Citation
Comparative Analysis of Growth and Development in the Hands of Primates

Material Information

Title:
Comparative Analysis of Growth and Development in the Hands of Primates
Creator:
Levin, Naomi Simcha
Publication Date:
Language:
English

Notes

Abstract:
Much of the scientific literature and osteological research on primates focuses primarily on chimpanzee, human, and macaque. There are few comparative analyses across the primate clades of Strepsirrhini and Haplorhini. Early studies on Macaca mulatta noted the pattern of ossification to be metacarpals first, followed progressively by more distal elements. However, humans appear to be distinct. Schaefer et al. 2009 show that ossification first appears in the distal row of phalanges, metacarpals, proximal phalanges, and lastly the intermediate phalanges. There are some similarities in the ossification sequence between man and macaque: in both species, MC1 is the last of the metacarpals to ossify. I have conducted a unique, comparative analysis of bone growth and development in the hand across a broader range of non-human primates. I addressed the following questions: Are there differences in ossification and fusion patterns across non-human primates at birth? Do these differences correspond to phylogeny or locomotor behavior? I used virtual reconstructions based on CT imaging and radiographs of the hand taken at birth and juvenile stages from multiple primate species, noting the patterns of ossification and fusion. I hypothesized that ossification of hand elements would be affected by phylogenetic group, but not locomotor behavior. ( en )
General Note:
Awarded Bachelor of Arts, magna cum laude, on May 8, 2018. Major: Anthropology
General Note:
College or School: College of Liberal Arts and Sciences
General Note:
Advisor: Valerie DeLeon. Advisor Department or School: Anthropology

Record Information

Source Institution:
University of Florida
Holding Location:
University of Florida
Rights Management:
Copyright Naomi Simcha Levin. Permission granted to the University of Florida to digitize, archive and distribute this item for non-profit research and educational purposes. Any reuse of this item in excess of fair use or other copyright exemptions requires permission of the copyright holder.

UFDC Membership

Aggregations:
UF Undergraduate Honors Theses

Downloads

This item is only available as the following downloads:


Full Text

PAGE 1

Gr owth in the Hands of Primates Comparative Analysis of Growth and Development in the Hands of Primates Author: Naomi S. Levin ompidompi@ufl.edu Advisor: Dr. Valerie DeLeon Ph. D University of Florida Turlington Hall, 330 Newell Dr, Gainesville, FL 32603

PAGE 2

Gr owth in the Hands of Primates Abstract Much of the scientific literature and osteological research on primates focuses primarily on chimpanzee, human, and macaque. There are few comparative analyses across the primate clades of Strepsirrhini and Haplorhini. Early studies on Macaca mulatta noted the pattern of ossification to be metacarpals first, followed progressively by more distal elements. However, humans appear to be distinct. Schaefer et al. 2009 show that ossification first appears in the distal row of phalanges, metacarpals proximal phalanges, and lastly the intermediate phalanges. There are some similarities in the ossification sequence between man and macaque: in both species, MC1 is the last o f the metacarpals to ossify. I have conducted a unique, comparative analysis o f bone growth and development in the hand across a broader ra nge of non human primates. I addressed the following questions: Are there differences in ossification and fusion patterns across non human primates at birth? Do these differences correspond to ph ylogeny or locomotor behavior? I used virtual reconstructions based on CT imaging and radiographs of the hand taken at birth and juvenile stages from multiple primate species, noting the pattern s of ossification and fusion. I hypothesized that ossification of hand elements would be affected by phylogenetic group, but not locomotor behavior.

PAGE 3

Gr owth in the Hands of Primates Introduction The scientific literature for human ossification and development through fetal and juvenile stages is significant and vast, comparatively there is considerably less written in the scientific literature on such patterns and observations across the Primate o rder. Biological anthropologists and evolutionary biologists have long been fascinated by evolutionary relationships among the different primate species. Debates continue today as to which species are more closely related on the evolutionary tree as new discoveries and techniques provide unique insight into this topic. Early studies on non human primates were focused mainly on Macaca mulatta Macaca nemestrina (Ruppenthal and Reese 1979; Van Wagenen and Hebert 1964), Pan troglodytes (Gavan 1953; Nissen an d Riesan 1949), and Callithrix jacchus (Phillips 1976). Comparatively, our egocentric mentality has led to countless studies, papers, articles, and books on the growth and development of humans, useful in forensics and pediatrics. Bones form through the pr ocess of ossification which can occur through two methods: endochondral and cartilaginous ossification. The hand materials ossify via endochondral ossification. This process results in growth and healing of bone materials. This process first begins with a r esses, the cartilaginous model is perforated by blood vessels and osteoclasts (which remove bone an d cartilage), bone marrow and osteoblasts (which lay down new bone cells, thus resulting in ossification). As this process continues, the materials (phalanges and metacarpals) grow out from the center of the primary ossification center, the shaft. A second ossification center forms proximally for the phalanges and metacarpal (MC) 1 and distally for MCs 2 5 However, it should be noted there was variation in the location of the secondary ossific ation center for MC 1 for some of the non human specimens in thi s study. See Results

