Canine sexual dimorphism in Ardipithecus ramidus was nearly human-like.

Body and canine size dimorphism in fossils inform sociobehavioral hypotheses on human evolution and have been of interest since Darwin's famous reflections on the subject. Here, we assemble a large dataset of fossil canines of the human clade, including all available Ardipithecus ramidus fossils recovered from the Middle Awash and Gona research areas in Ethiopia, and systematically examine canine dimorphism through evolutionary time. In particular, we apply a Bayesian probabilistic method that reduces bias when estimating weak and moderate levels of dimorphism. Our results show that Ar. ramidus canine dimorphism was significantly weaker than in the bonobo, the least dimorphic and behaviorally least aggressive among extant great apes. Average male-to-female size ratios of the canine in Ar. ramidus are estimated as 1.06 and 1.13 in the upper and lower canines, respectively, within modern human population ranges of variation. The slightly greater magnitude of canine size dimorphism in the lower than in the upper canines of Ar. ramidus appears to be shared with early Australopithecus, suggesting that male canine reduction was initially more advanced in the behaviorally important upper canine. The available fossil evidence suggests a drastic size reduction of the male canine prior to Ar. ramidus and the earliest known members of the human clade, with little change in canine dimorphism levels thereafter. This evolutionary pattern indicates a profound behavioral shift associated with comparatively weak levels of male aggression early in human evolution, a pattern that was subsequently shared by Australopithecus and Homo.

New femoral remains of Nacholapithecus kerioi: Implications for intraspecific variation and Miocene hominoid evolution.

The middle Miocene stem kenyapithecine Nacholapithecus kerioi (16-15 Ma; Nachola, Kenya) is represented by a large number of isolated fossil remains and one of the most complete skeletons in the hominoid fossil record (KNM-BG 35250). Multiple fieldwork seasons performed by Japanese-Kenyan teams during the last part of the 20th century resulted in the discovery of a large sample of Nacholapithecus fossils. Here, we describe the new femoral remains of Nacholapithecus. In well-preserved specimens, we evaluate sex differences and within-species variation using both qualitative and quantitative traits. We use these data to determine whether these specimens are morphologically similar to the species holotype KNM-BG 35250 (which shows some plastic deformation) and to compare Nacholapithecus with other Miocene hominoids and extant anthropoids to evaluate the distinctiveness of its femur. The new fossil evidence reaffirms previously reported descriptions of some distal femoral traits, namely the morphology of the patellar groove. However, results also show that relative femoral head size in Nacholapithecus is smaller, relative neck length is longer, and neck-shaft angle is lower than previously reported for KNM-BG 35250. These traits have a strong functional signal related to the hip joint kinematics, suggesting that the morphology of the proximal femur in Nacholapithecus might be functionally related to quadrupedal-like behaviors instead of more derived antipronograde locomotor modes. Results further demonstrate that other African Miocene apes (with the exception of Turkanapithecus kalakolensis) generally fall within the Nacholapithecus range of variation, whose overall femoral shape resembles that of Ekembo spp. and Equatorius africanus. Our results accord with the previously inferred locomotor repertoire of Nacholapithecus, indicating a combination of generalized arboreal quadrupedalism combined with other antipronograde behaviors (e.g., vertical climbing).

Variation of bony labyrinthine morphology in Mio-Plio-Pleistocene and modern anthropoids.

OBJECTIVES: The bony labyrinth of the inner ear has special relevance when tracking phenotypic evolution because it is often well preserved in fossil and modern primates. Here we track the evolution of the bony labyrinth of anthropoid primates during the Mio-Plio-Pleistocene-the time period that gave rise to the extant great apes and humans. MATERIALS AND METHODS: We use geometric morphometrics to analyze labyrinthine morphology in a wide range of extant and fossil anthropoids, including New World and Old World monkeys, apes, and humans; fossil taxa are represented by Aegyptopithecus, Microcolobus, Epipliopithecus, Nacholapithecus, Oreopithecus, Ardipithecus, Australopithecus, and Homo. RESULTS: Our results show that the morphology of the anthropoid bony labyrinth conveys a statistically significant phylogenetic signal especially at the family level. The bony labyrinthine morphology of anthropoids is also in part associated with size, but does not cluster by locomotor adaptations. The Miocene apes examined here, regardless of inferred locomotor behaviors, show labyrinthine morphologies distinct from modern great apes. DISCUSSION: Our results suggest that labyrinthine variation contains mixed signals and alternative explanations need to be explored, such as random genetic drift and neutral phenotypic evolution, as well as developmental constraints. The observed pattern in fossil and extant hominoids also suggests that an additional factor, for example, prenatal brain development, could have potentially had a larger role in the evolutionary modification of the bony labyrinth than hitherto recognized.

