Of all the monkeys and apes scientists have studied, no species shows a population-level hand preference. Individual primates often have strong biases, and chimpanzees, gorillas, and a few other species lean slightly to the right or left as groups, but not nearly as much as humans. Among humans, 90% are right-handed, and that holds true in every culture ever studied, past and present.
Why do humans stand out with such a strong preference for right-handedness? A new study in PLOS Biology by Thomas Püschel, Rachel Hurwitz, and Chris Venditti at the University of Reading analyzed over 2,000 primates and concluded that two factors explain it: our big brains and our bipedalism. Plug both into their model, and the predicted right-hand bias lines up almost exactly with what's observed. According to the researchers, walking upright freed the arms from locomotor duties, giving people room to specialize one hand for fine work such as tool use and throwing.
In almost all vertebrate species studied, the left brain is mainly responsible for sequencing motor planning and controls the right side of the body. In humans, language is also in the left hemisphere. So as human brains got bigger, dedicating one hemisphere to both fine-motor and linguistic work proved to be an efficient arrangement, and right-handedness got hardwired in our species. The study predicts the same arc across earlier hominins, with a weak preference in Australopithecus, stronger in Homo ergaster, stronger still by Neanderthals, and at the modern level in us. The exception is Homo floresiensis, the small-brained island species with a partly tree-climbing build, which the model puts back near Australopithecus, well below other Homo species.
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