Muscles Lost in Our Adult Primate Ancestors Still Imprint in Us: on Muscle Evolution, Development, Variations, and Pathologies

Comparative physiological anthropogeny: exploring molecular underpinnings of distinctly human phenotypes

Anthropogeny is a classic term encompassing transdisciplinary investigations of the origins of the human species. Comparative anthropogeny is a systematic comparison of humans and other living nonhuman hominids (so-called "great apes"), aiming to identify distinctly human features in health and disease, with the overall goal of explaining human origins. We begin with a historical perspective, briefly describing how the field progressed from the earliest evolutionary insights to the current emphasis on in-depth molecular and genomic investigations of "human-specific" biology and an increased appreciation for cultural impacts on human biology. While many such genetic differences between humans and other hominids have been revealed over the last two decades, this information remains insufficient to explain the most distinctive phenotypic traits distinguishing humans from other living hominids. Here we undertake a complementary approach of "comparative physiological anthropogeny," along the lines of the preclinical medical curriculum, i.e., beginning with anatomy and considering each physiological system and in each case considering genetic and molecular components that are relevant. What is ultimately needed is a systematic comparative approach at all levels from molecular to physiological to sociocultural, building networks of related information, drawing inferences, and generating testable hypotheses. The concluding section will touch on distinctive considerations in the study of human evolution, including the importance of gene-culture interactions.

Morphological variations of the calcaneal tendon: clinical significance

The calcaneal tendon, the largest and strongest in the human body, is created by the common junction of tendons of the gastrocnemius and soleus muscles. It is not a homogenous structure, being represented by layers in various arrangements. Morphological variability can be seen in the connection between the aponeurosis of the gastrocnemius muscle and the soleus muscle. Some types of plantaris tendon can be associated with a higher possibility of Achilles tendinopathy. Moreover, the presence of accessory structures, such as an accessory soleus muscle or additional gastrocnemius muscle heads may result in symptomatic pathologies. The main aim of this review is to summarize the current state of knowledge regarding the calcaneal tendon. Another aim is to present morphological variations of the calcaneal tendon and their clinical significance. Such information may be useful for clinicians, especially orthopedists, and surgeons. This review also provides an overview of embryological development and morphological variation among fetuses. Materials and methods: review was conducted according to PRISMA guidelines. An electronic search was conducted in five databases. Top quality tools were used to assess the quality of evidence in the studies reviewed. Research papers that made up the database of this review were analyzed, selected and assessed by two independently working researchers.

Gross anatomical description of the extrinsic and intrinsic scapular and brachial muscles of Sapajus apella (Linnaeus, 1758)

BACKGROUND

The robust brown capuchin monkey (Sapajus apella) is a South American primate with preferences for arboreal locomotion, which requires specific thoracic limb muscle adaptations. The present investigation studied the gross anatomy of the extrinsic and intrinsic scapular and brachial muscles.

METHODS

Gross dissections were performed in both thoracic limbs of four formaldehyde-fixed specimens.

RESULTS

Three rhomboideus muscles were present (capitis, cervicis, and thoracis). The trapezius muscle was divided into two parts (cervicis and thoracis). The pectoralis abdominalis and omotransversarius muscles were present. The anconeus muscle was found as an individual muscle or fused to the caput mediale of the triceps brachii muscle. The brachialis muscle had among one and two heads. The anconeus epitrochlearis was absent.

CONCLUSION

These muscles of Sapajus apella are adapted for arboreal locomotion and some terrestrial habits, since these have many similarities with other primates with a similar locomotor patterns.

Muscle spindles in the rhesus monkey platysma

The platysma of the rhesus monkey consists of two parts: a platysma myoides located similar to the human platysma, and a platysma cervicale passing the dorsal cervical region and being in contact with the cheek pouch. Our investigation showed that the muscle fiber morphology was comparable in both parts. Muscle spindles were only present in regions connected to the cheek pouch and contained only nuclear chain fibers. It is tempting to speculate that they sense the filling of the cheek pouch rather than mimic activities.