Sexual dimorphism of body size in an African fossil ape, Nacholapithecus kerioi.

Sexual size dimorphism in the African fossil ape Proconsul nyanzae (18 million years ago, 18 Ma) has been previously documented. However, additional evidence for sexual dimorphism in Miocene hominoids can provide great insight into the history of extant hominoid mating systems. The present study focused on body mass (BM) sexual dimorphism in Nacholapithecus kerioi from the Middle Miocene (16-15 Ma) in Africa. Bootstrap analysis revealed that P. nyanzae BM sexual dimorphism was lower than that in Pan troglodytes, which exhibits moderate sexual dimorphism, as reported previously. The same simulation revealed that BM sexual dimorphism of N. kerioi was comparable with that in Gorilla spp.; i.e., the males were approximately twice as large as the females. High sexual dimorphism in extant apes is usually indicative of a polygynous social structure (gorilla) or solitary/fission-fusion social system (orangutan). However, because of the high proportion of adult males in this fossil assemblage, the magnitude of dimorphism inferred here cannot be associated with a gorilla-like polygynous or oranguran-like solitary/fission-fusion social structure, and may reflect either taphonomic bias, or some other social structure. Extant hominoids have a long evolutionary history owing to their deep branching, comprising only a few existing members of the original highly successful group. Therefore, it is not surprising that the mating systems of extant hominoids fail to provide fossil apes with a perfect "model". The mating systems of extinct hominoids may have been more diverse than those of extant apes.

Sacral vertebral remains of the Middle Miocene hominoid Nacholapithecus kerioi from northern Kenya.

This study describes two new sacral specimens of Nacholapithecus kerioi, KNM-BG 42753I and KNM-BG 47687A, from the Aka Aiteputh Formation in Nachola, northern Kenya, excavated in 2002. They are of roughly equal size and are considered to belong to males. When scaled by body mass, the lumbosacral articular surface area of the better preserved specimen, KNM-BG 42753I, is smaller than that in Old World monkeys but similar to that in extant great apes and New World monkeys, as well as Proconsul nyanzae. The relatively narrow dimensions of the first sacral vertebral body in the transverse and sagittal planes are characteristics of N. kerioi and P. nyanzae and similar to those of extant great apes. In N. kerioi, lumbosacral surface area relative to body mass is small. This may simply be an extension of a trend from the previously reported small thoracolumbar vertebrae to the sacrum. ​The first sacral vertebrae of N. kerioi and Epipliopithecus vindobonensis have a higher craniocaudal vertebral body reduction (CVR; a higher CVR indicates a wider cranial width relative to a narrower caudal width), similar to that in Old World monkeys. Old World monkeys have a higher CVR, and usually have three sacral vertebrae, fewer than seen in extant great apes, which have a lower CVR and four to six (sometimes as many as eight) sacral vertebrae. New World monkeys have a lower CVR than Old World monkeys, but generally possess only three sacral vertebrae, and have a large caudal articular surface, which may be related, at least in the Atelidae, to the grasping ability of their tails. The possibility that N. kerioi had only three sacral vertebrae cannot be ruled out, because E. vindobonensis and Old World monkeys, with higher CVRs, have sacra consisting of three sacral vertebrae.

Kantis: A new Australopithecus site on the shoulders of the Rift Valley near Nairobi, Kenya.

Most Plio-Pleistocene sites in the Gregory Rift Valley that have yielded abundant fossil hominins lie on the Rift Valley floor. Here we report a new Pliocene site, Kantis, on the shoulder of the Gregory Rift Valley, which extends the geographical range of Australopithecus afarensis to the highlands of Kenya. This species, known from sites in Ethiopia, Tanzania, and possibly Kenya, is believed to be adapted to a wide spectrum of habitats, from open grassland to woodland. The Kantis fauna is generally similar to that reported from other contemporaneous A. afarensis sites on the Rift Valley floor. However, its faunal composition and stable carbon isotopic data from dental enamel suggest a stronger C4 environment than that present at those sites. Although the Gregory Rift Valley has been the focus of paleontologists' attention for many years, surveys of the Rift shoulder may provide new perspective on African Pliocene mammal and hominin evolution.

