The trapezius evolved separately, but the other three muscles in this group evolved from the first eight or ten ribs and the transverse processes of the cervical vertebrae (homologous to the ribs). The serratus anterior formed the basal unit for these three muscles. In higher primates it has evolved into two separate muscles — serratus anterior and levator scapulae — by concentration of the proximal and distal fibers and progressive reduction of the intermediate fibers.