gms | German Medical Science

Objectives

Background: The thumb (DI) stands out as qualitatively different in every respect. Its huge cortical representation reflects million years of evolution developing into the most relevant part of the human hand. A systematic review is still lacking.

Aim: A systematic search and review to elucidate the evolution of the human thumb focused on four crucial aspects and the status quo of current knowledge.

Methods: Literature was searched using PubMed, Scopus, Google Scholar, ResearchGate. Search terms: thumb and evolution. 4 domains were defined: molecular biology/genetics, neurophysiology, anatomy (osteology/myology), grips/tools. Research covering DI exclusively or as a critical part of the hand was eligible and assigned to the respective domain, followed by best evidence synthesis.

Results: Of 315 entries (PubMed, 1927–2022) 68 were eligible: biology/genetics 7, neurophysiology 5, anatomy 32, grips/tools 22. Most studies were published in the last decades. Due to this interdisciplinary topic study designs/methods vary considerably, including reviews, comparative, quantitative, experimental, in-vivo, morphometrics, biomechanics, kinetics, dissections (cadaveric, fossils), imaging, virtual modelling. DI evolution followed a distinct path (vs. DII-V): pre-axial (metapterygium), selected Hox genes, positive genomic selection, morphological design. DI inputs are processed independently in primary somatosensory cortex areas (2, 3b), boosting hand dexterity in primates (e.g. opposable DI). DI features 1 metacarpal and 2 phalanges (1 of 4 hypotheses). Structure/function of the TMC saddle joint is pivotal for gripping. A broad distal phalanx (PDP) is a recent acquisition. DI osteology shows asymmetry in favor of flexibility (e. g. pad-to-pad-interaction). DI myology features a true flexor longus (FPL), FPL independence, a flexor profundus (FDP), and large intrinsic muscles. Opposable DI is essential for cupping, precision/power grips, tool handling. Hypotheses concerning human-like DI/hand capabilities million years prior to human stone tool manufacture are discussed.

Conclusions: Results support more advanced manipulative achievements and tool-related dexterity in hominid hands than previously thought. Fossil evidence and new methodologies provide a more holistic view of modern human DI/hand functions, and how early ancestors balanced arboreal locomotion and tool-related behaviours.