A female bonobo, Lingoye, on the climbing frame in her enclosure with the pressure mat (white) on the vertical beam. Photo credit: Diana Samuel.
From touch typing to the mastery of musical instruments, the dexterity of the human hand is unique. While many of our close relatives are also equipped with an opposable thumb for gripping, their hands are suited for walking on all fours and manoeuvring between branches in the arboreal world that they inhabit. How modern human hands evolved from limbs that were adapted for moving through forests intrigues Tracy Kivell from the University of Kent, UK. She says, ‘Understanding how these abilities evolved requires a better understanding of what fossil human (hominin) ancestors may have been doing with their hands’. So, Kivell and Diana Samuel from the University of Kent and Sandra Nauwelaerts and Jeroen Stevens from the University of Antwerp, Belgium, turned to one of our closest relatives, bonobos, whose hands may be similar to those of our ancient ancestors, in a bid to understand how moving through trees may have shaped our predecessors’ hands.
Only 10 zoos in Europe house bonobos (120 animals in total), so Samuel and Kivell joined Stevens and Nauwelaerts at the Planckendael Zoo in Belgium during early spring 2015 to work with the zoo's troop of nine bonobos. ‘When temperatures are above 12°C, the bonobos have access to a large outdoor island … so it was best for us to collect data during colder months when they were inside’, says Stevens. The team wrapped a 35 cm-long mat fitted with pressure sensors around a 12 cm-diameter trunk in the animals’ enclosure, and focused three high-speed cameras on the mat to capture the apes’ manoeuvres as they climbed the vertical trunk, and swung beneath or knuckle-walked on all fours on the trunk when it was repositioned horizontally. ‘The bonobos were very curious about this new structure that had suddenly appeared inside their enclosure, which was beneficial for us as it meant they made regular contact with the pressure mat’, recalls Samuel. After 9 months, the team had almost 40 clear shots of the bonobos’ hands as the animals climbed, 16 as they swung beneath the trunk and another 11 as they knuckle-walked along the top of the log.
Comparing the position of the bonobos’ thumbs as they clambered up the vertical trunk and swung beneath it, it was clear that the thumb contributed little to the animals’ grip. Instead of grasping on the opposite side of the trunk from the palm and fingers, the thumb often remained beside the palm as the hand formed a hook which the bonobos slung over the trunk when swinging. In addition, even though the thumb was in contact with the mat, it experienced little force, in contrast to human thumbs when we grip objects. The team also analysed the forces exerted on the bonobos’ hands as they scaled the vertical log and moved across – above or beneath – the horizontal log and it was clear that the pressures exerted on the backs of the fingers as the animals knuckle-walked were much higher than the pressures exerted on the palm and fingers while climbing and swinging. The area of the knuckles in contact with the log when the bonobos knuckle-walked was also as large as the contact area when they swung beneath the log, probably because the animals grip only with their fingertips and part of the palm when swinging, while the pads on the backs of the apes’ knuckles expand the contact area when they walk on all fours.
But what does all of this mean for the evolution of our hominin ancestors’ hands? According to Kivell, they probably climbed and swung beneath tree limbs instead of knuckle-walking and she suspects that the relative versatility of their thumbs may have freed their hands to develop the manual dexterity that is the hallmark of modern humans.
