Biomedical subjects

Nathan M Young

Publications and source records attributed to Nathan M Young.

6 recordsLinked to original sources

PubMed2006

Primate molecular divergence dates.

With genomic data, alignments can be assembled that greatly increase the number of informative sites for analysis of molecular divergence dates. Here, we present an estimate of the molecular divergence dates for all of the major primate groups. These date estimates are based on a Bayesian analysis of approximately 59.8 kbp of genomic data from 13 primates and 6 mammalian outgroups, using a range of paleontologically supported calibration estimates. Results support a Cretaceous last common ancestor of extant primates (approximately 77 mya), an Eocene divergence between platyrrhine and catarrhine primates (approximately 43 mya), an Oligocene origin of apes and Old World monkeys (approximately 31 mya), and an early Miocene (approximately 18 mya) divergence of Asian and African great apes. These dates are examined in the context of other molecular clock studies.

Animals↗

PubMed2006

Function, ontogeny and canalization of shape variance in the primate scapula.

Primates have shoulders adapted to a wide range of locomotor functions from terrestrial pronograde quadrupedalism to highly arboreal suspensory behaviours. The shape of the scapula tightly follows these functional differences. Previous analyses of primate postcrania, including the scapula, indicate that quadrupedal monkeys are less variable than non-quadrupeds. It was previously suggested that this difference was due to a relationship between the strength of stabilizing selection and the functional demands of the upper limb. Here it is shown that intraspecific scapular shape variance is highly correlated with the degree of committed quadrupedalism. Primates that engage in frequent suspensory behaviours (e.g. apes and ateline monkeys) average twice the amount of shape variance as quadrupeds (e.g. Old World monkeys and Saimiri). Because this difference in intraspecific shape variance is apparent in infants and does not increase or decrease appreciably over ontogeny, it is not likely that differences in postnatal growth, neuromuscular control or environmental factors such as habitat structure/composition are the primary contributors to differences in adult shape variance. Instead variance in embryonic factors that affect the shape/size of the scapula or epigenetic factors associated with muscle attachments are more likely candidates. In particular, the heterogeneous functional demands of the non-quadrupedal shoulder probably reduce the stringency of stabilizing selection, resulting in the persistence into adulthood of increased amounts of embryonically generated scapular shape variance.

Animals↗

PubMed2005

Serial homology and the evolution of mammalian limb covariation structure.

The tetrapod forelimb and hindlimb are serially homologous structures that share a broad range of developmental pathways responsible for their patterning and outgrowth. Covariation between limbs, which can introduce constraints on the production of variation, is related to the duplication of these developmental factors. Despite this constraint, there is remarkable diversity in limb morphology, with a variety of functional relationships between and within forelimb and hindlimb elements. Here we assess a hierarchical model of limb covariation structure based on shared developmental factors. We also test whether selection for morphologically divergent forelimbs or hindlimbs is associated with reduced covariation between limbs. Our sample includes primates, murines, a carnivoran, and a chiropteran that exhibit varying degrees of forelimb and hindlimb specialization, limb size divergence, and/or phylogenetic relatedness. We analyze the pattern and significance of between-limb morphological covariation with linear distance data collected using standard morphometric techniques and analyzed by matrix correlations, eigenanalysis, and partial correlations. Results support a common limb covariation structure across these taxa and reduced covariation between limbs in nonquadruped species. This result indicates that diversity in limb morphology has evolved without signficant modifications to a common covariation structure but that the higher degree of functional limb divergence in bats and, to some extent, gibbons is associated with weaker integration between limbs. This result supports the hypothesis that limb divergence, particularly selection for increased functional specialization, involves the reduction of developmental factors common to both limbs, thereby reducing covariation.

Animals↗

PubMed2004

Genomic data support the hominoid slowdown and an Early Oligocene estimate for the hominoid-cercopithecoid divergence.

