The evolution of the upright posture and gait--a review and a new synthesis

Abstract

During the last century, approximately 30 hypotheses have been constructed to explain the evolution of the human upright posture and locomotion. The most important and recent ones are discussed here. Meanwhile, it has been established that all main hypotheses published until the last decade of the past century are outdated, at least with respect to some of their main ideas: Firstly, they were focused on only one cause for the evolution of bipedality, whereas the evolutionary process was much more complex. Secondly, they were all placed into a savannah scenario. During the 1990s, the fossil record allowed the reconstruction of emerging bipedalism more precisely in a forested habitat (e.g., as reported by Clarke and Tobias (Science 269:521-524, 1995) and WoldeGabriel et al. (Nature 412:175-178, 2001)). Moreover, the fossil remains revealed increasing evidence that this part of human evolution took place in a more humid environment than previously assumed. The Amphibian Generalist Theory, presented first in the year 2000, suggests that bipedalism began in a wooded habitat. The forests were not far from a shore, where our early ancestor, along with its arboreal habits, walked and waded in shallow water finding rich food with little investment. In contrast to all other theories, wading behaviour not only triggers an upright posture, but also forces the individual to maintain this position and to walk bipedally. So far, this is the only scenario suitable to overcome the considerable anatomical and functional threshold from quadrupedalism to bipedalism. This is consistent with paleoanthropological findings and with functional anatomy as well as with energetic calculations, and not least, with evolutionary psychology. The new synthesis presented here is able to harmonise many of the hitherto competing theories.

Fig. 1

Phyletic diagram of apes and Homo comprising the last 20 Ma, with several fossils that appear in the text (changed after Sénut 2003). To the author's conviction, only the semiterrestrial ancestors of the human lineage are older than indicated here (see text)

Fig. 2

Baboon mother (Papio anubis) with infant riding on her back. In case of danger, the female baboon can escape with similar velocity as if without a riding offspring. Also note the limb proportions and the horizontal spine (cf. text)

Fig. 3

The molar teeth of a macaque (a M. nemestrina) are suitable for processing a great variety of food. Those of orang-utan (b M. nemestrina) or of gorilla (c M. nemestrina) are rather derived and specialized showing enamel ridges in order to break plant fibers (b), or they are massive and pointed to shred hard plant materials like wood (c). Besides new evolutionary acquisitions, the teeth of human beings (d H. sapiens) are much more conservative in many respects as they lack many specializations. No indications exist that these generalized functional features are secondary. It is much more likely that the basal anatomy of human teeth, like that of the intestinal tract, still resembles in several aspects that of an ancestral ecological generalist. The teeth of the macaque (a) may serve as a more or less appropriate model

Fig. 4

a Long-tailed macaque (also crab-eating monkey, M. fascicularis) female standing bipedally in the water and eating food retrieved from the ground. b Proboscis monkey (N. larvatus) wading bipedally. c Hanuman langurs (S. entellus) in Nepal, wading bipedally while collecting algae in a stream during the dry period. d Savannah baboons (P. cynocephalus) wading bipedally while picking flowers as food. e Chimpanzee (P. troglodytes) female wading bipedally with her infant drinking from her breast. All pictures are redrawn from photographs or films (see text and Table 1 for sources)

Fig. 5

Bornean orangutan (P. pygmaeus) hunting fish as food. The adult female is standing bipedally in hip-deep water near the steep shore holding onto a dead branch. In the situation shown, the prey was slain using a self-made club; the dead fish was picked from the water and eaten (Foto: Ullal)

Fig. 6

a At an angle of view of about 45°, two objects on the ground of a shallow water are noticed easily. b At an angle of about 35°, the objects are hardly to be seen. Hence, a higher position of the head may be of positive selective value (cf. text)

Fig. 7

Thermographic pictures of a a macaque (M. fuscata), b a pygmy chimpanzee (bonobo, Pan paniscus), c and d human beings. Besides their faces, both nonhuman primates have rather warm (orange and red) limbs, especially, however, the lower ones indicating that both primate species thermoregulate with these skin surfaces. In humans, the lower abdomen and the hind limbs are insulated well by a subcutaneous fatty layer. Human thermoregulatory surfaces are face, neck, shoulder, and the upper thoracic region. The evolutionary acquisition of this marked anatomical difference preconditions the need of our ancestors to insulate only the lower limbs and abdomen. To the author’s knowledge, no other cause is probable but a wading habit of the prehuman antecedents in the African waters (thermograms by Thomas Zimmermann, Thermographisches Institut Berlin)