PAGE 4

Gr owth in the Hands of Primates The hand is the distal extremity of the upper limb. Within the hand are bones classified as carpals, metacarpals, and phalanges (the fingers, which are again divided into three rows being proximal, intermediate, or distal phalanges). Appea rance times of hand materials are meticulously studied and recorded in human juveniles (e.g., Garn et al. 1967; Birkner 1978 ; r eviewed in Schaefer 2009). Garn and colleagues (1967) study included participants from the Fels Research Insti tute Program of Human Development and provides information for each ossification center in the hand and assessments for both male and female for the 5 th 50 th and 95 th percentiles. For example, they showed t hat the capitate and hamate are the first two car pals to (1978) images which show ossification and development of the hand and wrist elements. Females appear to develop more rapidly than males while there is some overlap between lower female and higher male pe rcentiles (Garn et al. 1967 ). These two studies were used to compare human growth and development patterns to those observed in the current study in non human primates. Ruppenthal and Reese (1979) state that evolutionary scale is inver sely proportional to ossification and maturation. The current study attempts to provide more information on this statement, and proves true when humans are included in the discussion. However, humans tend to be an anomaly in terms of evolution, being uni que in many attributes. Take humans out of the equation, and this pattern observed by Ruppenthal and Reese (1979) becomes more ambiguous. This work provides insight into important implications from studying non human hand and wrist ossification patterns as den metacarpal bones are significantly important and useful in age assessments of nonhuman materials. Bot h thi s book and observations from my study show significant acceleration of

PAGE 5

Gr owth in the Hands of Primates development and maturation of bone in nonhuman primates compared to humans Though not tested in this study, Ruppenthal and Reese (1979) also noted that monkeys with a longer gestation period tended to be heavier and have mor e advance d skeletal development. My study did not include gestation period nor body weight. In a study conducted in 1964 by Van Wagenen and Hebert on ossification patterns in Macaca mulatta the researchers documented the ages at whic h hand and wrist materials ossified through roentgenographic methods. This study was used for comparing ossification patterns in macaques and other specimens in this study. A pattern noted in the study by Van Wagenen and Hebert is that by days 70 72 gestation ossif ication centers in the metacarpals begin to appear and by days 120 125, MC 3 and MC 4 are the most developed in that row of elements (Van Wagenen and Hebert 1964). This pattern observed in their macaque specimens was also seen in some of my specimens. One of the specimens which presented a similar pattern of development as stated in the macaque is Galago moholi (P3132) aged 30 days. Observation taken based on the 3D scans show that all metacarpal epiphyses are pre sent with MC1 being the least developed and MC3 being the most. Another specimen which shows advanced development in MC3 and 4 is Propithecuus verrauxi (P5640). This specimen is the same age as G. moholi previously mentioned secondary epiphyses present onl y on MCs3 and 4 in this row. Note that both specimens are classified as strepsirrhines. Chimpanzees are of particular interest to researchers because of their close evolutionary relationship with humans Nissen and Riesen (1949) used serial x ray photography to study ossification patterns in nine male and seven female chimpanzees These data were used to estimate the ages at which ossification began, looking at 70 epiphyses and short bones in the left extremity. The researchers noted a difference in ossifica tion advancement between the male and

