The hominoid proximal radius: re-interpreting locomotor behaviors in early hominins

Studies of fossil hominins are traditionally taxonomically narrow and often exclude comparisons with hylobatids. Hence, results of functional analyses of postcrania, interpreted as indicating that early hominins are "African-ape-like" in their postcranial skeletons and positional behaviors, may reflect an artifact of inadequate taxonomic and morphological breadth of the comparative sample. To address this problem and better understand early hominin positional behaviors, this study included hylobatids in a comparative analysis, focusing on the hominoid elbow joint. Specifically, morphometric variables of the proximal radius were derived from measurements from a sample of all genera of extant hominoids and casts of extinct hominin species. Univariate and multivariate analyses were performed on these data. Results show that early hominins are morphologically diverse and are not, as a group, similar to any one extant group. Instead, the fossils resemble Pan, Gorilla, and Hylobates, and are not like modern Homo sapiens or Pongo. This suggests that the morphology of Hylobates may reflect a morphotype for all later hominoids, thus complicating the functional interpretations of fossil hominins. The implications of these results are that the proximal radius is not a sensitive indicator of locomotor behavior among hominoids since the morphology in hylobatids and Gorilla and Pan is similar despite widely varying positional repertoires. Furthermore, inferences of function from form in extinct hominins can be drastically affected by the comparative outgroup selection. A re-evaluation of the functional morphology of the proximal radius in early hominins is addressed.     

Life-History Inference in the Early Hominins Australopithecus and Paranthropus

The life histories of early hominins are commonly characterized as being like those of great apes. However, the life histories of the extant great apes differ considerably from one another. Moreover, the extent to which their life histories correlate with the two aspects of morphology used to infer the life histories of fossil species, brain size and dental development, has remained subject to debate. Increased knowledge of great ape life histories and, more recently, dental development —in particular ages at first molar emergence— now make it clearer that the latter is strongly associated with important life-history attributes, whereas brain size, as reflected by cranial capacity, is less informative. Here we estimate ages at M1 emergence in several infant/juvenile individuals of Australopithecus and Paranthropus based on previous estimates of ages at death, determined through dental histology. These are uniformly earlier than would be predicted either by adult cranial capacity or by comparison to ages at M1 emergence in free-living extant great apes. This suggests that either, 1) the life histories of the early hominins were faster than those of all extant great apes; 2) there was selection for rapid initial dental development and presumably early weaning, but that early hominin life histories were otherwise more prolonged and consistent with adult cranial capacities; or 3) the ages at death have been systematically underestimated, resulting in underestimates of the ages at M1 emergence. We investigate the implications of each of these alternatives and, where possible, explore evidence that might support one over the others.     

Evolution of life history and behavior in Hominidae: Towards phylogenetic reconstruction of the chimpanzee–human last common ancestor

The origin of the fundamental behavioral differences between humans and our closest living relatives is one of the central issues of evolutionary anthropology. The prominent, chimpanzee-based referential model of early hominin behavior has recently been challenged on the basis of broad multispecies comparisons and newly discovered fossil evidence. Here, we argue that while behavioral data on extant great apes are extremely relevant for reconstruction of ancestral behaviors, these behaviors should be reconstructed trait by trait using formal phylogenetic methods. Using the widely accepted hominoid phylogenetic tree, we perform a series of character optimization analyses using 65 selected life-history and behavioral characters for all extant hominid species. This analysis allows us to reconstruct the character states of the last common ancestors of Hominoidea, Hominidae, and the chimpanzee–human last common ancestor. Our analyses demonstrate that many fundamental behavioral and life-history attributes of hominids (including humans) are evidently ancient and likely inherited from the common ancestor of all hominids. However, numerous behaviors present in extant great apes represent their own terminal autapomorphies (both uniquely derived and homoplastic). Any evolutionary model that uses a single extant species to explain behavioral evolution of early hominins is therefore of limited use. In contrast, phylogenetic reconstruction of ancestral states is able to provide a detailed suite of behavioral, ecological and life-history characters for each hypothetical ancestor. The living great apes therefore play an important role for the confident identification of the traits found in the chimpanzee–human last common ancestor, some of which are likely to represent behaviors of the fossil hominins.     

