Evolution of hominid bipedalism and prehensile capabilities

Abstract

In this paper, I present an updated version of the hylobatian model for the proximate ancestors of the Hominidae. The hylobatians are hypothesized to have been relatively small creatures that were especially adapted for vertical climbing on tree trunks and vines and for bipedalism on horizontal boughs. They were no more disposed toward suspensory behaviours than are modern chimpanzees and bonobos. According to this evolutionary scenario, bipedalism preceded the emergence of the Hominidae. The earliest hominids would be recognized as diurnally terrestrial bipeds that stood with full extension of the knee joints and walked with greater extension of the lower limbs than is common in non-human primates that are induced to walk bipedally on the ground. The wealth of hominid fossils from the Hadar Formation, Ethiopia, and the Laetolil Formation, Tanzania, are generally compatible with the hylobatian model. They show that by ca . 4 Ma B.P. habitually terrestrial, bipedal hominids had evolved from arboreal ancestors. The Hadar hominids had curved fingers and toes, strong great toes and thumbs, and other features that suggest that they were rather recently derived from arboreal hominids and that they probably continued to enter trees, perhaps for night rest and some foraging. The hominid hand bones from Hadar evince no features that are distinctly related to knuckle-walking. They relate neatly to counterparts in the hand of O.H. 7, a specimen that was found with stone tools. However, there is no evidence that the Hadar hominids of 3 Ma ago engaged in tool behaviour.

Biomechanics of phalangeal curvature

“…In conclusion, the strain differences between curved and straight phalanges illustrated here support the common assertion that phalangeal shaft curvature is related to the strains associated with arboreal and especially suspensory activity (e.g., Oxnard, 1973;Preuschoft, 1973b;Susman, 1979;Tuttle, 1981;Rose, 1988;Sarmiento, 1988;Stern et al, 1995;Richmond, 1998), and may underlie changes in curvature during ontogeny in response to changes in mechanical environments of arboreal and terrestrial supports (Richmond, 1998(Richmond, , 2003.…”

Origin of human bipedalism: The knuckle-walking hypothesis revisited

“…However, Tuttle (1969a) noted that the relevant metacarpal and radius morphology was not preserved. The subsequent discovery that metacarpal heads of Australopithecus lacked knucklewalking structures, while many aspects of the skeleton point to arboreal climbing adaptations, seemed to confirm this view (Tuttle, 1981).…”

“…1) grew out of a better understanding of the biomechanics involved in these locomotor strategies, and the striking biomechanical similarities between climbing and bipedalism (e.g., Fleagle et al, 1981). In addition, these hypotheses are supported by the many apelike arboreal traits present in early hominin fossils (e.g., Tuttle, 1981;Stern and Susman, 1983;Senut and Tardieu, 1985;Hunt, 1998).…”

“…Thus, there is no clear "time limit" on the retention of ancestral traits (e.g., pentadactyly). It should be noted that this issue is not the same as the debate over the functional significance of primitive retentions, such as whether or not the "arboreal" traits retained in Australopithecus are indicative of arboreal locomotion (e.g., Stern and Susman, 1983;Tuttle, 1981;Latimer, 1991;Richmond, 1999). In other words, a key aspect about the debate over the origin of bipedalism is the persistence of primitive retentions, not their biological role.…”

“…Most researchers would expect the earliest bipeds to retain some aspects of the locomotor adaptations of their immediate nonbipedal ancestors. Some researchers are open to the possibility that such retentions, although perhaps altered, may be present in Australopithecus, or even modern humans, millions of years after the origins of bipedalism (Corruccini, 1978;Corruccini and McHenry, 2001;Tuttle, 1981;McHenry, 1984;Latimer, 1991;Begun, 1993a;Shea and Inouye, 1993;Gebo, 1996;Ward et al, 1999a;Richmond and Strait, 2000). These authors reason that, because natural selection acts on existing morphology, traces of former adaptations can persist in descendents.…”