PAGE 6

Gr owth in the Hands of Primates female specimens, females being more advanced than males, a pattern also observed in humans. They also noted patterns related to the length of the gestation period, with shorter periods leading to the retardation of ap pearance of ce nters (Nissen and Riesen 1949). They found no correlation between timing of appearance of centers and birth weight. However, Ruppenthal and Reese (1979) stated that monkeys that were heavier showed more advanced stages of develop ment. Both agree that gestation impacts progression of ossification and development. Phillips (1976) conducted a study o f skeletal development in fetal and neonatal marmosets He used methods of radiography and alizarin preparation of 30 Callithrix jacchus fetuses. These were at various stages of gestation. In addition, he studied 29 preserved neonates ranging in age from birth to 50 days. The sam e pattern was observed in the metacarpal row as was previously seen by Van Wagenen and Hebert (1964) in their macaques What these specimens have in common is that metacarpals are the first (if not among the first) elements in the hand to ossify beginning with the shaft From the radiographs in t he study, Phillips noted that metacarpal s were first visib le in fetuses less than 12 days before birth. At 105 to 115 days gestation, the primary ossification centers for MCs 2, 3, and 4 were seen, and MC1 proximal epiphysis was present last, in the oldest specimen of 32 days. Our knowledge of the evolution of p rimates is based on numerous traits such as encephalization, orthograde versus pronograde posture, derived dental formulae, increased parental investment, adult morphology, and locomotor behavior etc. Variations in these categories reflected throughout the taxa and numerous cl assification systems within the primate evolutionary tree. The bones of the hand and feet may provide significant taxonomic markers for l ater stages of specialized taxa

PAGE 7

Gr owth in the Hands of Primates Divergence between human ancestors and chimpanzees is estimated to be between 4 6 MYA (million year s ago); the divergenc e between gorillas and the human/ chimpanzee clade is roughly 7 9 MYA, and the hominoids and cercopithecoid divergence is about 30 40 MYA (Yoder & Ziheng 2000). With each divergence, new species emerge with either derived or ancestral traits which may be tra ced back to a last common ancestor (LCA). Humans share about 95% of their DNA with chimpanzees, our closest relative, a sister taxon (Britten 2002). This high percent of shared DNA shows there are fundamental similarities between the two species on a cellu lar level, in addition to the numerous behavioral and morphol ogical similarities. This led me to hypothesize that species closely related would be more similar in the ossification and development than species more distantly related, primarily focusing on t he hand and wrist. Also, as Beigert (2009) stated, the function of a bone or region of the body impacts the morphology, or shape or appearance, of the material(s). This led me to consider an alternative hypothesis that adult locomotor behavior also i nfluences the development of skeletal eleme nts in the body, as we are adapted to our environment. In addition to supplementing evolutionary analytical methods, the results from subsequent studies can be used in estimating age of primates, as the ossification and fusion of carpals and metacarpals are more accurate than dental age estimations (Ruppenthal and Reese 1979). Dental age estimations for humans have been studied extensively and can be accurate and useful in forensic contexts. Age estimations of non human primates through the ossification patterns in the hands and wrists may be useful in primatology and biological anthropological studies of development, shedding more light on the broader story of evolution. I have conducted a unique, comparative analysis of bone growth and development in the hand across a broad range on non human primates. Studies show that bone development is

PAGE 8

Gr owth in the Hands of Primates influenced by evolutionary history but animals are also skeletally adapted to their environment. I hypothesized that ossification of hand elements would be affected by phylogenetic group, not locomotor behavior. Materials and Methods Primate specimens were individually selected from the Newborn Primate Database of Dr. DeLeon and her colleague s which includes micro CT scans of primates at different developmental stages Spe cimens were grouped into H aplor hine and S trepsirrhine clades. There were more specimens in the dataset in the Strepsirrhini suborder (n=16) than in Haplorhini (n=8) I used virtual reconstructions based on those micro CT images of the hand taken at birth and juvenile stages from multiple primate species, not ing the pattern of ossification. Each micro CT images were uploaded to the Amira program, where they were virtually reconstructed by extracting isosurfaces based on density threshold Notes and observations on patterns were placed in an Excel chart for easy access and organizational purposes. Specimens were chosen based on the inclusion of an entire right hand, left hand or both hands in the micro CT scans of the specimen The left hand was preferred for consistency, but if it was unavailable, (i.e., not included in the scan or partially included in the scan) then the right hand was used. The majority of specimens were perinatal (late fetal through 6 days old). Percent total ossification (%TO) represents the number of ossified elements present divided by the total materials that would be expected to ossify later in li fe. Carpals, epiphyses, and shafts were considered separate elements. Results

