Nucleotide diversity in gorillas

Comparison of the levels of nucleotide diversity in humans and apes may provide valuable information for inferring the demographic history of these species, the effect of social structure on genetic diversity, patterns of past migration, and signatures of past selection events. Previous DNA sequence data from both the mitochondrial and the nuclear genomes suggested a much higher level of nucleotide diversity in the African apes than in humans. Noting that the nuclear DNA data from the apes were very limited, we previously conducted a DNA polymorphism study in humans and another in chimpanzees and bonobos, using 50 DNA segments randomly chosen from the noncoding, nonrepetitive parts of the human genome. The data revealed that the nucleotide diversity (pi) in bonobos (0.077%) is actually lower than that in humans (0.087%) and that pi in chimpanzees (0.134%) is only 50% higher than that in humans. In the present study we sequenced the same 50 segments in 15 western lowland gorillas and estimated pi to be 0.158%. This is the highest value among the African apes but is only about two times higher than that in humans. Interestingly, available mtDNA sequence data also suggest a twofold higher nucleotide diversity in gorillas than in humans, but suggest a threefold higher nucleotide diversity in chimpanzees than in humans. The higher mtDNA diversity in chimpanzees might be due to the unique pattern in the evolution of chimpanzee mtDNA. From the nuclear DNA pi values, we estimated that the long-term effective population sizes of humans, bonobos, chimpanzees, and gorillas are, respectively, 10,400, 12,300, 21,300, and 25,200.     

Interactions between zoo‐housed great apes and local wildlife

Although there are published reports of wild chimpanzees, bonobos, and orangutans hunting and consuming vertebrate prey, data pertaining to captive apes remain sparse. In this survey‐based study, we evaluate the prevalence and nature of interactions between captive great apes and various indigenous wildlife species that range into their enclosures in North America. Our hypotheses were threefold: (a) facilities housing chimpanzees will report the most frequent and most aggressive interactions with local wildlife; (b) facilities housing orangutans and bonobos will report intermediate frequencies of these interactions with low levels of aggression and killing; and (c) facilities housing gorillas will report the lowest frequency of interactions and no reports of killing local wildlife. Chimpanzees and bonobos demonstrated the most aggressive behavior toward wildlife, which matched our predictions for chimpanzees, but not bonobos. This fits well with expectations for chimpanzees based on their natural history of hunting and consuming prey in wild settings, and also supports new field data on bonobos. Captive gorillas and orangutans were reported to be much less likely to chase, catch and kill wildlife than chimpanzees and bonobos. Gorillas were the least likely to engage in aggressive interactions with local wildlife, matching our predictions based on natural history. However unlike wild gorillas, captive gorillas were reported to kill (and in one case, eat) local wildlife. These results suggest that some behavioral patterns seen in captive groups of apes may be useful for modeling corresponding activities in the wild that may not be as easily observed and quantified. Furthermore, the data highlight the potential for disease transmission in some captive settings, and we outline the associated implications for ape health and safety. Am. J. Primatol. 71:458–465, 2009. © 2009 Wiley‐Liss, Inc.     

Demographic history and genetic differentiation in apes

Comparisons of genetic variation between humans and great apes are hampered by the fact that we still know little about the demographics and evolutionary history of the latter species. In addition, characterizing ape genetic variation is important because they are threatened with extinction, and knowledge about genetic differentiation among groups may guide conservation efforts. We sequenced multiple intergenic autosomal regions totaling 22,400 base pairs (bp) in ten individuals each from western, central, and eastern chimpanzee groups and in nine bonobos, and 16,000 bp in ten Bornean and six Sumatran orangutans. These regions are analyzed together with homologous information from three human populations and gorillas. We find that whereas orangutans have the highest diversity, western chimpanzees have the lowest, and that the demographic histories of most groups differ drastically. Special attention should therefore be paid to sampling strategies and the statistics chosen when comparing levels of variation within and among groups. Finally, we find that the extent of genetic differentiation among "subspecies" of chimpanzees and orangutans is comparable to that seen among human populations, calling the validity of the "subspecies" concept in apes into question.     

