Graduating from a crawler to a toddler is a significant developmental milestone for every child. Tentative steps turn into more confident toddles, and soon they are scampering off in whatever direction their insatiable curiosity takes them. At the same time as kids are learning to place one foot reliably after the next, significant changes are also occurring in how they use their hands. Whereas crawlers are happy to grab with either hand, by the age of three, a preference for the left of right hand has usually been established. The transition from crawling on hand and knees as a quadruped, to walking bipedally on two feet, creates a strong bias that can be seen both at the individual level – the vast majority of us has a preference for one side over the other – and at the population level – only around one in 10 people are left-handed. So, too, if we look at handedness from an evolutionary perspective. In mammals, the more upright the posture, the more likely that the species has a left- or right-hand bias. Ambidexterity, it seems, remains the preserve of quadrupeds.
The relationship between posture and population handedness – or lateralisation, to make sure we include paws and hooves and other forelimbs – was first seen in primates. The more upright the primate, the stronger the lateralisation. Humans, being the most upright, are also the most lateralised – more so than our somewhat bipedal ape cousins, the gorilla and the gibbon, and even more so than orangutans, who walk on all fours much of the time. The question that researchers from Saint Petersburg State University wanted to answer was: is this only a primate phenomenon?
Their answer: apparently not.
The team chose to look at marsupials, a group of mammals with some species walking upright – think Kangaroo – and others getting around on all fours. They observed three different species of marsupial – the grey short-tailed opossum (Monodelphis domestica) and the sugar glider (Petaurus breviceps), both quadrupeds, and the bipedal red-necked wallaby (Macropus rufogriseus). Just like in the primates, group-level handedness was strongest in the bipeds. In the wallabies, around 80% of individuals showed a preference for left hand over right when grabbing food or grooming, or in the case of young wallabies, opening their mother’s pouch. In the sugar glider, the percentage was 70%, and in the opossum – the least vertical in posture of the three – just 60% of individuals had a forelimb bias.
In the case of the sugar gliders and opossums, which forelimb was favoured depended on sex: females in both species tended to prefer their left side, and male opossums were more right-handed. This sex effect is well-known, although it’s usually the males that tend towards the southpaw state. This is true for a range of primates – though not all – as well as some of our closest animal companions. Cats, dogs and horses all have pronounced differences between the sexes, with males preferring to use their left paw or hoof and females, their right.
The reason for marsupials displaying the exact opposite sex effect compared with other mammals studied is unknown, but the authors speculate that it could be down to the wiring of the marsupial brain. In placental mammals, the corpus callosum, a fleshy bundle of neurons running from one side of the brain to the other, allows communication between the left and right hemispheres. Marsupials lack this structure, the brain hemispheres instead communicating through an alternate bundle of fibres known as the anterior commissure.
And what about humans – why do we have such a strong bias towards right-handedness? The answer to this question could also lie in how the brain is organised. Just as we need fine motor skills for peeling an orange when we’re hungry, we also need fine motor skills to manipulate our tongue and lips to form words and communicate. Presumably, it is more efficient for a single hemisphere of the brain to control both of these similar tasks. Given that the left hemisphere controls speaking in most people, it follows that most people – where these tasks are co-located – will be right-handed.
But, why should walking upright make us less ambidextrous? The scientific jury is still out on this one, too. One suggestion is that our grip changes depending on whether we are upright versus on all fours. Another hypothesis is that during evolution, a division of labour was established between tasks requiring fine motor skills (right-hand, left hemisphere) and simpler grabbing tasks (no preference) – the greater the complexity of the tasks performed by a species, the stronger the right-hand dominance.
As is often the case in science, the wider we cast our investigative net, the more we observe phenomena we’ve seen before – bipedalism quells ambidexterity in primates as well as marsupials – and the more we note exceptions to the rule – left dominance isn’t always a male thing. Whether these observations immediately illuminate, rather than throw up more questions, is less often guaranteed .
- Giljov, Karenina & Malashichev. 2013. Forelimb preferences in quadrupedal marsupials and their implications for laterality evolution in mammals. BMC Evolutionary Biology 13:61. doi:10.1186/1471-2148-13-61
- Giljov, Karenina & Malashichev. 2012. Limb preferences in a marsupial, Macropus rufogriseus: evidence for postural effect. Animal Behaviour 2012, 83:525–534. doi:10.1016/j.anbehav.2011.11.031