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Abstract:The palmaris longus muscle is widely recognized for its notable variability in living humans. These variations include not only muscle belly reversal, distinct double muscle bellies, duplication and triplication, but also total uni-or bilateral agenesis (absence). A review of the literature and data novel to this study illustrate that different populations of humans exhibit remarkable variation in the frequency of palmaris longus agenesis, from less than 5% of Chinese to nearly 65% of Indians. Comparative dissection-derived data reveal substantial variation in palmaris longus agenesis (PLA) in populations of extant humans (H. sapiens), chimpanzees (Pan spp.), and gorillas (Gorilla spp.) -but not orangutans (Pongo spp.), which apparently always develop this muscle. From this pattern, we infer that palmaris longus is undergoing non-adaptive, stochastic evolution in the extant African Homininae, while it continues to have adaptive purpose in Pongo, likely due to the orangutans' greater degree of arboreality than the African apes and humans. Clinical evidence supports this conclusion, at least as it applies to humans. This study highlights the utility of comparative soft tissue data collection and interpretation in elucidating the evolution of anatomical structures that do not fossilize.