Leaf Surface Roughness Elicits Leaf Swallowing Behavior in Captive Chimpanzees (Pan troglodytes) and Bonobos (P. paniscus), but not in Gorillas (Gorilla gorilla) or Orangutans (Pongo abelii)

Researchers have described apparently self-medicative behaviors for a variety of nonhuman species including birds and primates. Wild chimpanzees, bonobos, and gorillas have been observed to swallow rough leaves without chewing, a behavior proposed to be self-medicative and to aid control of intestinal parasites. Researchers have hypothesized that the presence of hairs on the leaf surface elicits the behavior. We investigated the acquisition and the underlying mechanisms of leaf swallowing. We provided 42 captive great apes (24 chimpanzees, six bonobos, six gorillas, and six orangutans) with both rough-surfaced and hairless plants. None of the subjects had previously been observed to engage in leaf swallowing behavior and were therefore assumed naïve. Two chimpanzees and one bonobo swallowed rough-surfaced leaves spontaneously without chewing them. In a social setup six more chimpanzees acquired the behavior. None of the gorillas or orangutans showed leaf swallowing. Because this behavior occurred in naïve individuals, we conclude that it is part of the behavioral repertoire of chimpanzees and bonobos. Social learning is thus not strictly required for the acquisition of leaf swallowing, but it may still facilitate its expression. The fact that apes always chewed leaves of hairless control plants before swallowing, i.e., normal feeding behavior, indicates that the surface structure of leaves is indeed a determinant for initiating leaf swallowing in apes where it occurs.     

Hand preferences for coordinated bimanual actions in 777 great apes: implications for the evolution of handedness in hominins

Whether or not nonhuman primates exhibit population-level handedness remains a topic of considerable scientific debate. Here, we examined handedness for coordinated bimanual actions in a sample of 777 great apes including chimpanzees, bonobos, gorillas, and orangutans. We found population-level right-handedness in chimpanzees, bonobos and gorillas, but left-handedness in orangutans. Directional biases in handedness were consistent across independent samples of apes within each genus. We suggest that, contrary to previous claims, population-level handedness is evident in great apes but differs among species as a result of ecological adaptations associated with posture and locomotion. We further suggest that historical views of nonhuman primate handedness have been too anthropocentric, and we advocate for a larger evolutionary framework for the consideration of handedness and other aspects of hemispheric specialization among primates.     

DNA DIVERGENCE AMONG HOMINOIDS

We have determined the degree of single-copy DNA divergence among the extant members of the Hominoidea employing the technique of DNA-DNA hybridization. The species studied include humans, two species of chimpanzees, gorillas, two subspecies of orangutans, and two species of gibbons; as an outgroup we have used a member of the Old World monkeys (Cercopithecidae), the baboon. Our methods are different from those previously used and allow us to control for two factors other than base-pair mismatch that can affect the thermal stability of DNA duplexes: the base composition and duplex length. In addition, we have studied more than one individual for most species and thus are able to assess the effect of intraspecific variation on phylogenetic conclusions. The results indicate that the closest extant relatives of humans are the chimpanzees. Gorillas are the next closest, followed by orangutans and gibbons. This result is strongly supported statistically, as there is virtually no overlap in measurements between different taxa. Our conclusions are in agreement with a growing amount of molecular evidence supporting this pattern of relatedness. The data behave as a reasonably good molecular clock, and we do not see an indication of slowdown in molecular evolution in the clade containing humans and African apes, contrary to what has been documented for protein-coding regions. Because of the clocklike nature of the results, we have estimated that the divergence of humans and chimpanzees occurred about 6–8 million years ago. Results from orangutans indicate that the Borneo and Sumatra populations are genetically distinct, about as different as the named species of chimpanzees.     

Evolution of a HOXB6 intergenic region within the great apes and humans

Data accumulated over the past decade from several loci suggest that nonhuman primates have a greater amount of intraspecific genetic variation relative to humans. In phylogenetic reconstructions among primates that are based on genetic data, therefore, it becomes essential to adequately sample the population(s) being analyzed. Inadequate sampling may not only underestimate variation within any given population, but such an underestimate may confound any phylogenetic or population-specific conclusions implied by the data. Here we present inter- and intraspecific data on the molecular evolution of an approximately 1.0 kb intergenic HOXB6 sequence among humans, common chimpanzees, pygmy chimpanzees, gorillas and orangutans. To date, this study represents the most comprehensive investigation of a noncoding nuclear locus among the great apes and humans that examines the nature and amount of intraspecific variation in DNA sequences. Not only do these HOXB6 data continue to support earlier findings that Homo sapiens sapiens has less genetic variation than any great ape species (Ruano et al., 1992; Deinard & Kidd, 1995), but they strongly suggest that a high level of genetic polymorphism existed within the common ancestor of the African ape clade (Homo-Pan-Gorilla). Despite detecting two nucleotide substitutions linking Pan and Homo, we caution against concluding that the HOXB6 data definitively support a Homo-Pan clade to the exclusion of Gorilla. Rather, we believe that taking into consideration the level of genetic polymorphism that is likely to have existed within the common ancestor, the most prudent conclusion that can be made from all available data, including morphological, karyotypic and genetic data, may be that speciation among Homo-Pan-Gorilla is best represented by a "trichotomy".     

