Thermogenesis, muscle hyperplasia, and the origin of birds. The origin of birds from their reptilian ancestors has been classically considered to be driven by the evolution of flight. In this issue (pages 653–656) Stuart Newman presents a new theory of bird origins that emphasizes instead the adaptive role of enlarged skeletal muscles. The common ancestor of birds and lizards lost the vertebrate gene for uncoupling protein-1 (UCP1) the product of which is essential for the generation of heat by brown fat, a tissue that protects newborns of mammals from hypothermia. This created an imperative for the direct ancestors of birds (but not “cold-blooded” lizards) to increase the mass of skeletal muscle, the main source of body heat in modern birds and their young. Evolutionary expansion of the thigh muscles led as a side-effect to bipedality. This in turn provided the opportunity for expansion of the breast muscles and freed up the forelimbs for other adaptive changes, such as flying or swimming.
Cover by S. Newman and S. Seif