Reconstructing the Diet of an Extinct Hominin Taxon: The Role of Extant Primate Models

Modern humans represent the only surviving species of an otherwise extinct clade of primates, the hominins. As the closest living relatives to extinct hominins, extant primates are an important source of comparative information for the reconstruction of the diets of extinct hominins. Methods such as comparative and functional morphology, finite element analysis, dental wear, dental topographic analysis, and stable isotope biogeochemistry must be validated and tested within extant populations before they can be applied to extinct taxa. Here we review how these methods have and might be used to reconstruct the diet of a particular extinct hominin, Paranthropus boisei, which has no extant analogue for its highly derived masticatory morphology. Our review emphasizes the potential and limitations of using extant primates as models for the reconstruction of extinct hominin diets. We encourage paleoanthropologists and those who study the feeding behaviors of extant primates to work together to investigate and validate methods for interpreting the diets of all extinct primates, including hominins.     

The problems of the Pliopithecidae as a hylobatid ancestor

Various members of the Pliopithecidae (Pliopithecus, Laccopithecus) and the Proconsulidae (Micropithecus, Dendropithecus, Limnoputhecus, Dionysopithecus, and Platdontopithecus) have been proposed as the ancestral hylobatid (gibbon), based largely on small size and simple-cusped, ape-like molars. However, this ignores evidence presented in early anatomical studies of living brachiating primates. All apes and several South American monkeys show structural anatomical adaptations for brachiation. The Pliopithecidae show some ceboid-like features in the hindlimb which suggest that this genus may have been partly suspensory and possibly comparable to spider monkeys, but without a prehensile tail. They were basically arboreal quadrupedal monkeys without any of the brachiator specializations. Large bodied apes add more traits in order to handle great weight. Among the small-bodied brachiators, only the hylobatids possess these large-brachiator traits. Such modifications serve no purpose other than to support a weight greater than 30 kg. The hylobatid gestation time and longevity are also characteristic only of much larger animals. The ancestral gibbon must have been among the large-bodied sivapithecines. This relationship is supported by body size, geography, and biochemical timing (pliopithecids were probably a distinct lineage in the late Oligocene). If a memeber of the Pliopithecidae were the ancestor of extant hylobatids, it would have had to have grown large, became adapted to brachiation, and then grown small again.Laccopithecus has been newly proposed as the ancestral gibbon. If it is not a member of the pliopithecids, with an age of less than 8 mya, then it could be a fossil hylobatid. It would have had to have separated from the Asian great ape line approximately 15 mya, developed full brachiation, and undergone a reduction in body size and dental sexual dimorphism.     

Hominoid dispersal patterns and human evolution

Recent advances in DNA and isotope analyses have allowed tentative reconstructions of dispersal strategies of Plio-Pleistocene hominins.(1,2) Comparing their findings to dispersal patterns of some extant apes and humans suggested groups of related males and unrelated females in Neandertals indicating patrilocality(2) and Pan-like male philopatry in australopiths.(1) Here we review the demographic, ethnographic, and genetic evidence of dispersal patterns in extant apes and humans and compare the results to the suggestions for Plio-Pleistocene hominins. We find that alternative dispersal patterns, for example among gorillas or gibbons, could explain the findings of related or natal males in a confined geographic area. Based on sexual size dimorphism, we speculate that gorillas might currently be the best model for reconstructing dispersal in robust australopiths. Given that the sexual size dimorphism in other australopiths is still hotly debated, the question of which hominoid model best matches their dispersal pattern must remain unanswered. Neandertal dispersal patterns have been compared to patrilocality of modern humans. However, the latter is related to the advent of food production. Consequently, hunter-gatherers exhibiting primarily multilocality appear to be the better comparison for Neandertals. Overall, human-like patrilocality and Pan-like male philopatry appear to be poor models for the reconstruction of dispersal patterns in Plio-Pleistocene hominins.     

Where are inion and endinion? Variations of the exo- and endocranial morphology of the occipital bone during hominin evolution

The occipital bone is frequently investigated in paleoanthropological studies because it has several features that help to differentiate various fossil hominin species. Among these features is the separation between inion and endinion, which has been proposed to be an autapomorphic trait in (Asian) Homo erectus. Methodologies are developed here to quantify for the first time the location of these anatomical points, and to interpret their variation due to the complex interactions between exocranial and endocranial size and shape of the occipital and nuchal planes, as well as the occipital lobes and cerebellum. On the basis of our analysis, neither ‘the separation between inion and endinion’ nor ‘endinion below inion’ can be considered as an autapomorphic trait in H. erectus, since this feature is a condition shared by extant African great apes and fossil hominins. Moreover, our results show that the exo- and endocranial anatomy of the occipital bone differs between hominins (except Paranthropus boisei specimens and KNM-ER 1805) and great apes. For example, chimpanzees and bonobos are characterized by a very high position of inion and their occipital bone shows an antero-posterior compression. However, these features are partly correlated with their small size when compared with hominins. Asian H. erectus specimens have a thick occipital torus, but do not differ from other robust specimens, neither in this feature nor in the analysed exo- and endocranial proportions of the occipital bone. Finally, the apparent brain size reduction during the Late Pleistocene and variation between the sexes in anatomically modern humans (AMH) reflect that specimens with smaller brains have a relatively larger posterior height of the cerebellum. However, this trend is not the sole explanation for the ‘vertical shift’ of endinion above inion that appears occasionally and exclusively in AMH.     