Carpal bones of Nacholapithecus kerioi, a Middle Miocene Hominoid From Northern Kenya.

OBJECTIVES: The carpal bones of the middle Miocene hominoid Nacholapithecus kerioi are described based on new materials. MATERIALS AND METHODS: The materials comprise a trapezoid, three capitates, two hamates, a centrale, a lunate, a triquetrum, and a pisiform, collected during the 2001 and 2002 field seasons from Nachola, Kenya. We also describe a pisiform recently assigned to the type specimen of N. kerioi, KNM-BG 35250. RESULTS: In the Nacholapithecus wrist, the ulnar styloid process articulates with both the triquetrum and pisiform, and the triquetrum facet on the hamate is relatively proximodistally oriented in dorsal view. The Nacholapithecus capitate possesses a moderate distopalmar hook-like process and separated radial articular facets for the trapezoid and the second metacarpal due to the carpometacarpal ligament attachment that is absent in the Proconsul capitate. DISCUSSION: The carpal anatomy of Nacholapithecus is similar to that of the early Miocene hominoid Proconsul. However, Nacholapithecus wrist anatomy appears to exhibit slightly more emphasized stability. Am J Phys Anthropol 160:469-482, 2016. © 2016 Wiley Periodicals, Inc.

Morphology of the thoracolumbar spine of the middle Miocene hominoid Nacholapithecus kerioi from northern Kenya.

A new caudal thoracic and a new lumbar vertebra of Nacholapithecus kerioi, a middle Miocene hominoid from northern Kenya, are reported. The caudal thoracic vertebral body of N. kerioi has a rounded median ventral keel and its lateral sides are moderately concave. The lumbar vertebral body has an obvious median ventral keel. Based on a comparison of vertebral body cranial articular surface size between the caudal thoracic vertebrae in the present study and one discussed in a previous study (KNM-BG 35250BO, a diaphragmatic vertebra), N. kerioi has at least two post-diaphragmatic vertebrae (rib-bearing lumbar-type thoracic vertebrae), unlike extant hominoids. It also has thick, rounded, and moderately long metapophyses on the lumbar vertebra that project dorsolaterally. The spinous process bases of its caudal thoracic and lumbar vertebrae originate caudally between the postzygapophyses, as described previously in the KNM-BG 35250 holotype specimen. In other words, the postzygapophyses of N. kerioi do not project below the caudal border of the spinous processes, similar to those of extant great apes, and unlike small apes and monkeys, which have more caudally projecting postzygapophyses. Nacholapithecus kerioi has a craniocaudally expanded spinous process in relation to vertebral body length, also similar to extant great apes. Both these spinous process features of N. kerioi differ from those of Proconsul nyanzae. The caudal thoracic vertebra of N. kerioi has a caudally-directed spinous process, whose tip is tear-drop shaped. These features resemble those of extant apes. The morphology of the spinous process tips presumably helps vertebral stability by closely stacking adjacent spinous process tips as seen in extant hominoids. The morphology of the spinous process and postzygapophyses limits the intervertebral space and contributes to the stability of the functional lumbar region as seen in extant great apes, suggesting that antipronograde activity was included in the positional behavior of N. kerioi.

Intra- and interspecific variation in macaque molar enamel thickness.

Enamel thickness has played an important role in studies of primate taxonomy, phylogeny, and functional morphology, although its variation among hominins is poorly understood. Macaques parallel hominins in their widespread geographic distribution, relative range of body sizes, and radiation during the last five million years. To explore enamel thickness variation, we quantified average and relative enamel thickness (AET and RET) in Macaca arctoides, Macaca fascicularis, Macaca fuscata, Macaca mulatta, Macaca nemestrina, and Macaca sylvanus. Enamel area, dentine area, and enamel-dentine junction length were measured from mesial sections of 386 molars scanned with micro-computed tomography, yielding AET and RET indices. Intraspecific sex differences were not found in AET or RET. Macaca fuscata had the highest AET and RET, M. fascicularis showed the lowest AET, and M. arctoides had the lowest RET. The latitudinal distribution of macaque species was associated with AET for these six species. Temperate macaques had thicker molar enamel than did tropical macaques, suggesting that thick enamel may be adaptive in seasonal environments. Additional research is needed to determine if thick enamel in temperate macaques is a response to intensified hard-object feeding, increased abrasion, and/or a broader diet with a greater range of food material properties. The extreme ecological flexibility of macaques may prohibit identification of consistent trends between specific diets and enamel thickness conditions. Such complications of interpretation of ecological variability, dietary diversity, and enamel thickness may similarly apply for fossil Homo species.

