The Upright Ape: A New Origin of the Species

Do we really know when bipedalism emerged? It is credited by most scholars as the keystone event in human evolution and customarily employed as a hominin synapomorphy. Biologist and neurosurgeon Aaron G. Filler tries to demolish this view on the basis of vertebral anatomy. His views are both refreshing and provocative, and may cause astonishment and incredulity. The book nonetheless contains many accurate anatomical observations and elaborate biomechanic arguments that deserve further consideration. The book is intended for the general public. The first – essentially introductory – 8 chapters follow the Gouldian tradition by intermingling historical passages with recent scientific advancements, further vindicating the role of natural historians such as Goethe and Geoffroy Saint-Hilaire. The result is satisfactory in itself but disproportionately long with respect to the 2 main – and much more technical – chapters of the book, where the author expounds his heterodox views on hominoid evolution. The author does not deny adaptation through natural selection but stresses the role of constraints, and although his proposals are formulated in the light of recent discoveries of evolutionary developmental biology (‘evo-devo’), his radical views on the abrupt origin of new body plans are exceedingly close to Goldschmidt’s [Goldschmidt, 1940] hopeful monster. According to Filler’s ‘humanian’ hypothesis, upright bipedalism has characterized the main hominoid lineage from the Early Miocene onwards, being further enhanced in australopiths and humans, but secondarily lost in the several ape lineages. Filler proposes a chain of bipedal ancestors of humans and apes, with the ‘diagonograde’ body plan of extant great apes representing a secondary reversal to a quadrupedal condition. Filler’s anatomical arguments mainly rely on the modern appearance of the Moroto vertebra, which according to him belongs to an ‘upright biped’ (p. 253). This initial transformation favouring bipedalism would have subsequently acted as a powerful phylogenetic constraint requiring many secondary adaptive readjustments for stiffening the lumbar region when orang-utans and African apes reacquired a quadrupedal gait. Given the molecular phylogeny of extant hominoids, the author favours the hypothesis that knuckle-walking evolved twice. This is not new, but the author provides additional arguments based on lower back anatomy, thus contradicting the nowadays more widely (and uncritically) accepted troglodytian hypothesis, which postulates a knuckle-walking stage before the advent of bipedalism. Given the pervasive nature of homoplasy in hominoid evolution, and previous suggestions that suspension might have independently evolved in pongines and hominines, a separate origin of knuckle-walking in gorillas and chimps seems plausible, albeit difficult to test given the elusive nature of the African ape fossil record. Published online: April 24, 2008