Levels of Abstraction in Orangutan (Pongo abelii) Categorization

Levels of abstraction have rarely been manipulated in studies of natural concept formation in nonhumans. Isolated examples have indicated that animals, relative to humans, may learn concepts at varying levels of abstraction with differential ease. The ability of 6 orangutans (Pongo abelii) of various ages to make natural concept discriminations at 3 levels of abstraction was therefore investigated. The orangutans were rewarded for selecting photos of orangutans instead of humans and other primates (concrete level), primates instead of other animals (intermediate level), and animals instead of nonanimals (abstract level) in a 2-choice touch screen procedure. The results suggest that, like a gorilla (Gorilla gorilla gorilla) tested previously (Vonk & MacDonald, 2002), orangutans can learn concepts at each level of abstraction, and unlike other nonhumans, most of these subjects rapidly learned the intermediate level discrimination.

An Assessment of Generalization of Imitation in Two Enculturated Orangutans (Pongo pygmaeus)

Deferred imitation of object-related actions and generalization of imitation to similar but not identical tasks were assessed in two human-reared (enculturated) orangutans (Pongo pygmaeus). Each ape displayed low levels of deferred imitation but did not provide evidence of generalization of imitation. Results suggest that enculturated orangutans may not possess social-cognitive abilities similar to that which enculturated chimpanzees are assumed to possess.

Tool-use and tool-making by captive, group-living orangutans (Pongo pygmaeus abelii) at an artificial termite mound

The present study examined the use and making of tools to obtain foodstuffs in artificial-mound holes by five captive, group-living Sumatran orangutans (Pongo pygmaeus abelii). Three adult orangutans frequently stripped leaves and twigs from a branch provided (tool-making), and then inserted the tool into a hole to obtain foodstuffs (tool-using). A 5-year-old female juvenile usually used the tools that adult orangutans had previously used, but rarely made tools herself. A 2-year-old male infant did not use any tools. The adult orangutans tend to leave one to several leaves at the top of the branch than to leave many leaves on the branch or to strip all leaves. It seemed likely that tools with appropriate leaves are easier to insert into holes and obtain more foodstuffs, compared with branches with many leaves or sticks without any leaves. When the orangutans were unable to insert a tool into a hole, they usually modified the tool and/or changed their tool-using technique, such as changing how they grasped the tool. These findings are discussed from the perspectives of the orangutan's behavioral flexibility regarding tool-use skills and hierarchical organization in food-processing techniques.

NEGATIVE-PRESSURE PULMONARY EDEMA COMPLICATED BY ACUTE RESPIRATORY DISTRESS SYNDROME IN AN ORANGUTAN (PONGO PYGMAEUS ABELII)

A 22-yr-old, 86-kg, morbidly obese female orangutan (Pongo pygmaeus abelii) was immobilized and transported to the Denver Zoological Gardens hospital for a routine physical examination. Immediately after arriving at the hospital, cyanosis and apparent inadequate ventilatory efforts were noted. Clinically significant hypoxia occurred despite attempts to ventilate the orangutan through face mask, and attempts to place an endotracheal tube began. A large volume of pink-tinged frothy fluid flowed from the trachea when the laryngoscope was inserted into the oropharynx. Severe pulmonary edema due to negative-pressure pulmonary edema, precipitating life-threatening hypoxia was suspected. The orangutan was maintained on a mechanical ventilator using the neuromuscular blocking agent cisatracurium besylate and sedation with periodic doses of isoflurane and midazolam for 48 hr. Positive end-expiratory pressure was used while the orangutan was ventilated mechanically to improve respiratory function. The edema and hypoxia improved, but respiratory arrest ensued 30 min after extubation, when the orangutan was removed from mechanical ventilation. Necropsy and histopathology demonstrated that serious lung injury had led to acute respiratory distress syndrome.