PAGE 9

Gr owth in the Hands of Primates Observations on Ossification in Haplorhines General observations reported for the Haplorhine suborder include wide variation in perinatal primates, as well as continued greater development when compared to Strepsirrhines of similar age. Note these descriptions focus on secondary epiphyses and the presence of carpals because metacarpal and phalanx shafts are all present in these micro CT im ages. Their presence in not listed here for each individual, but were included in calculations and comparisons in this study. Aotus nancymaae stillborn, had no secondary ossification centers present in both the left and right hands. There were two carpals that had begun to ossify in the left and right hands (possibly three in the right). Callicebus cupres stillborn, similar to A. n ancymaae had no secondary ossification centers, or epiphyses, present in either hand. Saguinus midas stillborn, presented different patterns in its development than those observed in the other two stillborn Haplorhine individuals. S. midas epiphyses of the metacarpals. The right hand included th ree carpals, clear ossification of metacarpal distal epiphysis for ray two, three and four, with a possible epiphysis for the first metacarpal just beginning the process. Other Haplorhines included in the study were two Tarsius syrichta two Cebuella pygm aea and one Saguinus oedipus The fetal Philippine Tarsier showed no carpals or epiphyses present, similar to the pattern observed in A. nancymaae and C. cupre u s The second tarsier (age CT images include the presence of five carpals and epiphyses for the first, second, and third metacarpals in the right hand. In the left hand (which was used for %TO comparisons) includes five carpals and epiphyses for metacarpals one, three, and four.

PAGE 10

Gr owth in the Hands of Primates The n ewborn Cebuella pygmaea or pygmy marmoset, had both hands included in the scans. The right hand materials included five (possibly six) carpals ossified, all metacarpal epiphyses, proximal row epiphyses, and intermediate row epiphyses ossified. The left ha nd used for comparisons included five (possibly seven) carpals ossified. All metacarpal materials, epiphyses for proximal phalanges one four, all intermediate row epiphyses, and distal epiphyses for phalanges one, three, and four were present. The second C pygmaea was described as bones present anteriorly on all metacarpals. Placed in the same age category as the subadult C. pygmaea Saguinus oedipus a cotton to p tamarin also presented a high development of ossification. At age P27, all carpals are present. All metacarpal material, proximal row and intermediate row materials, and epiphyses for distal phalanges two, three and four are present. The epiphysis for th e distal phalanx in ray two appears to beginning its ossification process, while those for ray one and five have not yet begun at this age. The images in this section (1a h ) show the ossified elements of the Haplorhine specimens used in this study.

PAGE 11

Gr owth in the Hands of Primates 1a. P96 1b. C. cupres 1c. Smid6 1d. Aotus101 1e. P93 1f. Cp17 1g. So111 1h. Cp14

PAGE 12

Gr owth in the Hands of Primates Observations on Ossification in Strepsirrhines There were more specimens in the dataset i n the Strepsirrhine suborder, 16 individuals, than in Haplorhine. Perinatal Strepsirrhines : Two individuals, Hapalemur griseus and Propithecus coquereli ers present in either of hands, ap art from a possible carpal in the right hand of P. coquereli At the fetal stage, Loris tardigradus also presented no epiphyses or carpals. Also two and five. La te fetal strepsirrhines include Loris tardigradus Otolemur crassicaudatus and Lemur catta Observations on both left and right hands of these three specimens note the absence of ossified carpals and secondary centers. Cheirogaleus medius and Galago moho li micro CT images were taken at age P0. The right hand was used for analysis of C. medius and did not show any carpals or epiphyses. The left hand for G. moholi presented the same pattern. The second C. medius aged P1, and Eulemur mongoz aged P1, prese nted the same stage of development as observed at age P0. Juvenile Strepsirrhines: The left hand of Galago senegalensis at age P7 showed two carpals in the micro CT images but no other secondary centers appeared to be ossifying. At age P23, G. senegalensis secondary ossification centers were beginning to appear as the individual developed. The left hand included all carpals, epiphyses for metacarpals two and four, intermediate phalanges two, three, and four, as well as the epiphysis for distal phalanx one. There were no epiphyses present in the proximal row of phalanges, nor for the other four distal phalanges. Eulemur mongoz

