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The first feathered, birdlike fossils have all been classified as Archeopteryx and were discovered in Bavaria in the mid-nineteenth century. There is no fossil evidence of this creature having been descended from dinosaurs, nor do other birdlike creatures appear in the fossil record for another 10 million years. In fact, these specimens (of which there are only three completely) are not thought of as being birds but only lend scientists an idea of how the evolution of birds occurred.

The first birds are thought to have arisen from one of two groups of dinosaurs, which are classified according to their hip structure. Saurician dinosaurs have the same pelvic structure as modern day reptiles and are thought to have given rise to reptiles. Ornithichian dinosaurs have a pelvis more similar to modern day birds, and are thought to have given rise to this group.

Before the cretaceous period (135 to 65 million years ago) few birds are present in the fossil records. This suggests that it is during this period that the major diversification of this group took place.

The first birds were likely large, flightless, ground-dwelling creatures, resembling modern day ostriches and emus. Flight is what separates birds from most other organisms, and is a relatively modern adaptation that allowed this group to rapidly exploit a large number of new habitats. Flight also allowed birds to escape rapidly from terrestrial predators and is a faster means of travel, so greater distances can be covered more quickly in search of food or mates.

Flight was made possible through the evolution of feathers. Feathers are thought to be analogous to reptilian scales and may have first served as insulation before evolving for purposes of flight. Initial flights are thought to have actually been in animals that glided between places, and then eventually evolved into what we think of as flight today.

Hollow bone structure is another adaptation that birds have evolved in order to make flight more efficient. Thicker heavy bones are not suitable for flight, whereas the hollow bone structure is not only lighter, but its mere physiology helps to supply birds' muscles with the high concentrations of oxygen needed for continuous flight.

Homeothermy, or the ability to maintain a single body temperature, is another adaptation seen in birds. While some argue that dinosaurs may well have been homeotherms, it is likely that the transition from heterothermy to homeothermy occurred somewhere late in the evolution of reptiles (since mammals are homeotherms too). Homeothermy allowed birds to exploit an even greater number of habitats, as they were no longer constrained by environmental temperatures.

It is likely that the evolution of birds was no simple matter, and involved a number of complicated intermediate forms that have not been preserved (or discovered) in the fossil record. The presence of different species today having different feeding adaptations lends insight to the high propensity this group has to exploit niches and adapt to changing environments.

Learn more about this author, Tina Lehman.
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