Fission-fusion dynamics, behavioral flexibility, and inhibitory control in primates

The Machiavellian Intelligence or Social Brain Hypothesis explains the evolution of increased brain size as mainly driven by living in complex organized social systems in which individuals represent "moving targets" who can adopt multiple strategies to respond to one another. Frequently splitting and merging in subgroups of variable composition (fission-fusion or FF dynamics) has been proposed as one aspect of social complexity ( compare with) that may be associated with an enhancement of cognitive skills like inhibition, which allows the suppression of prepotent but ineffective responses in a changing social environment. We compared the performance of primates experiencing high levels of FF dynamics (chimpanzees, bonobos, orangutans, and spider monkeys) to that of species living in more cohesive groups (gorillas, capuchin monkeys, and long-tailed macaques) on five inhibition tasks. Testing species differing in diet, phylogenetic relatedness, and levels of FF dynamics allowed us to contrast ecological, phylogenetic, and socioecological explanations for interspecific differences. Spider monkeys performed at levels comparable to chimpanzees, bonobos, and orangutans, and better than gorillas. A two-cluster analysis grouped all species with higher levels of FF dynamics together. These findings confirmed that enhanced inhibitory skills are positively associated with FF dynamics, more than to phylogenetic relations or feeding ecology.     

Modern African ape populations as genetic and demographic models of the last common ancestor of humans, chimpanzees, and gorillas

In order to fully understand human evolutionary history through the use of molecular data, it is essential to include our closest relatives as a comparison. We provide here estimates of nucleotide diversity and effective population size of modern African ape species using data from several independent noncoding nuclear loci, and use these estimates to make predictions about the nature of the ancestral population that eventually gave rise to the living species of African apes, including humans. Chimpanzees, bonobos, and gorillas possess two to three times more nucleotide diversity than modern humans. We hypothesize that the last common ancestor (LCA) of these species had an effective population size more similar to modern apes than modern humans. In addition, estimated dates for the divergence of the Homo, Pan, and Gorilla lineages suggest that the LCA may have had stronger geographic structuring to its mtDNA than its nuclear DNA, perhaps indicative of strong female philopatry or a dispersal system analogous to gorillas, where females disperse only short distances from their natal group. Synthesizing different classes of data, and the inferences drawn from them, allows us to predict some of the genetic and demographic properties of the LCA of humans, chimpanzees, and gorillas.     

Problems with the use of cladistic analysis in palaeoanthropology

Cladistic analysis is a popular method for reconstructing evolutionary relationships on the human lineage. However, it has limitations and hidden assumptions that are often not considered by palaeoanthropologists. Some researchers who are opposed to its use regard cladistics as the preferred method for taxonomic «splitters» and claim it has lead to a revitalisation of typology. Typology remains a part of human evolutionary studies, regardless of the acceptance or use of cladistics. The assumption/preference for «splitting» over «lumping» in cladistics (alpha) taxonomy and the general failure to evaluate (post-hoc) such taxonomies have served to reinforce this assertion.

Researchers have also adopted a number of practices that are logically untenable or introduce considerable error. The evolutionary trend of human encephalisation, apparently isometric with body size, and concurrent reduction in the gut and masticatory apparatus, suggests continuous cladistic characters are biased by problems of body size.

The method suffers a logical weakness, or circularity, leading to bias when characters with multiple states are used. Coding of such characters can only be done using prior criteria, and this is usually done using an existing phylogenetic scheme. Another problem with coding character states is the handling of variation within species. While this form of variation is usually ignored by palaeoanthropologists, when characters are recognised as varying, their treatment as a separate state adds considerable error to cladograms.

The genetic proximity of humans, chimpanzees and gorillas has important implications for cladistic analyses. It is argued that chimpanzees and gorillas should be treated as ingroup taxa and an alternative outgroup such as orangutans should be used, or an (hypothetical) ancestral body plan developed. Making chimpanzees and gorillas ingroup taxa would considerably enhance the biological utility of anthropological cladograms.