PAGE 13

Gr owth in the Hands of Primates analysis and comparison. The right and left hand materials were at the same stage of development, and included five carpals, epiphyses for metacarpals two, three and four, all epiphyses in the proximal and intermediate rows. The distal row in the right hand did not present and epiphyses at this stage. Galago moholi and P ropithecus coquereli included images taken at age P30. All epiphyses for the metacarpals, and proximal and intermediate phalanges were present for G. moholi All carpals were also present, as well as epiphyses for distal phalanges one, three and four. It appears that some of these epiphyses have also begun the fusion process though they are still clear distinguishable from their respective shafts. For P. coqu ereli the right hand was used. This specimen was significantly less advanced in ossification; no carpals were present and no epiphyses in the proximal and distal rows. Those materials that did have ossifying epiphyses include metacarpals three and four an d intermediate phalanges three and four. The left and right hands for Galago moholi at age P97 were included in the scans. The right hand materials included all carpals and all secondary epiphyses. There were also two sesamoid bones present on each metacar pal shaft. The materials in the distal row were fused. Similarly, the left hand materials included all carpals, metacarpal sesamoid bones, and all epiphyses. The intermediate and distal materials appeared further developed because these materials were fuse d while materials in the proximal row have not yet completed fusion. The images in this section (2a p ) show the ossified elements of the Strepsirrhine specimens in this study.

PAGE 14

Gr owth in the Hands of Primates 2a. P696 2b. P2046 2c. P1308 2d. DLC6905 2e. P2934 2f. P2935

PAGE 15

Gr owth in the Hands of Primates 2g. P2810 2h. P6888 2i. P1600 2j. P6426 2k. Is1020 2l. Ry2018

PAGE 16

Gr owth in the Hands of Primates 2m. P6132 2n. P5640 2o. P6132 2p. P3015 Comparative Analysis Most of the primates in this study showed an MC1 epiphysis at the base rather than at the head, similar to the pattern seen in humans. Those which exhibit a distal MC1 different to humans include Cebue lla pygmaea (Cp17) and Saguinus midas (Smid6) This pattern is interesting because it is different from the pattern observed in humans but is rare in non human primates. This is of further interest because Cp17 and Cp14 are both of the same specie s but one shows a distal epiphysis while the other shows a proximal one. It is unclear if this is individual variation. A similarity between both phylogenetic groups, Haplorhini and Strepsirrhini, is the ossification pattern observed at birth. Primary ossification c enters (shafts) of the metacarpals and

PAGE 17

Gr owth in the Hands of Primates phalanges are present, with secondary centers (epiphyses) and carpals appearing in the older age groups. This study had comparatively more strepsirrhine specimens than haplorhines (16 specimens in Strepsirrhini and eight specimens in Haplorhini) Suborder Haplorhini showed more variability in ossification at birth, whereas the roles were switched in the older age groups with strepsirrhines showing more variability around P30 (day 30 after birth). Among perinatal specimens, strepsirrhines had a mean %TO of 40.62% and haplorhines had a mean %TO of 54.90% ( Table 1 ). This apparent difference in mean %TO shows that, on average, haplorhines show earlier ossification at or around the time of birth than do strepsirrhines. Among older juveniles (P11 P30), the strepsirrhine G. moholi that of haplorhine S. oedipus (97.9%) than it is to the more closely related G. senegalensis (80.9%), contrary to the hypothesis that ossification patterns are relate d to phylogeny ( Table 1 ). This could be due to individual variation. Also among older juveniles (P23 P30), the haplorhines still appear to be more advanced (97.9 100%) than strepsirrhines, although the sample size is very small. The strepsirrhines showed a high level of variation at this later stage, ranging from 53.2% in Propithecus to 96.7% in Galago ( Table 1 ). This may also be attributed to individual variation. An effect of locomotor behavior is not clear.

PAGE 18

Gr owth in the Hands of Primates Table 1 A chart showing each taxon grouped by age and suborder classification with %TO and locomotor behavior. Discussion Haplorhines appear to show a more advanced stage of ossification than strepsirrhines at birth. Haplorhines showed more variability at birth, whereas strepsirrhines showed more va riation at one month of age. My limited sample size of older juveniles did not allow me to discern significant patterns of ossification related to phylogeny or locomotor behavior. An interesting observation was the similarity between humans and most of the other primates in this study sh aring a ~40.40%TO at birth (Schaefer et al., 2009). Human development is particularly interesting, in that, due to the derived characteristic of having a prolonged gestation period and long childhood, human develo pment takes a considerably longer time to progress when compared with all other primates. As expected, when the stage of development between humans and other primates are compared at the same, or similar, ages, humans have a slower rate of

