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 suggest that it had some black colouration which included the coverts (or possibly the secondary remiges). Carney pointed out that this is consistent with what we know of modern flight characteristics, in that black melanosomes have structural properties that strengthen feathers for flight. The preservation of the fossil feather is unusual. In contrast to the feather imprints found on the full skeletons of "Archaeopteryx", the isolated feather is preserved as a dark film, which could be composed either of organic matter (as in most fossil feathers) or as inorganic minerals. Though it is theoretically possible to determine which it is,