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.
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.
What accounts for these parallelisms?
The striking similarities we found across animal species, which are so distant from an evolutionary standpoint -- they separated tens of millions of years ago, and birds from mammals perhaps 100 million years ago -- suggest that the neural circuits that generate locomotion emerged during evolution from a common ancestral network of neurons, existing in the remote past. Even today, newborn animals and children probably learn locomotion starting from similar or perhaps the same primitives.
Rats are considered the original mammal. Is the manner in which human babies first walk tied to their instincts as mammals and perhaps they learn to move differently as time passes because of socialization? How does evolution come into the play?
That's a very good point. Some paleoanthropologists believe that our style of locomotion was a crucial starting point in human evolution, when humans separated from the chimpanzees, five to seven million years ago. Our ancestors probably did not have much larger brains than the chimps, nor did they have much more sophisticated hands. What initially separated them and us from other primates was habitual erect, bipedal locomotion. This then allowed freeing the hands for other uses than for locomotion, such as developing and using tools, and for social activities and care of the small babies. In sum, our style of locomotion, together with our cognitive abilities, probably was instrumental for evolving our style of social life. As for the other way around, that is, whether socialization plays a major role in the development of locomotion in human children, I really don't know what to answer. I would be inclined to believe that it is much more a matter of neural maturation but the issue is wide open.
Conversely, when and why do human babies start to deviate from how other animals walk?
Development of locomotion in humans seems to diverge from that of other animals after the stage of independent walking. We don't know when this occurs exactly, because we still don't have a fine-grained enough map of development. As for why humans diverge, I guess this must be related to the fundamental biomechanical differences of our locomotion relative to that of other animals. Humans walk and reach, walk and throw, walk and carry loads, etc. In other words, humans coordinate locomotion of the lower limbs with voluntary movements of the upper limbs. This coordination and the maintenance of upright balance while walking, which is much more difficult than balance during quadrupedal locomotion, requires that we use pulses of activity that are timed at very specific moments of the gait cycle. So, human adults stand apart from other animals, but they use neural signals to control their muscles, which are largely derived from the primitive signals, as any other animal.
Could you explain the study methodology or any noteworthy technologies that led to these discoveries?
We were the first to use such an extensive recording of electrical activity from multiple muscles in newborn babies, coupled with kinematics, and kinetics monitoring. And all our recordings were completely safe and non-invasive. Also, we were the first to use in young babies appropriate mathematical techniques, such as factorization algorithms, to extract, so to speak, the common neural commands output to multiple muscles.
What are some implications of your team's work for people who have trouble walking or maybe even for parents who are eager to see their babies walk?
I think that there may be important clinical implications of what we've shown, although one needs to be very careful when one wants to go from the bench of basic research to the bedside. We hope that the discovery of the signals used by the central nervous system to control muscles and locomotion may help in designing better tools for rehabilitation, functional electrical stimulation, and prosthetics for patients with spinal cord injuries and other neurological diseases which impair locomotion. Moreover, the similarity we found in human and animal patterns strongly supports the use of animals as experimental models of motor disorders in humans.
For parents, watch the movements of your newborn baby with respect. Although they look clumsy and rudimentary, they are in fact the prototype of the sophisticated movements of the adult.
Images: 1. Stanislav Fridkin/Shutterstock; 2. B. Strauch/Science (c) 2011 AAAS; 3. Science/AAAS.
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