The Origin of Birds is an early synopsis of bird evolution written in 1926 by Gerhard Heilmann, a Danish artist and amateur zoologist. The book was born from a series of articles published between 1913 and 1916 in Danish, and although republished as a book it received mainly criticism from established scientists and got little attention within Denmark. The English edition of 1926, however, became highly influential at the time due to the breadth of evidence synthesized as well as the artwork used to support its arguments. It was considered the last word on the subject of bird evolution for several decades after its publication. Through the course of the research represented in the book, Heilmann considers and eventually rejects the possibility of all living and several extinct groups of reptiles as potential ancestors for modern birds, including crocodilians, pterosaurs and several groups of dinosaurs. Despite his acknowledgment that some of the smaller Jurassic theropods had many similarities to Archaeopteryx and modern birds, he determined that they were unlikely to be direct bird ancestors and that they were instead closely–related offshoots, and concluded that the similarities were a result of convergent evolution rather than direct ancestry. Based essentially on a process of elimination, Heilmann arrives at the conclusion that birds must be descended from thecodonts, a group of archosaurs that lived during the Permian and Triassic periods. Although this conclusion was later shown to be inaccurate, The Origin of Birds was regarded as a masterful piece of scholarship at the time and set the international agenda for research in bird evolution for nearly half a century, and much of its research remains of interest.
Background When Heilmann began his research in the early 1900s, the early bird Archaeopteryx was only known from three fossils found in the limestone quarries of Solnhofen near Eichstätt, Germany. The three fossils consisted of two nearly complete skeletons found in 1861 and 1877 and a single feather from 1860. They had been discovered just a few decades after the discovery of the dinosaurs, and as some dinosaurs appeared somewhat birdlike, Archaeopteryx was regarded as a possible "missing link" between reptiles and birds by many paleontologists at the time. The similarities between Archaeopteryx, known dinosaurs and extant birds were examined and emphasized, with Thomas Huxley championing the idea that Archaeopteryx as well as modern birds had more in common with theropod dinosaurs than any other group of animals. This was at the time in opposition to the view of anatomist Sir Richard Owen of the British Museum, who viewed Archaeopteryx as no different taxonomically from modern birds. Huxley's work was controversial, and this climate of uncertainty and contention about bird origins persisted well into the beginning of the 20th century. While the dinosaur-bird connection (or lack thereof) was being pursued in paleontology, the problem of the evolution of flight was under scrutiny as well. It was observed that a number of animals with moderate flying or gliding ability, such as bats, flying lizards and flying squirrels have arboreal lifestyles. This led to the idea that the ancestors of birds must have gradually acquired the ability to fly from leaping among branches in the tops of trees. The Hungarian paleontologist Franz Nopcsa proposed an alternate hypothesis in 1907, arguing that the ancestors of birds were fast-running, bipedal animals related to theropod dinosaurs. When Heilmann came onto the paleontology scene, these two sets of conflicting theories provided the framework for his research and eventual conclusions.
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