Humans develop manual dexterity skills after a cumulative sequence of steps that also take place in other primates, according to a new study.
However, our larger brains and longer childhoods enable us to develop finer skills – such as the ability to tie shoelaces, use a knife and fork and make intricate tools – than primates with smaller brains, which have long progressed to adulthood.
This finding, published in the journal Science Advances, challenges current notions of evolution, according to senior author Carel van Schaik from the University of Zurich in Switzerland, and gives weight to a previous theory.
“There used to be this idea – linked to Ernst Haeckel – that most evolutionary change, say when a new species evolves, is produced by adding new steps at the end of a sequence of steps,” she says.
“[N]ow most people argue that evolution can often insert steps [or] features pretty much anywhere – the modularity principle – so you would see a mosaic pattern rather than a series of rigid sequences.
“It looks therefore like we here hit on a strong example of the old Haeckel principle, which tells us a lot about brain development.”
Why should we care?
Technology, a hallmark of humans, relies on advanced manual dexterity to make and use tools – these manipulations come at the very end of the set sequence the researchers discovered across different primates.
“So, for species that will have to painfully learn their use of tools, rather than having it as some modular, largely innate trait, fancy technology can only evolve if there is enough time to go through all the steps,” says van Schaik.
Human evolution of fine dexterity couldn’t have happened in other species, therefore, simply because they develop too fast to acquire those skills.
To establish this, first author Sandra Heidstab and colleagues spent more than seven years studying food manipulation skills in 128 captive young primates from 36 different species from birth until adulthood.
They were surprised to find all species develop manual skills in exactly the same order of increasing complexity across primates.
Two species diverged from the trajectory, the silvery gibbon (Hylobates moloch) and pileated gibbon (H. pileatus), which use their long, hook-like hands and long limbs for swinging and climbing, likely constraining their manual skill development.
Stages were delineated according to use of forelimbs, hands (together then separately), then fingers, followed by manipulation of multiple objects.
The team’s observations also showed that primates with bigger brains, such as chimpanzees, macaques and gorillas, learned finer manual skills than their smaller brained cousins like lemurs or callitrichids.
“It is no coincidence that we humans are so good at using our hands and using tools, our large brains made it possible,” says Heldstab. “A big brain equals great dexterity.”
But there’s a trade-off: the larger the brain, the longer each stage takes, suggesting bigger brains take longer to develop. That’s likely why only longer-lived mammals like humans, with patient parents to look after them during a prolonged childhood, can achieve these fine motor skills.
The study is part of work with the primates that established the step-wise categories.
It was initially inspired by van Schaik’s observations that lemurs used their mouths to scoff food straight from the ground, and then had clumsy manipulation skills when they did use their hands. This was totally different to the apes and macaques she was used to studying.
That paper also reported that complex foraging and tool use were associated with cognitive test performance and larger brain size, which the authors say could have developed with the innovations inspired by living with two feet on the ground rather than in trees.
This “suggests that large brains could also have evolved to support living in a complex foraging niche,” says van Schaik, “not just to survive in a socially complex world.”
Related reading: Why do humans have bigger brains than apes?