Evolutionary history

Dating back to the year 1758 when the tenth edition of the Systema Naturae of the famous Carl Linnaeus was published, birds are referred to by zoologists as Aves. However, there is a dissent among scientists on the proper definition of the term. As first suggested by Jacques Gauthier (1986), I prefer considering Aves as being represented by the last (i.e. the most recent) common ancestor of all extant birds and all its descendants. According to this definition the term Aves is synonymous to modern (or crown) birds. In contrast, numerous avian researchers still adhere to the tradition of using the term Aves in a broader sense to include the famous Archaeopteryx and other bird-like fossils. In this case, modern birds are referred to as Neornithes. 

 

Fossils not included in modern or crown birds, but still closer to this group than to any other group of living organisms, are treated as members of the avian stem group. These fossils either pertain to the ancestral lineage, in which case they represent direct ancestors of the crown birds, or to extinct side branches. Stem-group and crown birds together constitute the pan-Aves, or total birds. The informal name "birds" should be restricted to crown-group taxa, while the term "protobirds" might be used for representatives of the stem group. Pan-Aves originated ~250 million years ago, when the last (most recent) common archosaurian ancestor split into two species, one giving rise to the crocodilian and the other to the bird lineage. 

 

Reconstructions of phylogenetic relationships among representatives of the avian stem group are based on fossilised bones. Nuclear and mitochondrial DNA sequences, routinely used to reconstruct relationships among extant groups, are usually not available. As a consequence, inferring relationships among members of the stem group is inherently problematic and the figure below depicts just one possible hypothesis. Despite this limitation, a relatively coherent picture of avian stem-group evolution has developed over the last decades:

 

Reconstruction of the evolutionary history of birds. Evolution from the earliest avian ancestors towards modern birds progresses along the ancestral avian lineage (red line). For many clades the supposed age is given in million years (Ma), mostly following Allen et al. (2021). Note that the popular Dinosauria clade comprises almost all stem-group taxa (except Pterosauromorpha, Lagerpetonidae and Marasuchus) as well as modern birds. This means that dinosaurs didn't become extinct (a popular misconception), but are represented in the present-day fauna by modern birds. In fact, dinosaurs probably never were as diverse as they are today being reperesented by approximately 10,500 bird species. Crocodiles, alligators, and caimans (Crocodilia) are the closest extant relatives of birds.

 


The successive transformation into more and more bird-like creatures started with the enlargement of the rear limbs. Initially, ancestral birds possibly ran on their hind legs only for short periods, while moving on all four legs most of the time. Bipedalism may have evolved in order to enhance the acceleratory phase when trying to escape from predators. In the course of evolution, ancestral birds became lighter and smaller, showing increased levels of activity and higher body temperatures. All living birds have body temperatures above 40°C.

 

Along with increased body-temperature, birds developed proto-feathers for better insulation and possibly display. Later the forelimbs became significantly longer than the hindlimbs, and the long bony tail disappeared. Ultimately, complex contour feathers developed and birds learned to fly in order to effectively escape from earth-bound predators. Considering the flight behaviour of extant tinamous and landfowl, avian flight almost certainly evolved from the ground with a subsequent gliding phase

 

Today, some 10,500 bird species are inhabiting the Earth, but almost six hundred species have already gone extinct in historic times as a result of human activities, primarily due to deforestation and hunting (Sayol et al., 2020). 

 

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