Origin of birds mitochondrial uncoupling protein, UCP1, which is thermogenic. In mammals, UCP1 functions within brown adipose tissue to protect newborns against hypothermia. In modern birds, skeletal muscle serves a similar function and is presumed to have done so in their ancestors. In this view, bipedality and other avian skeletal alterations were side effects of muscle hyperplasia, with further evolutionary modifications of the forelimbs, including adaptations for flight or swimming, and vestigiality, being secondary consequences of two-leggedness. "Archaeopteryx" has historically been considered the first bird, or "Urvogel". Although newer fossil discoveries filled the gap between theropods and "Archaeopteryx", as well as the gap
Specimens of Archaeopteryx under his control and full exposed, his subsequent study and monograph was the first comprehensive description of an "Archaeopteryx" specimen that had not been disarticulated. The Berlin specimen of "Archaeopteryx" is to date the most complete existing specimen of this genus, and is widely regarded as one of the most beautiful fossils in the world. Its well-preserved skeleton and the preserved feathers of its wings and tail have made it of considerable interest to a wide range of scientific study, beginning with Wilhelm Dames and Carl Vogt shortly following its discovery. This iconic fossil preserves the animal with an articulated