Digit ratios predict polygyny in early apes, Ardipithecus, Neanderthals and early modern humans but not in Australopithecus

Social behaviour of fossil hominoid species is notoriously difficult to predict owing to difficulties in estimating body size dimorphism from fragmentary remains and, in hominins, low canine size dimorphism. Recent studies have shown that the second-to-fourth digit ratio (2D : 4D), a putative biomarker for prenatal androgen effects (PAEs), covaries with intra-sexual competition and social systems across haplorrhines; non-pair-bonded polygynous taxa have significantly lower 2D : 4D ratios (high PAE) than pair-bonded monogamous species. Here, we use proximal phalanx ratios of extant and fossil specimens to reconstruct the social systems of extinct hominoids. Pierolapithecus catalaunicus, Hispanopithecus laietanus and Ardipithecus ramidus have ratios consistent with polygynous extant species, whereas the ratio of Australopithecus afarensis is consistent with monogamous extant species. The early anatomically modern human Qafzeh 9 and Neanderthals have lower digit ratios than most contemporary human populations, indicating increased androgenization and possibly higher incidence of polygyny. Although speculative owing to small sample sizes, these results suggest that digit ratios represent a supplementary approach for elucidating the social systems of fossil hominins.     

A partial skeleton of the fossil great ape Hispanopithecus laietanus from Can Feu and the mosaic evolution of crown-hominoid positional behaviors

The extinct dryopithecine Hispanopithecus (Primates: Hominidae), from the Late Miocene of Europe, is the oldest fossil great ape displaying an orthograde body plan coupled with unambiguous suspensory adaptations. On the basis of hand morphology, Hispanopithecus laietanus has been considered to primitively retain adaptations to above-branch quadrupedalism-thus displaying a locomotor repertoire unknown among extant or fossil hominoids, which has been considered unlikely by some researchers. Here we describe a partial skeleton of H. laietanus from the Vallesian (MN9) locality of Can Feu 1 (Vallès-Penedès Basin, NE Iberian Peninsula), with an estimated age of 10.0-9.7 Ma. It includes dentognathic and postcranial remains of a single, female adult individual, with an estimated body mass of 22-25 kg. The postcranial remains of the rib cage, shoulder girdle and forelimb show a mixture of monkey-like and modern-hominoid-like features. In turn, the proximal morphology of the ulna-most completely preserved in the Can Feu skeleton than among previously-available remains-indicates the possession of an elbow complex suitable for preserving stability along the full range of flexion/extension and enabling a broad range of pronation/supination. Such features, suitable for suspensory behaviors, are however combined with an olecranon morphology that is functionally related to quadrupedalism. Overall, when all the available postcranial evidence for H. laietanus is considered, it emerges that this taxon displayed a locomotor repertoire currently unknown among other apes (extant or extinct alike), uniquely combining suspensory-related features with primitively-retained adaptations to above-branch palmigrady. Despite phylogenetic uncertainties, Hispanopithecus is invariably considered an extinct member of the great-ape-and-human clade. Therefore, the combination of quadrupedal and suspensory adaptations in this Miocene crown hominoid clearly evidences the mosaic nature of locomotor evolution in the Hominoidea, as well as the impossibility to reconstruct the ancestral locomotor repertoires for crown hominoid subclades on the basis of extant taxa alone.     

