What do thumbs and brains have in common? According to recent research, their growth has been linked throughout the evolution of the primates, suggesting that the long thumbs and large brains of modern humans emerged concurrently over time.
The recent study in Communications Biology, supports the theory that the hands and heads of the primates coevolved. The growth of one occurred together with the growth of the other in lemurs, apes, even hominins, leaving our own species with two of our most unique traits.
“We’ve always known that our big brains and nimble fingers set us apart, but now we can see they didn’t evolve separately,” said Joanna Baker, a biologist at the University of Reading, according to a press release. “As our ancestors got better at picking up and manipulating objects, their brains had to grow to handle these new skills. These abilities have been fine-tuned through millions of years of brain evolution.”
Read More: An Introduction to the History of Human Evolution
The Coevolution of Hands and Heads
Our dexterous thumbs and big brains have benefitted our species in many ways. They've allowed us to create the tools, technologies, and cultures that have helped us thrive over thousands of years. But the history of how these traits evolved is still surprisingly murky.
Some scientists have suggested that these traits evolved together, for instance, emerging in concert over evolutionary time, as more complex movements required more sensorimotor control. But the evidence for this theory has remained relatively limited.
To fill this gap in research, Baker and colleagues analyzed the bones and tissues of 95 primates — including extant and extinct species and a handful of hominins, such as Homo sapiens, Homo neanderthalensis, and Homo naledi — to trace their evolution over millions of years.
Looking at the length of the thumb and the mass of the brain (either measured or estimated from the volume of the brain or brain cavity), the researchers revealed that the species that had longer thumbs — which are associated with increased dexterity — were also the species that had larger brains.
A Pattern Across Primates
According to Baker and colleagues, the hominins had the longest thumbs and largest brains by far. But the relationship between the length of the thumb and the mass of the brain was solid in the other primates, too, and it remained strong even when Homo sapiens were removed from the analysis entirely.
“Our results imply a robust association between brain size and manual dexterity,” the researchers wrote in their study. “We interpret our results to indicate sustained historical coevolution between brain size and dexterity across the primate order, reflecting significant neural costs of manipulation behaviours.”
Read More: Chimps May Continuously Learn as They Age, a Factor of Human Evolution
The Thumbs and the Brain
Of course, the size of our hands and heads have long been seen as a factor in our species’ success, but never before has the coevolution of these two traits been traced so thoroughly throughout the primate line.
Still, Baker and colleagues were surprised to find that the growth of the thumb was associated with the growth of the brain in only one of two expected areas.
In fact, the researchers had thought that longer thumbs would be linked to a larger cerebellum as well as a larger neocortex, since the former controls movement and coordination while the latter concentrates on sensory processing, cognition, and consciousness. But their results indicated that longer thumbs were only tied to a larger neocortex.
“We therefore reveal the intriguing possibility that neural processes implicated in the evolution of manual dexterity across primates primarily involve neocortical regions,” they wrote in their study. “An exciting avenue of future research would be to test this idea further.”
Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
- Communications Biology. Human Dexterity and Brains Evolved Hand in Hand