All published human cladograms fail to meet standard quality criteria indicating that none of them may be considered reliable. The continuing uncertainty over the number and composition of fossil human species is the largest single source of error for cladistics and human phylogenetic reconstruction.     

Ancestral Population Sizes and Species Divergence Times in the Primate Lineage on the Basis of Intron and BAC End Sequences

The effective sizes of ancestral populations and species divergence times of six primate species (humans, chimpanzees, gorillas, orangutans, and representatives of Old World monkeys and New World monkeys) are estimated by applying the two-species maximum likelihood (ML) method to intron sequences of 20 different loci. Examination of rate heterogeneity of nucleotide substitutions and intragenic recombination identifies five outrageous loci (ODC1, GHR, HBE, INS, and HBG). The estimated ancestral polymorphism ranges from 0.21 to 0.96% at major divergences in primate evolution. One exceptionally low polymorphism occurs when African and Asian apes diverged. However, taking into consideration the possible short generation times in primate ancestors, it is concluded that the ancestral population size in the primate lineage was no smaller than that of extant humans. Furthermore, under the assumption of 6 million years (myr) divergence between humans and chimpanzees, the divergence time of humans from gorillas, orangutans, Old World monkeys, and New World monkeys is estimated as 7.2, 18, 34, and 65 myr ago, respectively, which are generally older than traditional estimates. Beside the intron sequences, three other data sets of orthologous sequences are used between the human and the chimpanzee comparison. The ML application to these data sets including 58,156 random BAC end sequences (BES) shows that the nucleotide substitution rate is as low as 0.6–0.8 × 10^–9 per site per year and the extent of ancestral polymorphism is 0.33–0.51%. With such a low substitution rate and short generation time, the relatively high extent of polymorphism suggests a fairly large effective population size in the ancestral lineage common to humans and chimpanzees.[Reviewing Editor: Dr. Magnus Nordborg]     

Hypoglossal canal size in living hominoids and the evolution of human speech

The relative size of the hypoglossal canal has been proposed as a useful diagnostic tool for the identification of human-like speech capabilities in the hominid fossil record. Relatively large hypoglossal canals (standardized to oral cavity size) were observed in humans and assumed to correspond to relatively large hypoglossal nerves, the cranial nerve that controls motor function of the tongue. It was suggested that the human pattern of tongue motor innervation and associated speech potential are very different from those of African apes and australopithecines; the modern human condition apparently appeared by the time of Middle Pleistocene Homo. A broader interspecific analysis of hypoglossal canal size in primates conducted in 1999 has rejected this diagnostic and inferences based upon it. In an attempt to resolve these differences of opinion, which we believe are based in part on biased size-adjustments and/or unwarranted assumptions, a new data set was collected and analyzed from 298 extant hominoid skulls, including orangutans, gorillas, chimpanzees, bonobos, siamang, gibbons, and modern humans. Data on the absolute size of the hypoglossal nerve itself were also gathered from a small sample of humans and chimpanzee cadavers. A scale-free index of relative hypoglossal canal size (RHCS) was computed as 100 x (hypoglossal canal area(0.5)/oral cavity volume(0.333)). No significant sexual dimorphism in RHCS was discovered in any species of living hominoid, but there are significant interspecific differences in both absolute and relative sizes of the hypoglossal canal. In absolute terms, humans possess significantly larger canals than any other species except gorillas, but there is considerable overlap with chimpanzees. Humans are also characterized by large values of RHCS, but gibbons possess an even larger average mean for this index; siamang and bonobos overlap appreciably with humans in RHCS. The value of RHCS in Australopithecus afarensis is well within both human and gibbon ranges, as are the indices computed for selected representatives of fossil Homo. Furthermore, the size of the hypoglossal nerve itself, expressed as the mass of nerve per millimeter of length, does not distinguish chimpanzees from modern humans. We conclude, therefore, that the relative size of the hypoglossal canal is neither a reliable nor sufficient predictor of human-like speech capabilities, and paleoanthropology still lacks a quantifiable, morphological diagnostic for when this capability finally emerged in the human career.     

