The life history pattern of recent humans is uniquely derived in many of its aspects including an extended post-reproductive lifespan combined with short interbirth intervals. A number of theories have been proposed to explain the evolution of this unusual pattern. However most have been difficult to test due to the fragmentary nature of the hominin fossil record and the lack of methods capable of inferring such later life history events. In search of a method we tested the hypothesis that the physiologically impactful events of parturition and menopause are recorded in dental cementum microstructure. We performed histomorphological analyses of 47 teeth from 15 individuals with known life history events and were able to detect reproductive events and menopause in all females. Furthermore, we found that other stressful events such as systemic illnesses and incarceration are also detectable. Finally, through the development of a novel analytical method we were able to time all such events with high accuracy (R-squared = 0.92).
Many open questions in evolutionary studies relate to species' physiological adaptations, including the evolution of their life history and reproductive strategies. There are few empirical methods capable of detecting and timing physiologically impactful events such as weaning, parturition and illnesses from hard tissue remains of either extant or extinct species. Cementum is an incremental tissue with post eruption annual periodicity, which covers the tooth root and functions as a recording structure of an animal's physiology. Here we test the hypothesis that it is possible to detect and time physiologically impactful events through the analysis of dental cementum microstructure. Our sample comprises 41 permanent and deciduous teeth from male and female rhesus macaques (Macaca mulatta) with known medical, lifestyle and life history information. We develop a semi-automated method of cementum histological analysis for the purpose of event detection and timing, aimed at significantly reducing the amount of intraand interobserver errors typically associated with histological analyses. The results of our work show that we were able to detect known events including weaning, parturition, illness and physical trauma with high accuracy (false negative rate = 3.2%; n = 1), and to time them within an average absolute difference of 0.43 years (R 2 = .98; p < .05). Nonetheless, we could not distinguish between the several types of stressful events underlying the changes in cementum microstructure. While this study is the first to identify a variety of life history events in macaque dental cementum, laying foundations for future work in conservation and evolutionary studies of both primates and toothed mammals at large, there are some limitations. Other types of analyses (possibly chemical ones) are necessary to tease apart the causes of the stressors.
The adaptive value of facial expressions has been debated in evolutionary biology ever since Darwin's seminal work. Among mammals, primates, including humans, exhibit the most intricate facial displays. Although previous work has focused on the role of sociality in the evolution of primate facial expressions, this relationship has not been verified in a wide sample of species.Here, we examine the relationship between allomaternal care (paternal or alloparental) and the morphology of three orofacial brainstem nuclei (facial; trigeminal motor; hypoglossal) across primates to test the hypothesis that allomaternal care explains variation in the complexity of facial expressions, proxied by relative facial nucleus size and neuropil fraction. The latter represents the proportion of synaptically dense tissue and may, therefore, correlate with dexterity. We find that alloparental care frequency predicts relative neuropil fraction of the facial nucleus, even after controlling for social system organization, whereas allomaternal care is not associated with the trigeminal motor or hypoglossal nuclei. Overall, this work suggests that alloparenting requires increased facial dexterity to facilitate nonverbal communication between infants and their nonparent caregivers and/or between caregivers. Accordingly, alloparenting and complex facial expressions are likely to have coevolved in primates.
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