Shared pattern of endocranial shape asymmetries among great apes, anatomically modern humans, and fossil hominins

Anatomical asymmetries of the human brain are a topic of major interest because of their link with handedness and cognitive functions. Their emergence and occurrence have been extensively explored in human fossil records to document the evolution of brain capacities and behaviour. We quantified for the first time antero-posterior endocranial shape asymmetries in large samples of great apes, modern humans and fossil hominins through analysis of "virtual" 3D models of skull and endocranial cavity and we statistically test for departures from symmetry. Once based on continuous variables, we show that the analysis of these brain asymmetries gives original results that build upon previous analysis based on discrete traits. In particular, it emerges that the degree of petalial asymmetries differs between great apes and hominins without modification of their pattern. We indeed demonstrate the presence of shape asymmetries in great apes, with a pattern similar to modern humans but with a lower variation and a lower degree of fluctuating asymmetry. More importantly, variations in the position of the frontal and occipital poles on the right and left hemispheres would be expected to show some degree of antisymmetry when population distribution is considered, but the observed pattern of variation among the samples is related to fluctuating asymmetry for most of the components of the petalias. Moreover, the presence of a common pattern of significant directional asymmetry for two components of the petalias in hominids implicates that the observed traits were probably inherited from the last common ancestor of extant African great apes and Homo sapiens.These results also have important implications for the possible relationships between endocranial shape asymmetries and functional capacities in hominins. It emphasizes the uncoupling between lateralized activities, some of them well probably distinctive to Homo, and large-scale cerebral lateralization itself, which is not unique to Homo.     

A morphometric mapping analysis of lower fourth deciduous premolar in hominoids: Implications for phylogenetic relationship between Nakalipithecus and Ouranopithecus

Clarifying morphological variation among African and Eurasian hominoids during the Miocene is of particular importance for inferring the evolutionary history of humans and great apes. Among Miocene hominoids, Nakalipithecus and Ouranopithecus play an important role because of their similar dates on different continents. Here, we quantify the lower fourth deciduous premolar (dp4) inner morphology of extant and extinct hominoids using a method of morphometric mapping and examine the phylogenetic relationships between these two fossil taxa. Our data indicate that early Late Miocene apes represent a primitive state in general, whereas modern great apes and humans represent derived states. While Nakalipithecus and Ouranopithecus show similarity in dp4 morphology to a certain degree, the dp4 of Nakalipithecus retains primitive features and that of Ouranopithecus exhibits derived features. Phenotypic continuity among African ape fossils from Miocene to Plio-Pleistocene would support the African origin of African apes and humans (AAH). The results also suggest that Nakalipithecus could have belonged to a lineage from which the lineage of Ouranopithecus and the common ancestor of AAH subsequently derived.     

3D Morphometric Analysis Reveals Similar Ecomorphs for Early Kangaroos (Macropodidae) and Fanged Kangaroos (Balbaridae) from the Riversleigh World Heritage Area,Australia

Understanding feeding ecology of extinct kangaroos is fundamental to understanding the evolution of kangaroos and the Australia paleoenvironment during the Oligo-Miocene. Comparisons with extant species have suggested that the macropodiforms of the Oligo/Miocene (kangaroos and allies) from the Riversleigh World Heritage Area, northern Australia, were predominantly folivorous browsers or fungivores, unlike the majority of extant species. To further test this hypothesis, we investigate the relationship between variation in cranial and mandibular shape of extant and extinct macropodiforms and ecological factors such as diet, locomotion, and body mass using 3D geometric morphometric analysis of 42 living species and eight extinct species from two radiations (the extinct clade of Balbaridae and some early representatives of the extant Macropodidae. Dietary class (fungivore, browser, grazer, and mixed feeder) correlated strongly with variation in cranial shape (20–25% of variance explained). There was also significant association between cranial shape, and both locomotor mode and body mass. In a principal component analysis of shape variation for crania (including the shape of the molar row), Riversleigh macropodiforms cluster with extant folivorous browsers on principal components (PC) 1 and 3, providing support for previous interpretations of these species as browsing kangaroos. However, as a group and regardless of phylogenetic association, the shape centroid of extinct species differs significantly from that of extant species. Riversleigh macropodiforms cluster with regular hoppers or arboreal tree kangaroos, but this may be a result of the correlation between diet and locomotor mode in kangaroos. Their similarity to extant browsers supports previous interpretations of rainforest and woodland environments at Riversleigh during the early and middle Miocene, respectively. Procrustes ANOVA Analysis of the full shape dataset and diet also shows that diet accounts for a significant portion of variation; however, when phylogeny is taken into account these results become nonsignificant. In analyses of dentary shape, some balbarid species cluster with extant mixed feeders, although this may reflect phylogenetic differences rather than ecological signal.