Relative strength of the tibia and fibula and locomotor behavior in hominoids

The fibula has rarely been considered in comparative morphological studies, probably due to its relatively minor role in carrying mechanical loads. However, some differences in morphology (and inferred function) of the fibula between humans and apes, and within apes, have been noted and related to differences in positional behavior. Therefore, the study of tibiofibular relations may be useful in characterizing such differences. This study examines cross-sectional geometric (CSG) properties (cortical area and polar section modulus, Z(p)) of the tibia and fibula at mid-diaphysis across a sample (n=87) of humans, chimpanzees, gorillas, orangutans, and gibbons. The fibula is compared against the tibia in the different taxa. The results indicate that the robusticity of the fibula relative to that of the tibia can be explained in terms of differences in positional behavior. In particular, hominoids that are more arboreal (i.e., gibbons, orangutans, and chimpanzees) possess a relatively more robust fibula than do hominoids that are more terrestrial (i.e., gorillas and humans). The difference appears to be a consequence of the more mobile fibula and more adducted position of the hindlimb necessary in an arboreal environment. Apart from providing the first CSG data on the fibula, these results may be helpful in reconstructing the locomotor behavior of fossil hominoids.     

Nuclear insertions help and hinder inference of the evolutionary history of gorilla mtDNA

Numts are fragments of mitochondrial DNA (mtDNA) that have been translocated to the nucleus, where they can persist while their mitochondrial counterparts continue to rapidly evolve. Thus, numts represent 'molecular fossils' useful for comparison with mitochondrial variation, and are particularly suited for studies of the fast-evolving hypervariable segment of the mitochondrial control region (HV1). Here we used information from numts found in western gorillas (Gorilla gorilla) and eastern gorillas (Gorilla beringei) to estimate that these two species diverged about 1.3 million years ago (Ma), an estimate similar to recent calculations for the divergence of chimpanzee and bonobo. We also describe the sequence of a gorilla numt still possessing a segment lost from all contemporary gorilla mtDNAs. In contrast to that sequence, many numts of the HV1 are highly similar to authentic mitochondrial organellar sequences, making it difficult to determine whether purported mitochondrial sequences truly derive from that genome. We used all available organellar HV1 and corresponding numt sequences from gorillas in a phylogenetic analysis aimed at distinguishing these two types of sequences. Numts were found in several clades in the tree. This, in combination with the fact that only a limited amount of the extant variation in gorillas has been sampled, suggests that categorization of new sequences by the indirect means of phylogenetic comparison would be prone to uncertainty. We conclude that for taxa such as gorillas that contain numerous numts, direct approaches to the authentication of HV1 sequences, such as amplification strategies relying upon the circularity of the mtDNA molecule, remain necessary.     

Measurements of Salivary Alpha Amylase and Salivary Cortisol in Hominoid Primates Reveal Within-Species Consistency and Between-Species Differences

Salivary alpha amylase (sAA) is the most abundant enzyme in saliva. Studies in humans found variation in enzymatic activity of sAA across populations that could be linked to the copy number of loci for salivary amylase (AMY1), which was seen as an adaptive response to the intake of dietary starch. In addition to diet dependent variation, differences in sAA activity have been related to social stress. In a previous study, we found evidence for stress-induced variation in sAA activity in the bonobos, a hominoid primate that is closely related to humans. In this study, we explored patterns of variation in sAA activity in bonobos and three other hominoid primates, chimpanzee, gorilla, and orangutan to (a) examine if within-species differences in sAA activity found in bonobos are characteristic for hominoids and (b) assess the extent of variation in sAA activity between different species. The results revealed species-differences in sAA activity with gorillas and orangutans having higher basal sAA activity when compared to Pan. To assess the impact of stress, sAA values were related to cortisol levels measured in the same saliva samples. Gorillas and orangutans had low salivary cortisol concentrations and the highest cortisol concentration was found in samples from male bonobos, the group that also showed the highest sAA activity. Considering published information, the differences in sAA activity correspond with differences in AMY1 copy numbers and match with general features of natural diet. Studies on sAA activity have the potential to complement molecular studies and may contribute to research on feeding ecology and nutrition.     

Great apes' risk-taking strategies in a decision making task

We investigate decision-making behaviour in all four non-human great ape species. Apes chose between a safe and a risky option across trials of varying expected values. All species chose the safe option more often with decreasing probability of success. While all species were risk-seeking, orangutans and chimpanzees chose the risky option more often than gorillas and bonobos. Hence all four species' preferences were ordered in a manner consistent with normative dictates of expected value, but varied predictably in their willingness to take risks.     

