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Human newborns are considered altricial compared with other primates because they are relatively underdeveloped at birth. However, in a broader comparative context, other mammals are more altricial than humans. It has been proposed that altricial development evolved secondarily in humans due to obstetrical or metabolic constraints, and in association with increased brain plasticity. To explore this association, we used comparative data from 140 placental mammals to measure how altriciality evolved in humans and other species. We also estimated how changes in brain size and gestation length influenced the timing of neurodevelopment during hominin evolution. Based on our data, humans show the highest evolutionary rate to become more altricial (measured as the proportion of adult brain size at birth) across all placental mammals, but this results primarily from the pronounced postnatal enlargement of brain size rather than neonatal changes. In addition, we show that only a small number of neurodevelopmental events were shifted to the postnatal period during hominin evolution, and that they were primarily related to the myelination of certain brain pathways. These results indicate that the perception of human altriciality is mostly driven by postnatal changes, and they point to a possible association between the timing of myelination and human neuroplasticity.

 

A central challenge for biology is to reveal how different levels of biological variation interact and shape diversity. However, recent experimental studies have indicated that prevailing models of evolution cannot readily explain the link between micro- and macroevolution at deep timescales. Here, we suggest that this paradox could be the result of a common mechanism driving a correlated pattern of evolution. We examine the proportionality between genetic variance and patterns of trait evolution in a system whose developmental processes are well understood to gain insight into how such alignment between morphological divergence and genetic variation might be maintained over macroevolutionary time. Primate molars present a model system by which to link developmental processes to evolutionary dynamics because of the biased pattern of variation that results from the developmental architecture regulating their formation. We consider how this biased variation is expressed at the population level, and how it manifests through evolution across primates. There is a strong correspondence between the macroevolutionary rates of primate molar divergence and their genetic variation. This suggests a model of evolution in which selection is closely aligned with the direction of genetic variance, phenotypic variance, and the underlying developmental architecture of anatomical traits. 

Effective interpretation of historical selective regimes requires comprehensive in vivo performance evaluations and well-constrained ecomorphological prox- ies. The feeding apparatus is a frequent target of such evolutionary studies due to a direct relationship between feeding and survivorship, and the durability of craniodental elements in the fossil record. Among vertebrates, behaviors such as bite force have been central to evaluation of clade dynamics; yet, in the absence of detailed performance studies, such evaluations can misidentify potential selective factors and their roles. Here, we combine the results of a total-clade performance study with fossil-inclusive, phylogenetically informed methods to assess bite-force proxies throughout mesoeucrocodylian evolution. Although bite-force shifts were previously thought to respond to changing rostrodental selective regimes, we find body-size dependent conservation of performance proxies throughout the history of the clade, indicating stabilizing selection for bite-force potential. Such stasis reveals that mesoeucrocodylians with dietary ecologies as disparate as herbivory and hypercarnivory maintain similar bite-force-to-body-size relationships, a pattern which contrasts the pre- cept that vertebrate bite forces should vary most strongly by diet. Furthermore, it may signal that bite-force conservation supported mesoeucrocodylian craniodental disparity by providing a stable performance foundation for the exploration of novel ecomorphospace.