A study of host defence peptide beta-defensin 3 in primates

We have investigated the molecular evolution of the gene coding for beta-defensin 3 (DEFB103) in 17 primate species including humans. Unlike the DEFB4 genes (coding for beta-defensin 2) [Boniotto, Tossi, Del Pero, Sgubin, Antcheva, Santon and Masters (2003) Genes Immun. 4, 251-257], DEFB103 shows a marked degree of conservation in humans, Great Apes and New and Old World monkeys. Only the Hylobates concolor defensin hcBD3 showed an amino acid variation Arg17-->Trp17 that could have a functional implication, as it disrupts an intramolecular salt bridge with Glu27, which locally decreases the charge and may favour dimerization in the human congener hBD3. This is thought to involve the formation of an intermolecular salt bridge between Glu28 and Lys32 on another monomer [Schibli, Hunter, Aseyev, Starner, Wiencek, McCray, Tack and Vogel (2002) J. Biol. Chem. 277, 8279-8289]. To test the role of dimerization in mediating biological activity, we synthesized hBD3, hcBD3 and an artificial peptide in which the Lys26-Glu27-Glu28 stretch was replaced by the equivalent Phe-Thr-Lys stretch from human beta-defensin 1 and we characterized their structure and anti-microbial activity. Although the structuring and dimerization of these peptides were found to differ significantly, this did not appear to affect markedly the anti-microbial potency, the broad spectrum of activity or the insensitivity of the anti-microbial action to the salinity of the medium.

Microarray analysis of nonhuman primates: validation of experimental models in neurological disorders

Nonhuman primates (NHPs) have provided robust experimental animal models for many human-related diseases due to their similar physiologies. Nonetheless, profound differences remain in the acquisition, progression, and outcome of important diseases such as AIDS and Alzheimer's, for which the underlying basis remains obscure. We explored the utility of human high-density oligonucleotide arrays to survey the transcription profile of NHP genomes. Total RNA from prefrontal cortices of human (Homo sapiens), common chimpanzee (Pan troglodytes), cynomolgous macaque (Macaca fascicularis), and common marmoset (Callithrix jacchus) was labeled and hybridized to Affymetrix U95A GeneChip probe arrays. Corresponding data obtained previously from common chimpanzee and orangutan (Pongo pygmaeus) were added for comparison. Qualitative (present or not detected) and quantitative (expression level) analysis indicated that many genes known to be involved in human neurological disorders were present and regulated in NHPs. A gene involved in dopamine metabolism (catechol-O-methyltransferase) was absent in macaque and marmoset. Glutamate receptor 2 was up-regulated, and transcription-associated genes were down-regulated in NHPs compared with humans. We demonstrate that transcript profiling of NHPs could provide comparative genomic data to validate and better focus experimental animal models of human neurological disorders.

Scoliosis in an Orangutan

STUDY DESIGN

The first case of scoliosis in an Orangutan spine is reported.

OBJECTIVES

To study the nature of scoliosis in the spinal specimen, and to determine the etiologic significance to human idiopathic scoliosis.

SUMMARY OF BACKGROUND DATA

Idiopathic scoliosis has not been observed in primates other than man. Previous studies highlighted the importance of erect posture in the development of idiopathic scoliosis in man.

METHODS

A spinal model of an orangutan spine was studied in great detail to determine its nature. The methods used included plain radiographs, computed tomography scans, a three-dimensional plastic model using rapid prototyping, and dissection of the frozen specimen.

RESULTS

Features similar to human idiopathic scoliosis were noted including a right-side curve, vertebral rotation to convexity, displacement of the spinal cord to the concavity, and an equal number of ribs on either side. No evidence of congenital anomaly was found. Atypical findings included male gender, a short curve, and kyphosis at the apex.

CONCLUSIONS

Although scoliosis found in the orangutan has features similar to idiopathic scoliosis, there also are some dissimilar features, making this diagnosis unlikely. The features observed in this spine suggest that erect posture is important in the morphology of human idiopathic scoliosis.

Statistical alignment based on fragment insertion and deletion models

MOTIVATION

The topic of this paper is the estimation of alignments and mutation rates based on stochastic sequence-evolution models that allow insertions and deletions of subsequences ('fragments') and not just single bases. The model we propose is a variant of a model introduced by Thorne et al., (J. Mol. Evol., 34, 3-16, 1992). The computational tractability of the model depends on certain restrictions in the insertion/deletion process; possible effects we discuss.

RESULTS

The process of fragment insertion and deletion in the sequence-evolution model induces a hidden Markov structure at the level of alignments and thus makes possible efficient statistical alignment algorithms. As an example we apply a sampling procedure to assess the variability in alignment and mutation parameter estimates for HVR1 sequences of human and orangutan, improving results of previous work. Simulation studies give evidence that estimation methods based on the proposed model also give satisfactory results when applied to data for which the restrictions in the insertion/deletion process do not hold.

AVAILABILITY

The source code of the software for sampling alignments and mutation rates for a pair of DNA sequences according to the fragment insertion and deletion model is freely available from http://www.math.uni-frankfurt.de/~stoch/software/mcmcsalut under the terms of the GNU public license (GPL, 2000).

