The furcula, the forked chest bone that children snap for luck, has long carried an evolutionary mystery: anatomists could not agree whether it formed from the two clavicles alone or incorporated a third, central element. A study led by Sebastián Jiménez-Villalón, who completed his doctorate at the Universidad de Chile, with Alexander Vargas of the Faculty of Sciences, Joao Botelho of the Pontificia Universidad Católica and colleagues at the Universidade de São Paulo, has now answered the question by watching the bone assemble itself in bird embryos.
Published in Science Advances under the title Embryos resolve the enigmatic origin of the theropod wishbone, the work compared embryonic development in four species: the chicken, the zebra finch, the budgerigar and the Chilean tinamou. The tinamou proved decisive. Its furcula forms from three cartilaginous elements, two lateral clavicles and a medial interclavicle, the central chest bone familiar from reptiles and dinosaurs. The elements fuse before ossification, leaving no visible sutures in the adult skeleton.
That invisible seam explains why the fossil record has been so unhelpful: the fusion happens too early to leave a trace. Chickens, by contrast, build their furcula from the clavicles alone, suggesting the central component was later lost in some lineages. The authors propose the three-way fusion occurred in early bipedal theropods whose reduced forelimbs placed little mechanical demand on the shoulder girdle, long before powered flight existed.
Only later did the enlarged, V-shaped furcula connect to the sternum and take on its modern role as a spring that stores and releases energy with each wingbeat, a striking case of an evolutionary vestige recruited for a new function. The team is careful to frame the interpretation as testable rather than settled, pointing to the molecular signals controlling the fusion as the next target for investigation.