Human evolution: Thumbs up for efficiency

Evolutionary selection and morphological integration in the hand of modern humans

OBJECTIVES

To enhance our understanding of the evolutionary dynamics of the modern human hand by analyzing the degree of integration and ability to respond to selection pressures of each phalanx and metacarpal bone.

MATERIALS AND METHODS

The sample comprised 96 adult individuals, both female and male, from Euro-American, Afro-American, and European populations. We collected 10 linear measurements from the 19 metacarpals and proximal, middle, and distal phalanges that constitute the five digits of the hand. Using these data, we constructed variance/covariance matrices to quantify the degree of integration and assess the hand ability to respond to selective pressures.

RESULTS

Distal phalanges are the most evolvable and flexible elements, while being the least integrated and constrained. The thumb is similarly integrated as the second and third rays, while medial rays (fourth and fifth digits) are more integrated. However, the thumb presents different integration and response to selection patterns. No significant relationship was found between functionality and the indices of selection and integration. Finally, the correlation between hand and foot indices yielded significant results for conditional evolvability and flexibility.

DISCUSSION

The findings suggest different evolutionary trajectories for the metacarpal and distal phalanx in the modern human thumb, likely reflecting varying functional and developmental pressures. The first metacarpal, characterized by high flexibility and low evolvability, appears to have reached a stable, optimal morphology, under stabilizing selection. In contrast, the distal phalanx seems to have undergone directional evolution, suggesting specialization for a specific function. Comparisons between hands and feet suggest that these structures evolve differently under directional selection but similarly under stabilizing selection.

Step by step enhancement of aesthetics for distal phalangeal prosthetic replacement using neoteric stamp technique: A case report

This case study delineates the proficient creation of a silicone finger prosthesis, tailored for a patient contending with partial digit amputation. The prosthesis was devised with the overarching goal of reinstating not only the physiological dexterity of the hand but also its aesthetic integrity and the patient's psychological equilibrium. The crafting process entailed a meticulous technique to replicate the intricate texture of the skin in order to guarantee a near normal appearance. Post-prosthesis integration, the patient exhibited enhancements in manual functionality and articulated a heightened self-assuredness because of the indiscernible prosthesis. This illustrative case underscores the efficacy of silicone finger prosthetics in conferring both functional and aesthetic restitution to those afflicted with partial digit amputations.