We may be the only animal to move around upright on two legs, but babies learn to walk in the same way that rats, cats, monkeys and birds do.
Francesco Lacquaniti at the University of Rome Tor Vergata, Italy, and colleagues used an electromyograph (EMG) to record the activity of 20 muscles in the arms, legs and torso during walking.
They took measurements from adults, preschoolers, toddlers and even newborns, who take baby steps when supported and made to walk forwards. The EMG picked up two patterns of activity during newborn "walking": one pattern causes the legs to flex and extend, the other makes sure that they move alternately as the baby moves forward.
Things become more complicated by the time toddlers are taking their first steps solo, though. Lacquaniti found four neural patterns at this stage: the two patterns seen in newborns, and two more that control subtle movements, such as shifting the weight from heel to toe before the leg leaves the ground and making sure that the knee bends only after the toes are lifted.
Although even as toddlers we are beginning to move in a way that no other animal does, the motor patterns controlling walking in other animals are nearly identical. When Lacquaniti's team looked at previously published results, they found that newborn rats show the same two patterns as newborn babies. By the time rats reach adulthood they show the four patterns seen in toddlers. Adult cats, rhesus monkeys and guinea fowl also show the four patterns of toddling.
"During evolutionary history, nature didn't scrap the old hardware," says Lacquaniti. "Instead it was modified and tuned to adapt to our needs."
Human adults differ, though. By adulthood, the four phases have changed subtly in a way not seen in other animals. This might allow us to accommodate complicated arm movements such as grasping objects as we walk, Lacquaniti says.
The results make sense, thinks Sten Grillner of the Karolinska Institute in Stockholm, Sweden. We might appear to stand apart from the rest of the animal kingdom because of our unusual posture and the fact that babies take so long to learn to walk – but it's all a matter of perspective.
"In fact, the reason humans take so long before starting to walk is that the human baby is born very early in the maturation process," Grillner says. "If age is measured from conception rather than birth, the relationship between brain size and walking age is linear, and humans follow this relationship closely."
Journal reference: Science, DOI: 10.1126/science.1210617