teeth but still had a long bony tail. Other fossils show birds had lost their bony tail but still had teeth and claws on their wings.
At the end of the Cretaceous period 66 million years ago when the bird fauna consisted of birds with long bony feathery tails, birds that still had teeth, birds that had claws on their wings and birds more like todays birds, there is evidence of a significant increase in carbon dioxide in the atmosphere due to extensive volcanic eruptions. At the same time, to make matters a lot worse, it is now clear the Earth was hit by an enormous asteroid at least 10 kilometres in length traveling at about 17,000 miles an hour. The impact was on the edge of the Caribbean and was like a huge nuclear bomb exploding. It shot enormous amounts of dust and debris into the atmosphere causing a nuclear winter such that the sun was obscured for probably hundreds of years causing photosynthesis to collapse. This in turn caused the extinction of about 70% of life on Earth. All the Ornithischian and Sauropod dinosaurs disappeared along with almost all the theropod dinosaurs including all the birds that had teeth and long bony tails. The only dinosaurs left were birds that looked not dissimilar to our birds today. It is thought that gamebirds, geese such as the magpie goose from Australia, and some waders, at least, survived the effects of the asteroid impact. It is also thought that most of the animals and plants that survived did so in the southern hemisphere especially Australia, New Zealand and Antarctica. In other words, as far from the asteroid impact as it is possible to be. At the end of the Cretaceous period Antarctica was still joined to Australia, and was quite warm with forests and no hint of ice. It is thought some life could still just about hang on in these regions.
Modern DNA analysis of our northern hemisphere birds indicates that quite a number are very distantly related to birds in these southern continents, and that many of our present day birds are probably descended from the ancient survivors that gradually spread across the world and recolonised it after the disaster of the asteroid impact.
Sometimes, our arrogance and belief in human supremacy over the natural world, leads us to look down on birds and other animals, but birds are better than humans in so many ways. They have a very different and much more efficient breathing system which enables them to undertake long distant migration, for example, to enable the bar headed geese to fly across the Himalayas. They have better hearing and digestive systems than us and, of course, their navigation abilities never cease to amaze us. It is very unlikely that these extraordinary capabilities have all evolved since the asteroid disaster They have to be further examples of their dinosaur inheritance.
The story of bird evolution is a fascinating one. There is still a lot to find out. However, we are beginning to have a few stakes in the ground in contrast to all the time leading up to about the 1970s when we knew almost nothing about where our birds came from, and what we thought we knew was mostly wrong. There are still many important questions to answer, for example, did all our birds evolve from a single line of maniraptoran dinosaurs or did they evolve from more than one line? What groups of birds survived and came through the catastrophe of the asteroid impact and which groups have evolved since then? We have some hints of incomplete answers to a few of these questions but further discoveries about bird evolution will continue to amaze us for a long time into the future.