A different kind of developmental recombination is represented by the famous two-legged goat described by the Dutch morphologist Slijper (1942), in which a correlated shift in morphology and behavior accommodated an induced abnormality, leading to the well coordinated production of a complex and individually advantageous adjustment, producing a novel phenotype with little or no genetic change (Rachootin and Thomson, 1981). Slijper’s two-legged goat was born with a congenital defect of the front legs so that it could not walk on all fours, and so it learned to walk and run by using its hind legs alone. Then, when it died an accidental death, Slijper dissected it and documented remarkable changes in muscle and bone, including striking changes in the bones of the hind legs; the leg muscles, including a greatly thickened and elongated gluteal tongue and an innovative arrangement of small tendons, a modified shape of the thoracic skeleton, and extensive modifications of the pelvis (West-Eberhard, 2003, p. 53).
It is not known whether the bipedal goat’s abnormal front legs were due to a genetic or an environmentally induced defect, but in either case the inducer acted as a novel switch mechanism that in effect controlled the expression of a whole suite of correlated and adaptive changes in behavior, muscle, and bone. Even though the event that caused these changes was random with respect to adaptation, the phenotypic result was not a random variant. Rather, it was an adaptive accommodation of a random input, the result of pushing to extremes developmental plasticity in behavior, muscle, and bone that had already been subjected to a long history of selection and adaptive evolution. Similar effects on behavior and morphology are quite common in quadripedal mammals, including primates, forced or trained to walk upright (West-Eberhard, 2003, p. 42, Figure 3.12 on bipedal baboon; Hirasaki et al., 2004; see also descriptions of a bipedal macaque in Waldman, 2004, and a bipedal dog in KFOR, 2003). These observations raise the possibility that the two-legged-goat effect, or “phenotypic accommodation” (West-Eberhard, 2003, 2005), has played a role in the evolution of bipedal locomotion in vertebrates, including humans, as suggested by Slijper, who noted that some of the novel morphological features of the two-legged goat resembled those of kangaroos and of other bipedal species such as orangutans (Slijper, 1942). Japanese macaques experimentally taught to walk upright develop humanlike gait characteristics (Hirasaki et al., 2004), suggesting that the evolution of bipedalism in humans might not be as difficult or as large an evolutionary step as some anthropologists have believed. The distinctive anatomical features of humans compared with other primates that are associated with bipedal running include changes in muscle mass, tendon length, and thorax and pelvis shape (Bramble and Lieberman, 2004), the same features that underwent striking alterations in the bipedal goat