     

Testing for size and allometric differences in fossil hominin body mass estimation

Body size reconstructions of fossil hominins allow us to infer many things about their evolution and lifestyle, including diet, metabolic requirements, locomotion, and brain/body size relationships. The importance of these implications compels anthropologists to attempt body mass estimation from fragmentary fossil hominin specimens. Most calculations require a known “calibration” sample usually composed of modern humans or other extant apes. Caution must be taken in these analyses, as estimates are sensitive to overall size and allometric differences between the fossil hominin and the reference sample. Am J Phys Anthropol 151:215–229, 2013. © 2013 Wiley Periodicals, Inc.     

Shared Pattern of Endocranial Shape Asymmetries among Great Apes,Anatomically Modern Humans,and Fossil Hominins

Anatomical asymmetries of the human brain are a topic of major interest because of their link with handedness and cognitive functions. Their emergence and occurrence have been extensively explored in human fossil records to document the evolution of brain capacities and behaviour. We quantified for the first time antero-posterior endocranial shape asymmetries in large samples of great apes, modern humans and fossil hominins through analysis of “virtual” 3D models of skull and endocranial cavity and we statistically test for departures from symmetry. Once based on continuous variables, we show that the analysis of these brain asymmetries gives original results that build upon previous analysis based on discrete traits. In particular, it emerges that the degree of petalial asymmetries differs between great apes and hominins without modification of their pattern. We indeed demonstrate the presence of shape asymmetries in great apes, with a pattern similar to modern humans but with a lower variation and a lower degree of fluctuating asymmetry. More importantly, variations in the position of the frontal and occipital poles on the right and left hemispheres would be expected to show some degree of antisymmetry when population distribution is considered, but the observed pattern of variation among the samples is related to fluctuating asymmetry for most of the components of the petalias. Moreover, the presence of a common pattern of significant directional asymmetry for two components of the petalias in hominids implicates that the observed traits were probably inherited from the last common ancestor of extant African great apes and Homo sapiens.These results also have important implications for the possible relationships between endocranial shape asymmetries and functional capacities in hominins. It emphasizes the uncoupling between lateralized activities, some of them well probably distinctive to Homo, and large-scale cerebral lateralization itself, which is not unique to Homo.     

Mandibular size and shape variation in the hominins at Dmanisi, Republic of Georgia

The hominin fossils of Dmanisi, Republic of Georgia, present an ideal means of assessing levels of skeletal size and shape variation in a fossil hypodigm belonging to the genus Homo because they have been recovered from a spatially and temporally restricted context. We compare variation in mandible size and shape at Dmanisi to that of extant hominoids and extinct hominins. We use height and breadth measurements of the mandibular corpus at the first molar and the symphysis to assess size, and analyze shape based on size-adjusted (using a geometric mean) versions of these four variables. We compare size and shape variation at Dmanisi relative to all possible pairs of individuals within each comparative taxon using an exact resampling procedure of the ratio of D2600 to D211 and the average Euclidean distance (AED) between D2600 and D211, respectively. Comparisons to extant hominoids were conducted at both the specific and subspecific taxonomic levels and to extinct hominins by adopting both a more, and less speciose, hominin taxonomy. Results indicate that the pattern of variation for the Dmanisi hominins does not resemble that of any living species: they exhibit significantly more size variation when compared to modern humans, and they have significantly more corpus shape variation and size variation in corpus heights and overall mandible size than any extant ape species. When compared to fossil hominins they are also more dimorphic in size (although this result is influenced by the taxonomic hypothesis applied to the hominin fossil record). These results highlight the need to re-examine expectations of levels of sexual dimorphism in members of the genus Homo and to account for marked size and shape variation between D2600 and D211 under the prevailing view of a single hominin species at Dmanisi.     

Cope's rule in cryptodiran turtles: do the body sizes of extant species reflect a trend of phyletic size increase?

Cope's rule of phyletic size increase is questioned as a general pattern of body size evolution. Most studies of Cope's rule have examined trends in the paleontological record. However, neontological approaches are now possible due to the development of model-based comparative methods, as well as the availability of an abundance of phylogenetic data. I examined whether the phylogenetic distribution of body sizes in extant cryptodiran turtles is consistent with Cope's rule. To do this, I examined body size evolution in each of six major clades of cryptodiran turtles and also across the whole tree of cryptodirans (n = 201 taxa). Extant cryptodiran turtles do not appear to follow Cope's rule, as no clade showed a significant phyletic body size trend. Previous analyses in other extant vertebrates have also found no evidence for phyletic size increase, which is in contrast to the paleontological data that support the rule in a number of extinct vertebrate taxa.     

