Study Links Long Thumbs to Bigger Brains in Primates
Primates with longer thumbs also tend to have larger brains, according to new research that suggests intelligence and manual dexterity may have evolved together.Scientists at the University of Reading compared thumb proportions and estimated brain mass across 94 primate species, including humans, other hominins and lemurs. Their findings, published in Communications Biology, indicate that longer thumbs are consistently associated with greater brain size, particularly in the neocortex the region linked to planning and cognition.
“We see a pattern across all primates: species with longer thumbs relative to hand size generally have bigger brains,” said lead researcher Dr Joanna Baker. “It supports the idea that thinking ability and hand dexterity developed side by side.”
Humans and their close relatives do possess unusually long thumbs and large brains, but once the link between the two traits is accounted for, they no longer appear to be evolutionary outliers. The only exception identified in the study was Australopithecus sediba, which had a disproportionately long thumb even after brain size was considered, possibly due to its mixed arboreal and terrestrial lifestyle.
While opposable thumbs and tool use have long been seen as hallmarks of human evolution, the research suggests thumb length alone does not predict tool use. “The relationship holds whether or not a primate uses tools,” Baker noted.
Experts say the study adds to evidence that hand and brain features likely co-evolved, but also caution that other factors such as hand structure, biomechanics, and neural mechanisms are needed to fully explain human-like dexterity.
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