Evolution Of Hominid Bipedalism Research Articles

The upright posture improves plantar stepping and alters responses to serotonergic drugs in spinal rats

Recent studies on the restoration of locomotion after spinal cord injury have employed robotic means of positioning rats above a treadmill such that the animals are held in an upright posture and engage in bipedal locomotor activity. However, the impact of the upright posture alone, which alters hindlimb loading, an important variable in locomotor control, has not been examined. Here we compared the locomotor capabilities of chronic spinal rats when placed in the horizontal and upright postures. Hindlimb locomotor movements induced by exteroceptive stimulation (tail pinching) were monitored with video and EMG recordings. We found that the upright posture alone significantly improved plantar stepping. Locomotor trials using anaesthesia of the paws and air stepping demonstrated that the cutaneous receptors of the paws are responsible for the improved plantar stepping observed when the animals are placed in the upright posture.We also tested the effectiveness of serotonergic drugs that facilitate locomotor activity in spinal rats in both the horizontal and upright postures. Quipazine and (±)-8-hydroxy-2-(dipropylamino)tetralin hydrobromide (8-OH-DPAT) improved locomotion in the horizontal posture but in the upright posture either interfered with or had no effect on plantar walking. Combined treatment with quipazine and 8-OH-DPAT at lower doses dramatically improved locomotor activity in both postures and mitigated the need to activate the locomotor CPG with exteroceptive stimulation. Our results suggest that afferent input from the paw facilitates the spinal CPG for locomotion. These potent effects of afferent input from the paw should be taken into account when interpreting the results obtained with rats in an upright posture and when designing interventions for restoration of locomotion after spinal cord injury.

Bipedality in chimpanzee (Pan troglodytes) and bonobo (Pan paniscus): Testing hypotheses on the evolution of bipedalism

A host of ecological, anatomical, and physiological selective pressures are hypothesized to have played a role in the evolution of hominid bipedalism. A referential model, based on the chimpanzee (Pan troglodytes) and bonobo (Pan paniscus), was used to test through experimental manipulation four hypotheses on the evolution of hominid bipedalism. The introduction of food piles (Carry hypothesis) increased locomotor bipedality in both species. Neither the introduction of branches (Display hypothesis) nor the construction of visual barriers (Vigilance hypothesis) altered bipedality in either species. Introduction of raised foraging structures (Forage hypothesis) increased postural bipedality in chimpanzees. These experimental manipulations provided support for carrying of portable objects and foraging on elevated food-items as plausible mechanisms that shaped bipedalism in hominids.

The evolution of bipedalism in hominids and reduced group size in chimpanzees: alternative responses to decreasing resource availability

One hypothesis for the evolution of hominid bipedalism is that bipedalism was more efficient than quadrupedalism for long-distance terrestrial locomotion, and was favored when resources became scarcer and more widely separated during the drying of African forests in the Miocene. Here we extend this scenario for the evolution of bipedalism based on principles of behavioral ecology of extant primates. Daily travel distance is not only an increasing function of resource scarcity, but also of group size (because of intragroup scramble competition). When faced with Miocene drying, hominoids were forced to evolve either energetically more efficient locomotory abilities or smaller group sizes. The cost of the latter strategy is that smaller groups are less successful than larger groups in intergroup contest competition (smaller groups are supplanted from limiting resources). Among the earliest hominids, bipedalism may have been favored over small group size as an alternative energetic response to decreased resource availability. The alternative was to maintain quadrupedal locomotion but evolve fission–fusion grouping to reduce daily travel distance for individuals and hence, their energetic costs of travel. We suggest that this strategy represents the evolutionary pathway taken by chimpanzees. This divergence of strategies may have been a result of inherent differences in feeding ecology, but could also have been enhanced by the pre-empting of niche space by the two closely related and presumably competing hominoid ancestors of humans and chimpanzees. If so, it could have been a potential factor in the speciation process that led to modern humans and chimpanzees.

Chimpanzee Hand Preference in Throwing and Infant Cradling: Implications for the Origin of Human Handedness

Calvin (i983) has hypothesized that the neurophysiological, perceptual, and cognitive demands of throwing may have served as important evolutionary precursors to a variety of traits( e.g., handedness, tool use, and language processing) in early hominids. Eighty-eight percent of humans throw with their right hands (Healey, Liederman, and Geschwind I986), and Calvin has argued that this right-handed throwing evolved as a result of a left-hemisphere specialization for planned sequential movements. He has further suggested that right-handed throwing would have been more prevalent in females than in males because females predominantly carried infants on the left arm, leaving the right hand available for other actions. Infant carrying has also been hypothesized as a major selective feature in the evolution of bipedalism in hominids (Leakey I976).

Surface electromyography of superficial back muscles in human children: Functions during vertical climbing and suspension and implications for the evolution of hominid bipedalism

Surface electromyograms were taken of electrical activity in the trapezius, latissimus dorsi and caudal erector spinae muscles of human children as they elevated their superior extremities overhead and engaged in suspensory behaviors on a cargo net. Muscle activity patterns were determined and results were compared with electromyography (EMG) data derived from studies of juvenile pongid apes (Tuttle & Basmajian, 1976) in order to assess the problem of muscular economy and efficiency. Results from this preliminary investigation qualitatively verified Prost's (1980) hypothesis concerning the relative inefficiency of suspensory behaviors in man. Based on EMG data, these behaviors were less economical in Homo compared to pongid apes since selection presumably favored adaptations for efficient bipedalism at the expense of adaptations for efficacious climbing during the hominid evolutionary career. Some of these changes involved energy sparing regimes of superficial back muscle activity important for economical hanging and climbing. Relationships between the mode of attachment of the latissimus dorsi muscle and observed hominoid positional behaviors were elucidated within the context of lower back stability.

Evolution of hominid bipedalism and prehensile capabilities

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.