Dehydroepiandrosterone-sulfate (DHEA-S), sex,and age in zoo-housed western lowland gorillas (<Emphasis Type="Italic">Gorilla gorilla gorilla</Emphasis>)

Among humans, dehydroepiandrosterone-sulfate (DHEA-S) declines with age and is hypothesized to be involved in somatic maintenance and healthy aging. Men have significantly higher DHEA-S than women, contradicting longer lifespans in the latter. Declines of DHEA-S with age also are observed in chimpanzees. In both chimpanzees and bonobos, males and females show no differences in DHEA-S production. Based on human and chimpanzee data, gorillas were predicted to show declining DHEA-S with age. Similar to chimpanzees and bonobos, it also was predicted DHEA-S would not be significantly different between males and females. DHEA-S was assayed from serum banked during physical examinations of gorillas housed at three North American zoos (n = 63). Gorillas ranged from 6 to 52 years of age. Differences between males and females were examined using t tests. Linear regression was used to determine the relationship of DHEA-S with age. There was no significant difference in DHEA-S between males and females. Additionally, there was no significant relationship between DHEA-S and age. As predicted, there were no sex-based differences in DHEA-S in gorillas, which is similar to chimpanzees and bonobos but different from modern humans. Unlike chimpanzees and humans, there was no significant relationship between DHEA-S and age in gorillas. The absence of a relationship between age and DHEA-S may be due to the lack of gorillas under age 6 years in this sample as declines in chimpanzees occur prior to age 5 years, more rapid growth and development among gorillas compared with other African hominoids, or a unique pattern of DHEA-S production.     

Evolution of nuclear gene families in primates,Copy-number variation in the argininosuccinate synthetase (ASS) pseudogene family and the anonymous DNA sequence,D1S1

Changes in the copy number of nuclear genes provide the raw material for the creation of new gene functions. To better understand the mechanisms for such events, and their physiologic and evolutionary consequences, it is valuable to study a well characterized and closely related group of species such as primates. Fortuitously, most of the powerful molecular techniques and DNA probes developed for research in humans are equally applicable to non-human primates. We review what is known of copy number variation in primates and describe two informative DNA probes: pAS-1, a cDNA probe to the human urea cycle enzyme argininosuccinate synthetase (ASS), and an anonymous DNA probe, D1S1.In additon to the ASS structural locus on human chromosome 9, pAS-1 detects at least 14 dispersed, processed pseudogenes in humans. The number of pseudogene copies appears to be approximately the same in humans, chimpanzees, gorillas, orangutans and baboons; less in marmosets; and least in some rodents. Chimpanzees and gorillas appear to have all of the human pseudogenes though an Xp copy may be missing from gorillas. The Y pseudogene is apparently absent from orangutans and baboons, and, finally, a comparison of humans and chimpanzees revealed that the number of nucleotide substitutions in the Y chromosome pseudogenes is approximately 1 per 100.D1S1 maps to human chromosome 3 but also detects a high homology copy on chromosome 1. Chimpanzees, gorillas and orangutans all appear to have only the chromosome 3 homolog suggesting that this is the ancestral sequence and that the duplication occurred after separation of humans and the great apes.Both the ASS pseudogene family and the D1S1 system provide valuable information on the evolution of nuclear gene families in primates.     

Variation in anthropoid vertebral formulae: implications for homology and homoplasy in hominoid evolution

Variation in vertebral formulae within and among hominoid species has complicated our understanding of hominoid vertebral evolution. Here, variation is quantified using diversity and similarity indices derived from population genetics. These indices allow for testing models of hominoid vertebral evolution that call for disparate amounts of homoplasy, and by inference, different patterns of evolution. Results are interpreted in light of "short-backed" (J Exp Zool (Mol Dev Evol) 302B:241-267) and "long-backed" (J Exp Zool (Mol Dev Evol) 314B:123-134) ancestries proposed in different models of hominin vertebral evolution. Under the long-back model, we should expect reduced variation in vertebral formulae associated with adaptively driven homoplasy (independently and repeatedly reduced lumbar regions) and the relatively strong directional selection presumably associated with it, especially in closely related taxa that diverged relatively recently (e.g., Pan troglodytes and Pan paniscus). Instead, high amounts of intraspecific variation are observed among all hominoids except humans and eastern gorillas, taxa that have likely experienced strong stabilizing selection on vertebral formulae associated with locomotor and habitat specializations. Furthermore, analyses of interspecific similarity support an evolutionary scenario in which the vertebral formulae observed in western gorillas and chimpanzees represent a reasonable approximation of the ancestral condition for great apes and humans, from which eastern gorillas, humans, and bonobos derived their unique vertebral profiles. Therefore, these results support the short-back model and are compatible with a scenario of homology of reduced lumbar regions in hominoid primates. Fossil hominin vertebral columns are discussed and shown to support, rather than contradict, the short-back model.