Several lines of indirect evidence suggest that hominoids (apes and humans) and cercopithecoids (Old World monkeys) diverged around 23-25 Mya. Importantly, although this range of dates has been used as both an initial assumption and as a confirmation of results in many molecular-clock analyses, it has not been critically assessed on its own merits. In this article we test the robusticity of the 23- to 25-Mya estimate with approximately 150,000 base pairs of orthologous DNA sequence data from two cercopithecoids and two hominoids by using quartet analysis. This method is an improvement over other estimates of the hominoid-cercopithecoid divergence because it incorporates two calibration points, one each within cercopithecoids and hominoids, and tests for a statistically appropriate model of molecular evolution. Most comparisons reject rate constancy in favor of a model incorporating two rates of evolution, supporting the "hominoid slowdown" hypothesis. By using this model of molecular evolution, the hominoid-cercopithecoid divergence is estimated to range from 29.2 to 34.5 Mya, significantly older than most previous analyses. Hominoid-cercopithecoid divergence dates of 23-25 Mya fall outside of the confidence intervals estimated, suggesting that as much as one-third of ape evolution has not been paleontologically sampled. Identifying stem cercopithecoids or hominoids from this period will be difficult because derived features that define crown catarrhines need not be present in early members of these lineages. More sites that sample primate habitats from the Oligocene of Africa are needed to better understand early ape and Old World monkey evolution.

Animals↗

PubMed2004

The phylogenetic position of Morotopithecus.

The phylogenetic relationship of the Ugandan Miocene hominoid Morotopithecus bishopi to fossil and living hominoids remains to be determined. In a cladistic approach to this question, we used three published Miocene character sets as the basis of a phylogenetic analysis: J. Hum. Evol. 29 (1995) 101; Function, Phylogeny, and Fossils: Miocene Hominoid Evolution and Adaptations, 1997, 389. Because these datasets often describe the same anatomy using different characters and states, three different datasets were created to reflect these alternatives. In addition, new postcranial characters describable in Morotopithecus were added to each of the above datasets and a fourth dataset was created using only postcranial characters. The most parsimonious tree(s) recovered in all analyses consistently placed Morotopithecus as a sister taxon to the extant great apes, with Hylobates sister to this clade. Morotopithecus was also consistently more derived than Proconsul, Afropithecus, and Kenyapithecus (as defined prior to the description of Equatorius), but less derived than Oreopithecus, Sivapithecus (only craniodentally) and Dryopithecus. These results imply that Morotopithecus is more derived than Hylobates. However, gibbons are believed to have branched off by at least 18 Ma while Morotopithecus is dated at >20.6 Ma. Possible explanations include: (1) the dating of the Morotopithecus material is too old; (2) the Hylobates divergence time has been underestimated; (3) the great ape condition, and not that of Hylobates, is primitive for hominoids; (4) the similarities of Morotopithecus and great apes are homoplasies. Given current evidence, the first possibility is unlikely, but it is not possible to choose definitively between the latter three possibilities. This conclusion is supported by the fact that despite the consistencies of the analyses, the addition of Morotopithecus and the use of different characters had a large effect on the placement of other Miocene taxa. This raises questions as to the robustness of the connections between Miocene taxa and extant hominoids since different results can be achieved by changing either a few characters, or by adding a single taxon. Many of the characters used to estimate phylogeny may need to be reassessed before a reliable assessment of the phylogenetic position of Morotopithecus can be achieved.

Animals↗

PubMed2003

A reassessment of living hominoid postcranial variability: implications for ape evolution.

In an analysis of hominoid postcranial variation, 'Evol. Anthrop. 6 (1998) 87' argued that many purportedly unique features of the hominoid postcranium are actually much more variable than previously reported and in many instances overlap with both suspensory (Ateles) and non-suspensory primates. Based on these results, it was concluded that parallelism in the living ape postcranium was a plausible and even likely possibility given the Miocene hominoid postcranial record. However, this analysis did not distinguish whether within-hominoid variability or overlap with non-hominoids involved one or all ape taxa, a distinction which has potentially important effects on the interpretation of results. To address this issue, primate postcranial morphometric data from the trunk and forelimb were reanalyzed using three techniques: cladistic analysis, principle components analysis, and cluster analysis. Results reveal that these postcranial characters distinguish not only suspensory and quadrupedal primates but also discriminate hominoids and Ateles from all other taxa, great apes from lesser apes and Ateles, cercopithecines from colobines, and cercopithecoids from platyrrhines. The majority of hominoid variability and overlap with Ateles occurs with Hylobates humeral head and shoulder joint characters related to brachiation. This suggests that Hylobates' specializations may skew analyses of hominoid postcranial uniqueness and variability, and that great apes are relatively similar in their postcranium.

Anatomy, Comparative↗