A new study published in Science reveals that the neural mechanism behind our ability to move around is not as special as we might like
We haven't evolved as much as we'd like to believe. Our genome is 99 percent similar to apes, and new research in the journal Science suggests that the circuitry that enables us to walk early on comes from our earliest mammalian ancestor, the rat.
"Admittedly, humans excel in cognitive abilities and in skilled movements like writing or talking," says neuroscientist Sten Grillner in a commentary on the study. "But when it comes to the basic motor repertoire, such as walking, posture, or orienting movements, the situation is different."
Human locomotion had been presumed unique because we walk upright with two legs. When investigators led by Nadia Dominici analyzed the electrical activities produced by walking toddlers, preschoolers, and adults as well as newborns that were held upright and prodded to walk along a surface, however, they saw that we essentially follow the same chain of motor commands as several other animals, including rats, cats, monkeys, and guinea fowl. It's only after the toddler stage of independent locomotion that we build on this neural mechanism that has seemingly evaded vertebrate evolution and walk in our own special way.
In the Q&A below with neurologist and study co-author Francesco Lacquaniti, learn more about how humans first learn to move around, the role of animal evolution in locomotion, and the study's implications for clinical rehabilitation.
How did this research begin?
I've been working on locomotion over the last 15-plus years, but the idea for this specific work came only recently and rather casually. One day, I was watching the wobbly walking of one of my sons who is about one year old. He was trying to make his first steps on his own. I asked myself, 'Boy, why is it so complicated? How does it all start?' So I talked to my friend and the pediatrician of my sons, Dr. Ambrogio Di Paolo, and together with our teams we organized systematic recordings of neonates about three days old at the well-baby maternity ward of Sant'Eugenio Hospital. At the same time, I organized similar recordings in our laboratory at Santa Lucia Foundation for toddlers around one year old, ready to make their first unsupported steps. We also recorded older children, two to four years old, and adults for comparison. Our aim was to get a picture of the development of locomotion since its onset, right after birth.
"Nature tends not to scrap old hardware. Whenever possible, she reutilizes it for new functions by adapting it." -- Lacquaniti
What were your key findings?
Before our work, most neurologists and physiologists thought that the primitive patterns of neural control of locomotion expressed by newborn babies are discarded during the subsequent development, replaced by entirely new patterns. Well, when we compared the patterns of neonates with those of toddlers and older children, we were surprised to find that the primitive patterns of the neonate are not discarded at all, but they are retained and tuned, while few new patterns are added during development.
Could you explain this further, with an analogy perhaps?
An analogy? The first that comes to my mind is a beginner driving a car who tends to use only the low gears, first and second, but after some learning he or she starts using higher gears also, third, fourth, and so on. However, the low gears are still there, and are used whenever necessary.
Why are these discoveries so significant?
As I said, people believed that neonate stepping is a primitive behavior, and that children must suppress the primitive patterns to learn how to walk in a functional manner. Instead we now showed that learning does not involve suppression. The primitive patterns are retained and fine-tuned, incremented by new patterns. The other significant discovery we made is that the patterns of children are not unique to the human species, but they are shared by other animal species that are completely different from us.
Did you expect these similarities?
Yes and no. No, because as a neurologist I knew the vast differences that exist between our locomotion and that of other animals. No animal walks or runs like us, using bipedal erect plantigrade locomotion. Moreover, our central nervous system is much more complicated. However, as a physiologist I did expect some similarity because I knew that there are many of the core processes that are conserved across animal species. Nature tends not to scrap old hardware; whenever possible, she reutilizes it for new functions by adapting it. Think of the evolution of fish fins into the limbs of terrestrial animals. But even so, honestly, I did not expect the almost perfect superposition of animal waveforms and child waveforms.