Understanding the dynamics of Spinocerebellar Ataxia 8 (SCA8) locus through a comparative genetic approach in humans and apes

Spinocerebellar Ataxia 8 (SCA8) is a neurodegenerative disorder caused by expansion of a trinucleotide repeat. We undertake a comparative genetic analysis among human populations and primate species in the normal variation range, where forces that shaped present diversity can be recognised. We determinate number of repeats of the short tandem repeat through allele length sizing and sequencing methods. Human allele distributions are very similar among populations, ruling out ethnicity as a genetic risk for allele expansion. Primate comparison shows human-specific features, with longer human alleles due to a novel variable trinucleotide repeat, not present in non-human primates, which increased the disease-causing expansion likelihood. SCA8 seems to be a human specific disease.

Orangutan cultures and the evolution of material culture

Geographic variation in some aspects of chimpanzee behavior has been interpreted as evidence for culture. Here we document similar geographic variation in orangutan behaviors. Moreover, as expected under a cultural interpretation, we find a correlation between geographic distance and cultural difference, a correlation between the abundance of opportunities for social learning and the size of the local cultural repertoire, and no effect of habitat on the content of culture. Hence, great-ape cultures exist, and may have done so for at least 14 million years.

Refinement of a chimpanzee pericentric inversion breakpoint to a segmental duplication cluster

BACKGROUND

Pericentric inversions are the most common euchromatic chromosomal differences among humans and the great apes. The human and chimpanzee karyotype differs by nine such events, in addition to several constitutive heterochromatic increases and one chromosomal fusion event. Reproductive isolation and subsequent speciation are thought to be the potential result of pericentric inversions, as reproductive boundaries form as a result of hybrid sterility.

RESULTS

Here we employed a comparative fluorescence in situ hybridization approach, using probes selected from a combination of physical mapping, genomic sequence, and segmental duplication analyses to narrow the breakpoint interval of a pericentric inversion in chimpanzee involving the orthologous human 15q11-q13 region. We have refined the inversion breakpoint of this chimpanzee-specific rearrangement to a 600 kilobase (kb) interval of the human genome consisting of entirely duplicated material. Detailed analysis of the underlying sequence indicated that this region comprises multiple segmental duplications, including a previously characterized duplication of the alpha7 neuronal nicotinic acetylcholine receptor subunit gene (CHRNA7) in 15q13.3 and several Golgin-linked-to-PML, or LCR15, duplications.

CONCLUSIONS

We conclude that, on the basis of experimental data excluding the CHRNA7 duplicon as the site of inversion, and sequence analysis of regional duplications, the most likely rearrangement site is within a GLP/LCR15 duplicon. This study further exemplifies the genomic plasticity due to the presence of segmental duplications and highlights their importance for a complete understanding of genome evolution.

Referential Understanding of Videos in Chimpanzees (Pan troglodytes), Orangutans (Pongo pygmaeus), and Children (Homo sapiens)

Performance on identical search tasks based on cues directly perceived or indirectly perceived through video were compared among a group of 4 adult chimpanzees (Pan troglodytes), a group of 2 adult orangutans (Pongo pygmaeus), and a group of 36 children (between 2 and 3 years of age). Children comprehended directly perceived cues but had difficulty with video cues. In contrast, chimpanzees and 1 orangutan were successful in using video to guide their search for a hidden object. Two follow-up studies with 3-year-old children demonstrated the importance of more distinct perceptual and verbal cues in aiding children's understanding of video as referring to real-world events.

