The evolution of birds

It is generally believed that birds evolved from reptiles and are the nearest living creatures to the dinosaurs, the last of which suffered extinction some 65 million years ago. However, the exact evolutionary route from Tyrannosaurus Rex to house sparrow is not so easy to work out.

Dinosaurs to birds

It is known that the skies 150 million years ago were patrolled by massive winged reptiles called pterosaurs, but these were not the ancestors of modern birds. For one thing, they did not have feathers and, for another, they belong to a different branch of the evolutionary tree.

Dinosaurs, by which is meant land-dwelling reptiles of the Triassic, Jurassic and Cretaceous eras (251 to 65 million years ago) can be divided into two main branches, the Saurischians and the Ornithischians, based on their hip structure. Confusingly enough, birds evolved from the first-named group, not the second. They are therefore more closely related to carnivorous reptiles such as Tyrannosaurus and Velociraptor than herbivores such as Iguanodon and Stegosaurus.

Archaeopteryx

The idea that birds evolved from dinosaurs was first proposed after the discovery in Germany in 1861 of the fossilised impression of a creature from 150 million years that was clearly a reptile but which also had feathers. This was named Archaeopteryx (meaning “ancient feather”) and was widely hailed as being an intermediary between dinosaurs and birds, given that the structure of its bones and the arrangement of the feathers made it entirely possible for Archaeopteryx to fly. It would have been about the size of a modern raven. The discovery came only two years after the publication of Darwin’s “The Origin of Species” and was seen as confirmation of Darwin’s conjecture that species could evolve into new ones.

However, there are doubts over whether Archaeopteryx was the direct ancestor of modern birds, or whether that honour belongs to a creature related to Archaeopteryx that has not, as yet, turned up in the fossil record. Another complication has been the discovery of fossils (in China in 1998) of dinosaurs that had feathers but clearly were not suited for flight.

The suggestion is therefore that feathers evolved for a purpose other than enabling a creature to fly. It is quite possible that the original function of feathers was to provide insulation or to attract mates, and that Archaeopteryx and its relatives and descendants adapted them as a means of achieving flight.

One theory is that an ancestor of Archaeopteryx might have hopped along the ground and grabbed insects out of the air. Those that jumped higher would get the most insects and the ability to flap the wings and take off altogether evolved from that activity.

So where might birds have come from?

The fossil record includes later specimens that show how early bird species might have developed. These include Ichthyornis and Hesperornis from the Upper Cretaceous (100 million years ago).

The virtual disappearance of the dinosaurs 65 million years leaves open the question of how the bird ancestors managed to survive, especially if the reason for the disappearance was a strike on Earth by a large meteor or small asteroid, resulting in worldwide destruction of plant life due to vast dust clouds preventing proper sunlight from reaching the surface for a number of years. It has been suggested that the small size of proto-birds might have been a factor, coupled with the insulation provided by feathers and their ability to fly to whatever food sources were available.

In the post-dinosaur world the evolution of birds took off in many directions, aided by the fact that winged flight enabled birds to reach locations that presented many different ecological challenges that in turn led to further evolutionary development. By 60 million years ago the ancestors of modern herons, vultures and kingfishers are known to have been living, and most of the bird families in existence today are known to have evolved by the end of the Miocene (11 million years ago).

Continuing evolution

Evolutionary changes are still taking place in bird populations. Charles Darwin noted how the finches of the Galapagos Islands had evolved different feeding strategies to cope with the variety of food available between different islands within the same group, and those changes would have taken place relatively recently in the geological timescale, given that the islands themselves had only emerged from the ocean within the past four million years.

An even more recent example of bird adaptability leading to species evolution is that of the feral or urban pigeon which has developed characteristics that are quite distinct from its rural cousin the woodpigeon, due to having abandoned the cliffs of coast and mountain for the manmade cliffs of city buildings.

What all this shows is that evolution is a process that affects all living creatures, as it has done ever since the only life forms on the planet were viruses and bacteria.

© John Welford