Late Miocene to Pliocene carbon isotope record of differential diet change among East African herbivores.

Stable isotope and molecular data suggest that C(4) grasses first appeared globally in the Oligocene. In East Africa, stable isotope data from pedogenic carbonate and fossil tooth enamel suggest a first appearance between 15-10 Ma and subsequent expansion during the Plio-Pleistocene. The fossil enamel record has the potential to provide detailed information about the rates of dietary adaptation to this new resource among different herbivore lineages. We present carbon isotope data from 452 fossil teeth that record differential rates of diet change from C(3) to mixed C(3)/C(4) or C(4) diets among East African herbivore families at seven different time periods during the Late Miocene to the Pliocene (9.9-3.2 Ma). Significant amounts of C(4) grasses were present in equid diets beginning at 9.9 Ma and in rhinocerotid diets by 9.6 Ma, although there is no isotopic evidence for expansive C(4) grasslands in this part of the Late Miocene. Bovids and hippopotamids followed suit with individuals that had C(4)-dominated (>65%) diets by 7.4 Ma. Suids adopted C(4)-dominated diets between 6.5 and 4.2 Ma. Gomphotheriids and elephantids had mostly C(3)-dominated diets through 9.3 Ma, but became dedicated C(4) grazers by 6.5 Ma. Deinotheriids and giraffids maintained a predominantly C(3) diet throughout the record. The sequence of differential diet change among herbivore lineages provides ecological insight into a key period of hominid evolution and valuable information for future studies that focus on morphological changes associated with diet change.

Functional morphology and anatomy of cervical vertebrae in Nacholapithecus kerioi, a middle Miocene hominoid from Kenya.

This paper describes the morphology of cervical vertebrae in Nacholapithecus kerioi, a middle Miocene primate species excavated from Nachola, Kenya in 1999-2002. The cervical vertebrae in Nacholapithecus are larger than those of Papio cynocephalus. They are more robust relative to more caudal vertebral bones. Since Nacholapithecus had large forelimbs, it is assumed that strong cervical vertebrae would have been required to resist muscle reaction forces during locomotion. On the other hand, the vertebral foramen of the lower cervical vertebrae in Nacholapithecus is almost the same size as or smaller than that of P. cynocephalus. Atlas specimens of Nacholapithecus resemble those of extant great apes with regard to the superior articular facet, and they have an anterior tubercle trait intermediate between that of extant apes and other primate species. Nacholapithecus has a relatively short and thick dens on the axis, similar to those of extant great apes and the axis body shape is intermediate between that of extant apes and other primates. Moreover, an intermediate trait between extant great apes and other primate species has been indicated with regard to the angle between the prezygapophyseal articular facets of the axis in Nacholapithecus. Although the atlas of Nacholapithecus is inferred as having a primitive morphology (i.e., possessing a lateral bridge), the shape of the atlas and axis leads to speculation that locomotion or posture in Nacholapithecus involved more orthograde behavior similar to that of extant apes, and, in so far as cervical vertebral morphology is concerned, it is thought that Nacholapithecus was incipiently specialized toward the characteristics of extant hominoids.

Earliest colobine skeletons from Nakali, Kenya.

Old World monkeys represent one of the most successful adaptive radiations of modern primates, but a sparse fossil record has limited our knowledge about the early evolution of this clade. We report the discovery of two partial skeletons of an early colobine monkey (Microcolobus) from the Nakali Formation (9.8-9.9 Ma) in Kenya that share postcranial synapomorphies with extant colobines in relation to arboreality such as mediolaterally wide distal humeral joint, globular humeral capitulum, distinctly angled zona conoidea, reduced medial trochlear keel, long medial epicondyle with weak retroflexion, narrow and tall olecranon, posteriorly dislocated fovea on the radial head, low projection of the femoral greater trochanter, wide talar head with a greater rotation, and proximodistally short cuboid and ectocuneiform. Microcolobus in Nakali clearly differs from the stem cercopithecoid Victoriapithecus regarding these features, as Victoriapithecus is postcranially similar to extant small-sized terrestrial cercopithecines. However, degeneration of the thumb, a hallmark of modern colobines, is not observed, suggesting that this was a late event in colobine evolution. This discovery contradicts the prevailing hypothesis that the forest invasion by cercopithecids first occurred in the Plio-Pleistocene, and shows that this event occurred by the late Miocene at a time when ape diversity declined.