Localized hypoplasia of the primary canine in bonobos, orangutans, and gibbons

This paper extends observations by Lukacs ([1999] Am. J. Phys. Anthropol. 110:351-363; [2001] Am. J. Phys. Anthropol. 116:199-208) of localized hypoplasia of the primary canine (LHPC) among large apes to gibbons, bonobos, and orangutans. LHPC is a roughly circular area of deficient enamel on the labial surface of primary canine teeth which, on current evidence from humans, forms several months after birth due to malnutrition-induced craniofacial osteopenia, leading to crypt fenestration that exposes the dental follicle and more deep-sited ameloblasts to minor physical traumata during normal motor infant development. Our goal was to determine the prevalence of LHPC among a variety of apes which differ in body mass and socioecology, with a view to elucidating the etiology of the defect. We examined juvenile dentitions from 122 animals from three taxa: 8 Hylobates lar, 75 Pongo pygmaeus from Borneo and Sumatra, and 39 Pan paniscus from central Africa. Reported variables include taxon, sex, arcade, side, and tooth size. Presence/absence and ordinal severity of defect expression were recorded by description, microphotography, and scanning electron microscopy. Molds were taken in high-resolution dental impression materials and cast in epoxy resin. There are clear taxonomic, but no sex, differences. Prevalence ranged from 0.0% in gibbons to 61.5% in bonobos and 85.3% in orangutans. The result for orangutans is similar to that reported by Lukacs ([1999] Am. J. Phys. Anthropol. 110:351-363), while bonobos are much more affected than were the common chimpanzees (22%) described by Lukacs ([1999] Am. J. Phys. Anthropol. 110:351-363). There are no significant antimeric differences, but the lower canine is much more affected than the upper by LHPC. We show that larger teeth are more affected by LHPC and have more severe defects. Also, we encountered several instances of patent or healing canine crypt fenestrations, occasionally in direct association with LHPC. Location of the defect indicates that LHPC may occur perinatally but more usually several months postnatally. Histological examination showing the neonatal line and LHPC is required to resolve the issue of timing. We concur with Lukacs ([1999] Am. J. Phys. Anthropol. 110:351-363) that taxonomic, anatomical, and environmental variables combine to determine the occurrence and appearance of LHPC. Nevertheless, we conclude that LHPC probably reflects deficient growth of the arcades in infant apes and humans.

Variation of variable number of tandem repeat sequences in the 3'-untranslated region of primate dopamine transporter genes that affects reporter gene expression

Genetic polymorphism has been reported in the 3'-untranslated region (3'-UTR) of the human dopamine transporter (DAT) gene and the variable number of tandem repeat (VNTR) polymorphism has been proposed to be associated with normal personal traits or psychoneurological disorders. To assess the variation of this region in nonhuman primates, we amplified the VNTR regions by the polymerase chain reaction in several species of apes and monkeys, and determined their DNA sequences. The 3'-UTR of the chimpanzee DAT gene was also polymorphic and alleles with one or two unit(s) of a 40 bp sequence were found, while all gorillas and orangutans examined had only 2-repeat allele. Cynomolgus macaques and African green monkeys shared 11- or 12-repeat and 5-repeat alleles, respectively. By performing transient transfection assay, we found that most of the VNTR sequences of nonhuman primates exhibited higher activities on reporter gene assay as compared to those of human 9-, 10- and 11-repeat sequences.

Quantifying temporal bone morphology of great apes and humans: an approach using geometric morphometrics

The hominid temporal bone offers a complex array of morphology that is linked to several different functional systems. Its frequent preservation in the fossil record gives the temporal bone added significance in the study of human evolution, but its morphology has proven difficult to quantify. In this study we use techniques of 3D geometric morphometrics to quantify differences among humans and great apes and discuss the results in a phylogenetic context. Twenty-three landmarks on the ectocranial surface of the temporal bone provide a high level of anatomical detail. Generalized Procrustes analysis (GPA) is used to register (adjust for position, orientation and scale) landmark data from 405 adults representing Homo, Pan, Gorilla and Pongo. Principal components analysis of residuals from the GPA shows that the major source of variation is between humans and apes. Human characteristics such as a coronally orientated petrous axis, a deep mandibular fossa, a projecting mastoid process, and reduced lateral extension of the tympanic element strongly impact the analysis. In phenetic cluster analyses, gorillas and orangutans group together with respect to chimpanzees, and all apes group together with respect to humans. Thus, the analysis contradicts depictions of African apes as a single morphotype. Gorillas and orangutans lack the extensive preglenoid surface of chimpanzees, and their mastoid processes are less medially inflected. These and other characters shared by gorillas and orangutans are probably primitive for the African hominid clade.

Genomic structure and evolution of the ancestral chromosome fusion site in 2q13-2q14.1 and paralogous regions on other human chromosomes

Human chromosome 2 was formed by the head-to-head fusion of two ancestral chromosomes that remained separate in other primates. Sequences that once resided near the ends of the ancestral chromosomes are now interstitially located in 2q13-2q14.1. Portions of these sequences had duplicated to other locations prior to the fusion. Here we present analyses of the genomic structure and evolutionary history of >600 kb surrounding the fusion site and closely related sequences on other human chromosomes. Sequence blocks that closely flank the inverted arrays of degenerate telomere repeats marking the fusion site are duplicated at many, primarily subtelomeric, locations. In addition, large portions of a 168-kb centromere-proximal block are duplicated at 9pter, 9p11.2, and 9q13, with 98%-99% average sequence identity. A 67-kb block on the distal side of the fusion site is highly homologous to sequences at 22qter. A third ~100-kb segment is 96% identical to a region in 2q11.2. By integrating data on the extent and similarity of these paralogous blocks, including the presence of phylogenetically informative repetitive elements, with observations of their chromosomal distribution in nonhuman primates, we infer the order of the duplications that led to their current arrangement. Several of these duplicated blocks may be associated with breakpoints of inversions that occurred during primate evolution and of recurrent chromosome rearrangements in humans.