PAGE 19

Gr owth in the Hands of Primates development. In comparison, Galago (P97 with 100%TO) appeared to have all elements ossified ( Figure 2 ). Figure 2 also presents data contradicting the stated hypothesis that ossification patterns are related to phylogeny. Here, Galago moholi a strepsirrhine specie s (95.7%TO) is closer in development to Saguinus oedipus a haplorhine specie s (97.9%TO) than to the more closely related Galago senegalensis (80.9%TO) It is unclear if this pattern relates to function of the hand. Figure 2. A bar graph showing the %TO observed in the older juvenile age group. Conclusions This study was restricted by the limited data set and could not fully answer the questions put forth in the beginning. While ther e were some patterns and observations which can be attributed to phylogeny, supported the initial hypothesis, there were also those that did not agree with the hypothesis. There is yet to be a clear explanation for these and other outliers that were present. To begin with, more specimens would need to be added to the existing Newborn Primate

PAGE 20

Gr owth in the Hands of Primates Database from which these data were collected. Including apes would help in determining the potential influence of phylogenetic relationships between humans and the other primates. Bei ng the only habitual bipedal primate did not allow me to determine if the locomotor behavior was related to ossification patterns in humans. With more specimens added to the database, researchers could better control for interspecific variation in ossification. There should also be background knowledge of each birth of the specimens, such as the health of the mother, the duration of the pregnancy, the sex of the baby, and how the individual died. These variables could have also factore d into the advancement or delay of ossification in these primates. It is unclear why some of the spec imens were stillborn, leading to the question whether a complication with the pregnancy led to a delay in development, thus producing a lower %TO. This is just one part in the greater picture of evolution. Primate hand ossification is helpful for age estimations and would be useful in biological anthropology, ecology, and primatology. This and other research from this database could help us better understand the specifics of ossification and development in the hand and wrist as related to evolution.

PAGE 21

Gr owth in the Hands of Primates References Birkner, R. Normal Radiographic Patterns and Variances of the Human Skeleton An X ray Atlas of Adults and Children. Baltimore (Munich): Urban and Schwarzenberg. 1978. Proceedings of the National Academy of Sciences vol.99, no.21, 2002, pp. 13633 13635., doi:10.1073/pnas.172510699. Garn, S.M., Rohm ann, C.G., and Silverman, F.N. Radiographic standards for postnatal ossification and tooth calcification. M edical Radiography and Photography 43 : 45 66. 1967. Gavan, J.A. Growth and Development of the Chimpanzee: A longitudinal and comparative study. Hum. Biol. 25:93 143, 1953 Gisler, D. B., Wilson, S. G., and Hekhuis, G. L. Correlati on of skeletal growth and epiphyseal ossification with age of monkeys. Ann. NY Acad. Sci. 85:64 66, 1962 Mackie, E. J., et al. Multi Functional Complex Organ. The Growth Plate Journal of Endocrinology vol. 211, no. 2, Mar. 2011, pp. 109 121., doi10.1530/joe 11 0048. Menees, T. O. and Holly, L. E. The ossification in the extremities of the newborn. American Journal of Roentgenology 28:389 390, 1932 Nissen H. W. and Riesen, A. H. Onset of ossification in the epiphyses and short bones of extremities in chimpanzees. Growth 13:45 70, 1949 Phillips, T. R. Skeletal development in the fetal and neonatal marmoset ( Callithrix jacchus ). Lab. Anim. Sci. 10:317 333, 1976 Ruppenthal, G. C., and Reese D. J Nursery Care of Nonhuman Primates Plenum, 1979. Schaefer, M. et al. Juvenile Osteology: a Laboratory and Field Manual Academic Press Elsevier, 2009. Van Wagenen, G. and Hebert, C. N. Ossification in the fetal monkey ( Macaca mulatta ) estimation of age and progress of gestation by roentgenography. American Journal of Anatomy 114:107 125, 1964

PAGE 22

Gr owth in the Hands of Primates Yoder, A. D., and Yang Z Molecular Biology and Evolution vol. 17, no. 7, 2000, pp. 1081 1090., doi:10.1093/oxfordjournals.molbev.a026389.