The evolution of hominin ontogenies

Since the beginnings of paleoanthropology, immature fossil hominin specimens have marked important but highly contested cornerstones of research. Long deemed as not representative of a fossil species’ morphology, immature hominins are now in the center of scientific attention, and an increasing interest in evolutionary developmental questions has made developmental paleoanthropology a vibrant field of research. Here we report on recent advances in this field, which result from a combination of new methods to reconstruct fossil ontogenies with insights from evo-devo research on extant species.     

Microanatomical diversity of amniote ribs: an exploratory quantitative study

Bone microanatomical diversity in extant and extinct tetrapods has been studied extensively, using increasingly sophisticated quantitative methods to assess its ecological, biomechanical and phylogenetic significance. Most studies have been conducted on the appendicular skeleton, and a strong relationship was found between limb bone microanatomy and habitat preferences. Few comparative studies have focused on the microanatomy of the axial skeleton and its ecological signal. In the present study, we propose the first exploratory study of the microanatomical diversity of amniote ribs. Our comparative sample comprises 155 species of extant amniotes and encompasses the taxonomic, ecological, and body size diversity of this group. We standardized our sampling location to the midshaft of mid‐dorsal ribs. Transverse sections were obtained from classical petrographic methods, as well as by X‐ray microtomography. Most of the microanatomical and size characters of the ribs display a phylogenetic signal, which is an expected result and is also observed in amniote limb bones and vertebrae. We found a significant relationship between rib cortical thickness, global compactness, and lifestyle. As for the vertebrae, the development of the spongiosa in the medullary region appears to be strongly correlated with size. Even though an ecological signal was found in the inner structure of the ribs, additional work is needed to document the intra‐individual variability of the rib microanatomy along the rib cage and within a single element.     

Ecological and Phylogenetic Correlates to Body Size in the Indriidae

I investigated ecological and phylogenetic correlates to body size variations in 10 taxa of extant Indriidae (Indri, Avahi, and Propithecus). I also tested for phylogenetic niche conservatism as a model for the evolution of indriid body size. Phylogenetic niche conservatism refers to the shared attributes that related taxa have acquired because they tend to have occupied similar niches during their evolutionary history. I collected species-specific data on body mass, climate, density, and chemical properties of food items from the literature. I used 2 phylogenies in independent contrasts methods to control for phylogenetic relationships (Indri and Propithecus as sister taxa vs. Indri basal taxa to all indriids). Multivariate models indicated that lemur density and resource quality are the strongest ecological correlates to indriid body size variations. Partitioning methods revealed that 52.4–67% of indriid body size variation is explained by phylogenetic niche conservation. Thus, indriid body size variations may be the result of stabilizing selection. Though it is possible to identify constraints on lower than average body size, there are few data on selection against larger than average body size in indriids. Large body size in subfossil lemurs further complicates identification of constraints on larger than average body size in extant indriids. Researchers using independent contrast methods to control for phylogeny should be aware that some ecology-phenotype relationships are best explained as the result of the synergistic effects of ecology and phylogeny.     

犬齿窝与人类中面部骨骼的演化

犬齿窝是包括现代人类在内的许多人族成员面部骨骼的重要性状,但在分类学上的意义仍存在争议。有学者认为该性状是一个发生于基础面形的近祖性状,除了一些例外,在灭绝和现生的大猿及人属中都存在。另有学者认为,犬齿窝是仅存在于智人及其直系祖先的衍生性状,在发育上与颧齿槽突嵴有关。这种关系并非总是成立,在智人中存在着明显的差异：弧型颧齿槽突嵴和直斜型颧齿槽突嵴与犬齿窝有时共存,有时不共存。我们由此推测,犬齿窝的发生和形态与上颌窦的前部发育有关,颧齿槽突嵴的形态与鼻窦的侧面发育有关。在人类演化的过程中,犬齿窝经历了不同的变形,比如上颌沟(如南方古猿非洲种、傍人粗壮种)、上颌小窝(如傍人粗壮种)、上颌沟(如匠人)或犬齿窝缺如(如傍人埃塞俄比亚种、傍人鲍氏种、肯尼亚扁脸人、人属鲁道夫种)。犬齿窝消失的原因各类群并不相同,如中新世和早更新世人属以及中更新世人属(如人属海德堡种/人属罗得西亚种、人属尼安德特种)。人属罗得西亚种具有弱化的犬齿窝,不具备演化为智人的可能,因此被排除在智人的演化支之外。