Human-specific organization of primary visual cortex: alternating compartments of dense Cat-301 and calbindin immunoreactivity in layer 4A

There is evidence that the cortical anatomy of the magnocellular (M) visual pathway, which carries information about motion and luminance contrast, was modified in human evolution. Recent results indicate that layer 4A of humans contains a meshwork of tissue bands that stain densely for nonphosphorylated neurofilament (NPNF), a protein that is preferentially expressed in elements of the M pathway, whereas apes and monkeys lack a comparable pattern. Here we examined the distribution of staining for Cat-301 -- a monoclonal antibody well established to stain M-related structures preferentially -- in area V1 of humans, apes (chimpanzees, orangutan), Old World monkeys (macaques) and New World monkeys (spider monkeys, squirrel monkeys). Single-staining experiments, using a peroxidase-tetramethylbenzidine (TMB) reaction, revealed alternating zones of dark and light staining for Cat-301 in layer 4A of humans, similar to those observed with NPNF. Double-staining studies in humans revealed that Cat-301-immunoreactive somas and neuropil were localized within the same tissue bands that stained strongly for NPNF and, furthermore, that these bands alternated with irregularly shaped territories that stained very strongly for calbindin. Nonhuman primates, by contrast to humans, displayed weak Cat-301 and calbindin staining in layer 4A. The co-localization of Cat-301 and NPNF in human layer 4A, and the weak staining for these molecules in layer 4A of other primates, suggests that the cortical representation of the M channel was modified in recent human evolution. The calbindin-rich compartments in human layer 4A cannot be related to a particular geniculostriate pathway on neurochemical grounds; they may constitute an interneuronal population that increased in human evolution.

Molecular basis of evolutionary loss of the alpha 1,3-galactosyltransferase gene in higher primates

Galactose-alpha1,3-galactose (alphaGal) epitopes, the synthesis of which requires the enzyme product of alpha1,3-galactosyltransferase (alpha1,3GT), are sugar chains on the cell surface of most mammalian species. Notable exceptions are higher primates including Old World monkeys, apes, and humans. The alphaGal-negative species as well as mice with deletion of the alpha1,3GT gene produce abundant anti-alphaGal antibodies. The evolutionary loss of alphaGal epitopes has been attributed to point mutations in the coding region of the gene. Because no transcripts could be found in the higher primate species with Northern blot analysis, a potential alternative explanation has been loss of upstream regulation of the gene. Here, we have demonstrated that the rhesus promoter is functional. More importantly, a variety of full-length transcripts were detected with sensitive PCR-based methods in the tissues of rhesus monkeys, orangutans, and humans. Five crucial mutations were delineated in the coding region of the human and rhesus and three in the orangutan, any one of which could be responsible for inactivation of the alpha1,3GT gene. Two of the mutations were shared by all three higher primates. These findings, which elucidate the molecular basis for the evolutionary loss of alphaGal expression, may have implications in medical research.

Pneumatized spaces, sinuses and spongy bones in the skulls of primates

The earliest attempts to understand the "pneumatized spaces" in the skulls of primates in general were focussed on the hollow spaces and the epithelium which covers their surfaces. More recent approaches consider the sinuses as a means to optimise skull architecture. Still, many attempts to get hold of the meaning of the intriguing pneumatized spaces circle around the air filled volumes they enclose. Here, we would like to reverse the approach and focus our biomechanic interpretation on the walls surrounding the big, empty, or at least not mechanically resistant spaces, and their mechanical properties. As a working hypothesis, we consider not only the walls of the more or less closed cavities, or sinuses, but also the braincase, the orbits, and the nasal channel as thin-walled shells of which we know that they can carry surprisingly large loads with a minimum of material. Details of the wall's profiles fit with this approach. From the same viewpoint, the bubble-like, air-filled cavernous systems in the ethmoid or temporal bones, and the marrow-filled spongy substance in the upper jaw are looked at as honeycomb-structures, which provide mechanical properties that are biologically advantageous and allow the saving of weight.

