Wednesday, 26 October 2011

Archaeopteryx Is Restored To Its Evolutionary Perch As The First Bird

A holotype fossil of the magpie sized Archaeopteryx lithographica
On the 28th of July 2011, a fossil discovery in China rocked the palaeontological world, destabilising Archaeopteryx lithographica's position as the first ever bird on Earth. Xiaotingia zhengi was, without a doubt a dinosaur,  but its morphology was so similar to that of Archaeopteryx that it led scientists to believe that the so called 'first bird' was actually more closely related to dinosaurs. However a recent study of the family tree of the avian dinosaurs has dispelled this mildly unwelcome development.

Phylogenetic trees are often redefined and now Dr Michael Lee from the University of Adelaide and Dr Trevor Worthy from the University of New South Wales, upon reorganising the evolutionary relationships between the first birds and their ancestral dinosaurs, has found new evidence which counters the difficulty of Xiaotingia. When they began to put together a new tree, they encountered a problem. In the new format, Archaeopteryx would have been placed with flightless dinosaurs or gliders which did not glide like birds.

Such a relationship would have meant that bird flight evolved more than once. As flight is a very special ability, it is highly unlikely that it evolved in bird-like dinosaurs and true birds separately. Therefore Archaeopteryx cannot be placed with the flightless dinosaurs or dissimilar gliders. Lee and Worthy were then able to use this new information in conjunction with other methods of analysis to view Archaeopteryx's true place in the evolution of birds.

When the duo looked at the creature's morphological traits, they found that the number of traits for dinosaurs and the number of traits for birds were equal. Yet just looking at the evolutionary tree is useless as there is a 50:50 chance for either relationship. A new technique was needed. Lee and Worthy used a method of analysis called 'maximum likelyhood.' This determines the rate of evolution in different traits by using genetic evidence applied to fossil evidence.

A trait such as a particular fur colour can evolve multiple times and therefore is not very reliable for forming solid evolutionary trees. Yet a unique trait, such as a backbone, takes far longer to evolve. It is less likely to evolve multiple times in separate animal groups, and is therefore, much more reliable for studying evolutionary relationships. When this method is applied to flight, it shows that this trait must only have evolved once. Archaeopteryx, with its wing structure and musculature must have flown like a bird.

By the law of maximum likelyhood Archaeopteryx is more closely related to birds than dinosaurs and so can be counted as the first ever avian on Earth. While it is possible that flight did evolve twice in two different animal groups, a form known as convergent evolution, such a scenario is unlikely and not very elegant in evolutionary terms. Lee finally stated that he would like to reapply the maximum likelyhood method to other animal groups, such as mammals, to dispel other doubts about genetic relationships and equal chance.