Morphological variability of the plantaris tendon in the human fetus

Muscular anatomy often differs between species and individuals. In particular, the plantaris muscle (PM) demonstrates great morphological variability in its course and its proximal and distal attachments. The aim of this study was to investigate the morphological variation of the PM tendon in human fetuses. Forty-six spontaneously aborted human fetuses (23 male, 23 female) aged 18-38 weeks of gestation were studied. Morphology of the attachment of the PM was assessed in both lower extremities (n = 92). The PM was present in 72 lower limbs (78.26%) and absent in 20 (21.74%). Eight types of PM distal attachment were identified. We propose an eight-fold classification of PM insertion in fetuses. Leg length, length of tendon, extension point (ExP) from the calcaneus, and ExP thickness differed significantly among types of PM insertion.

Morphological variability of the plantaris muscle origin in human fetuses

INTRODUCTION

The plantaris muscle (PM) is a small, fusiform muscle located between the gastrocnemius muscle (GM) and soleus muscle (SM). PM supports movements of the knee and ankle. This muscle presents a great variability, and also has a high clinical significance. Nevertheless, data concerns morphology and morphometry of the origin of PM in human fetuses are scarce.

MATERIAL AND METHODS

Forty-seven spontaneously-aborted human fetuses (23 male, 24 female) aged 18-38 weeks of gestation were examined. The morphology and morphometry of the origin of PM were evaluated.

RESULTS

PM was present in 74 lower limbs (78.7%), and absent on 20 limbs (21.3%). We distinguished VI types of the proximal attachment of PM. Belly width and thickness, as well as thickness of the tendon and MT junction differed significantly between types of PM origin.

CONCLUSIONS

We distinguished six (I-VI) types of origin of PM in human fetuses. The most common type was type Ia, characterized by an attachment to the lateral head of GM, lateral femoral condyle and to the knee joint capsule. Our results of PM anatomical variation in fetuses will pave the way for detailed comparisons with studies carried out on adult cadavers.

The Visible Ape Project: A free, comprehensive, web-based anatomical atlas for scientists and veterinarians designed to raise public awareness about apes

The Visible Ape Project (VAP) is a free online platform providing unprecedented access to a suite of resources designed to comprehensively illustrate and educate about the anatomy of our closest relatives, the apes. It contains photographs, magnetic resonance images, and computed tomography scans, as well as three-dimensional models that can be manipulated to explore homologies and variations in soft and hard tissues in hylobatids, orangutans, gorillas, chimpanzees, and bonobos. Based at Howard University, a historically black university, it aims to reach communities underrepresented in anthropology and evolutionary biology, providing educational materials appropriate for K-12 and college classrooms in both English and Spanish. Accordingly, VAP incorporates outreach activities to disseminate science and promote awareness of apes, forming partnerships with veterinarians and conservationists in Africa and Asia. In this paper, we present an introduction to the website to illustrate how this accessible, evolving resource can support evolutionary anthropology and related disciplines.

Embryonic evidence uncovers convergent origins of laryngeal echolocation in bats

Bats are the second-most speciose group of mammals, comprising 20% of species diversity today. Their global explosion, representing one of the greatest adaptive radiations in mammalian history, is largely attributed to their ability of laryngeal echolocation and powered flight, which enabled them to conquer the night sky, a vast and hitherto unoccupied ecological niche. While there is consensus that powered flight evolved only once in the lineage, whether laryngeal echolocation has a single origin in bats or evolved multiple times independently remains disputed. Here, we present developmental evidence in support of laryngeal echolocation having multiple origins in bats. This is consistent with a non-echolocating bat ancestor and independent gain of echolocation in Yinpterochiroptera and Yangochiroptera, as well as the gain of primitive echolocation in the bat ancestor, followed by convergent evolution of laryngeal echolocation in Yinpterochiroptera and Yangochiroptera, with loss of primitive echolocation in pteropodids. Our comparative embryological investigations found that there is no developmental difference in the hearing apparatus between non-laryngeal echolocating bats (pteropodids) and terrestrial non-bat mammals. In contrast, the echolocation system is developed heterotopically and heterochronically in the two phylogenetically distant laryngeal echolocating bats (rhinolophoids and yangochiropterans), providing the first embryological evidence that the echolocation system evolved independently in these bats.