A new Late Miocene great ape from Kenya and its implications for the origins of African great apes and humans.

Extant African great apes and humans are thought to have diverged from each other in the Late Miocene. However, few hominoid fossils are known from Africa during this period. Here we describe a new genus of great ape (Nakalipithecus nakayamai gen. et sp. nov.) recently discovered from the early Late Miocene of Nakali, Kenya. The new genus resembles Ouranopithecus macedoniensis (9.6-8.7 Ma, Greece) in size and some features but retains less specialized characters, such as less inflated cusps and better-developed cingula on cheek teeth, and it was recovered from a slightly older age (9.9-9.8 Ma). Although the affinity of Ouranopithecus to the extant African apes and humans has often been inferred, the former is known only from southeastern Europe. The discovery of N. nakayamai in East Africa, therefore, provides new evidence on the origins of African great apes and humans. N. nakayamai could be close to the last common ancestor of the extant African apes and humans. In addition, the associated primate fauna from Nakali shows that hominoids and other non-cercopithecoid catarrhines retained higher diversity into the early Late Miocene in East Africa than previously recognized.

An exploratory study on the combined effects of external and internal morphology on load dissipation in primate capitates: its potential for an understanding of the positional and locomotor repertoire of early hominins.

This pilot study explored whether the redirection of stress through trabeculae within morphologically constrained capitates provides information about habitual/positional behaviours unavailable from the study of external morphology alone. To assess this possibility, an experimental finite element approach was taken, whereby no attempt was made to reconstruct the actual magnitudes and loading conditions experienced by the capitates in vivo. Rather, this work addressed fundamental biological questions relating to bone plasticity, i.e. internal versus external bone morphology. The capitates of 7 species with different and - in the case of fossils - inferred locomotor behaviours were selected. Virtual models of capitates were created, scaled to the same size and subjected to the same theoretical load. In the first set of analyses, models were assigned the material properties of bone throughout, whereas in the second set, models were assigned 11 different material properties representing the trabecular architecture derived from high-resolution CT. Species with arboreal behaviours consistently redirected loads towards the ulnar aspect of the capitate when trabeculae were introduced, while terrestrial species, and the bipedal Homo, redirected stress towards the radial side. From these preliminary analyses, it is tentatively concluded that Australopithecus anamensis habitually engaged in arboreal behaviours, whereas Australopithecus afarensis did not.

An additional specimen of a large-bodied Miocene hominoid from Chiang Muan, northern Thailand.

Chiang Muan is the first Miocene fossil site in Southeast Asia, from which large-bodied Miocene hominoids have been discovered (Kunimatsu et al., Primate Res 16:299, 2000a). In this article, we describe a hominoid lower molar (CMu15-5'01) recovered from the Upper Lignite Member of Chiang Muan. The age of Chiang Muan is estimated to be latest Middle or earliest Late Miocene (around 11 Ma), based on the mammalian fauna.

Comparative and functional anatomy of phalanges in Nacholapithecus kerioi, a Middle Miocene hominoid from northern Kenya.

We describe phalanges of the KNM-BG 35250 Nacholapithecus kerioi skeleton from the Middle Miocene of Kenya. Phalanges of N. kerioi display similarities to those of Proconsul heseloni despite their enhanced robusticity. They do not show highly specialized features as in living suspensory primates. However, N. kerioi manifests several distinctive features that are observed in neither living arboreal quadrupeds nor P. heseloni or P. nyanzae. The most remarkable of them is its phalangeal elongation. N. kerioi phalanges (particularly pedal) are as long as those of Pan despite its much smaller body size. While lengthened digits enable a secure grip of supports and are especially adaptive for grasping large vertical trunks, the skeletal and soft tissues are subjected to greater stress. Probably, strong selective pressures favored powerful hallucal/pollical assisted grips. Although this functional adaptation does not exclude the possible use of the terrestrial environment, arboreal behavioral modes must have been crucial in its positional repertoire. N. kerioi is distinguished from P. heseloni in the greater size of its manual phalanges over its pedal phalanges. These derived features of N. kerioi suggest positional modes supporting more weight on the forelimb, and which occur more frequently on vertical supports. If Proconsul is referred to as an "above-branch arboreal quadruped" with a deliberate and effective climbing capability, N. kerioi may be thought of as an "orthograde climber". While living apes are powerful orthograde climbers, they are also more or less suspensory specialists. Suspensory behavior (plus climbing) and pronograde quadrupedalism (plus climbing) are the two main arboreal behavioral adaptations in living anthropoids. Thus, N. kerioi is an unusual fossil primate in that it cannot be incorporated into this dichotomy. It is plausible that a N. kerioi-like orthograde climber with large forelimbs and cheiridia was a precursor of suspensory living apes, and N. kerioi may demonstrate what an initial hominoid of this grade might have looked like.