Maintenance of self-imposed delay of gratification by four chimpanzees (Pan troglodytes) and an orangutan (Pongo pygmaeus)

Delay maintenance, which is the continuance over time of the choice to forgo an immediate, less preferred reward for a future, more preferred reward, was examined in 4 chimpanzees (Pan troglodytes) and 1 orangutan (Pongo pygmaeus). In the 1st experiment, the apes were presented with 20 chocolate pieces that were placed, one at a time, into a bowl that was within their reach. The apes could consume the available chocolate pieces at any time during a trial, but no additional pieces would be given. The total length of time taken to place the 20 items into the bowl ranged from 60 s to 180 s. All 5 apes delayed gratification on a majority of trials until all 20 chocolate pieces were presented. Unlike in most experiments with human children using this test situation, attention by the apes to the reward was not detrimental to delay maintenance. In a 2nd experiment with the chimpanzees only, 4 foods of differing incentive value were presented in different trials in the same manner as in Experiment 1. The chimpanzees were highly successful in obtaining all food pieces, and there was no difference in performance as a function of food type.

Human paralogs of KIAA0187 were created through independent pericentromeric-directed and chromosome-specific duplication mechanisms

KIAA0187 is a gene of unknown function that maps to 10q11 and has been subject to recent duplication events. Here we analyze 18 human paralogs of this gene and show that paralogs of exons 14-23 were formed through satellite-associated pericentromeric-directed duplication, whereas paralogs of exons 1-9 were created via chromosome-specific satellite-independent duplications. In silico, Northern, and RT-PCR analyses indicate that nine paralogs are transcribed, including four in which KIAA0187 exons are spliced onto novel sequences. Despite this, no new genes appear to have been created by these events. The chromosome 10 paralogs map to 10q11, 10q22, 10q23.1, and 10q23.3, forming part of a complex family of chromosome-specific repeats that includes GLUD1, Cathepsin L, and KIAA1099 pseudogenes. Phylogenetic analyses and comparative FISH indicates that the 10q23.1 and 10q23.3 repeats were created in 10q11 and relocated by a paracentric inversion 13 to 27 Myr ago. Furthermore, the most recent duplications, involving the KIAA1099 pseudogenes, have largely been confined to 10q11. These results indicate a simple model for the evolution of this repeat family, involving multiple rounds of centromere-proximal duplication and dispersal through intrachromosomal rearrangement. However, more complex events must be invoked to account for high sequence identity between some paralogs.

Tripedal knuckle-walking: a proposal for the evolution of human locomotion and handedness

A comparative morphological analysis of human and non-human hominoids was conducted in an attempt to determine the mode of locomotion of the protohominid. Although the generalized hominoid anatomy permits variation of locomotion: brachiation, knuckle-walking, etc., minor variations in structure determine which behavior is favored. Arboreal arm swinging requires a flexible forelimb while terrestrial fist or knuckle-walking demands more rigidity of the hand and wrist. It is demonstrated that the large human thumb accompanied by the strong adduction of the thenar, hypothenar, and palmar interosseous muscles offer powerful rigidity to the hand, while fusion of the os centrale with the scaphoid during gestation permits the formation of an arch of carpals which imbue the wrist with the stability necessary for weight bearing. Fascialization of the contrahentes and dorsiepitrochlearis muscles in the human as well as depilation of the middle phalanges; the webbing (syndactyly) of the palm; the direction of the fibers of the interosseous membrane of the forearm; the shape of the puerile annular ligament, and the direction of the human glenoid fossa strongly suggest that the ancestor of man used a knuckle-walking form of locomotion prior to becoming bipedal. A model is presented that suggests that bipedalism was attained through an intermediate stage of tripedalism. The model is based on the fact that man's anatomy is much more asymmetric than that of the great apes. A presumption is made that due to the absence of trees for climbing in the transition from forest to open plain, the protohominid needed to carry tools (stones) at all times for protection. Stones could be carried for long distances on the posterior iliac crest since the weight would be shifted posteriorly over the legs. Pick up, medial rotation and adduction of the stone would employ a two-muscle chain of biceps brachii and latissimus dorsi. On the iliac crest, the stone is posterior to the coronal plane of the glenohumeral joint, and with the contraction of this two-muscle chain, the shoulder on one side is moved posteriorly effecting a semi-erect posture. It is proposed that tripedalism of the protohominid may be an explanation for the handedness unique to hominids.