Preliminary analysis of Nacholapithecus scapula and clavicle from Nachola, Kenya.

The Miocene ape Nacholapithecus is known from rather complete skeletons; some of them preserve the shoulder joint, identified by three scapulae and one clavicle. Comparisons made with other Miocene and living apes ( Proconsul, Equatorius, Ugandapithecus) suggest that the mobility of the scapulohumeral joint was important, and scapular features such as the morphology and position of the spine and the morphology of the acromion and axillary border resemble those of climbing arboreal primates except for chimpanzees, gorillas, or orang-utans. From the size of the scapula (male Nasalis size), it is clear that the animal is smaller than an adult chimpanzee, but the clavicle is almost as relatively long as those of chimpanzees. Some features closer to colobine morphology reinforce the hypothesis that Nacholapithecus was probably a good climber and was definitely adapted for an arboreal life.

Variability of tail length in hybrids of the Japanese macaque (Macaca fuscata) and the Taiwanese macaque (Macaca cyclopis).

In primates, tail length is subject to wide variation, and the tail may even be absent. Tail length varies greatly between each species group of the genus Macaca, which is explained by climatic factors and/or phylogeographic history. Here, tail length variability was studied in hybrids of the Japanese (M. fuscata) and Taiwanese (Macaca cyclopis) macaque, with various degrees of hybridization being evaluated through autosomal allele typing. Relative tail length (percent of crown-rump length) correlated well with the number of caudal vertebrae. Length profiles of caudal vertebrae of hybrids and parent species revealed a common pattern: the length of several proximal-most vertebrae do not differ greatly; then from the third or fourth vertebra, the length rapidly increases and peaks at around the fifth to seventh vertebra; then the length plateaus for several vertebrae and finally shows a gentle decrease. As the number of caudal vertebrae and relative tail length increase, peak vertebral length and lengths of proximal vertebrae also increase, except that of the first vertebra, which only shows a slight increase. Peak vertebral length and the number of caudal vertebrae explained 92 % of the variance in the relative tail length of hybrids. Relative tail length correlated considerably well with the degree of hybridization, with no significant deviation from the regression line being observed. Thus, neither significant heterosis nor hybrid depression occurred.

Systematic morphology and evolutionary anatomy of the autonomic cardiac nervous system in the lesser apes, gibbons (hylobatidae).

We examined the morphology of the autonomic cardiac nervous system (ACNS) on 20 sides of 10 gibbons (Hylobatidae) of three genera, and we have inferred the evolution of the anatomy of the primate ACNS. We report the following. (1) Several trivial intraspecific and interspecific variations are present in gibbons, but the general arrangement of the ACNS in gibbons is consistent. (2) Although the parasympathetic vagal cardiac nervous system is extremely consistent, the sympathetic cardiac nervous system, such as the composition of the sympathetic ganglia and the range of origin of the sympathetic cardiac nerves, exhibit topographical differences among primates. (3) The vertebral ganglion, seldom observed in the Old World monkeys (Cercopithecidae), was consistently present in gibbons as well as in humans. (4) There are fewer thoracic ganglia contributing to the cervicothoracic ganglion in humans than in gibbons and in gibbons than in Old World monkeys. (5) The superior cardiac nerve originating from the superior cervical ganglion, rarely observed in Old World monkeys but commonly observed in humans, was present in 13 of 20 sides (65%), mostly on the left. Accordingly, the ACNS morphology exhibits evolutionary changes within the primate lineage. These evolutionary differences between Old World monkeys, gibbons, and humans are most parsimoniously interpreted as resulting from regular changes in the lineages leading from their common ancestor to the extant species that we dissected. They include the reduction in the number of thoracic ganglia contributing to the cervicothoracic ganglion and the expansion of the range of the cardiac nervous origin.