Study Reveals Why Infants Can't Walk

Scientists have figured out the underlying reason why human babies can't walk at birth while foals and other hoofed animals get up and go within hours of being born. Turns out, all mammals essentially take their first steps at the same point in brain development.

A team of scientists has come up with a model that can predict the onset of those first steps with information on the weight of that animal's mature brain (which indicates brain development time) and whether the species stands with its heels touching the ground like us or on its tippy toes like cats and horses.

The results suggest "the neuronal mechanisms that underlie the onset of walking are very similar in different mammals, and that they are activated at a very similar relative time point during brain development," said lead researcher Martin Garwicz of Lund University in Sweden.

The upshot is that while humans might not walk until just under 1 year of age and an elephant shrew at just 1 day old, both organisms hit this milestone at the same point in their brain development.

The research is published this week in the journal Proceedings of the National Academy of Sciences.

Longstanding mystery

"It's something I've always wondered about," Garwicz told LiveScience. "Even children ask this question — How come a little foal can start walking straight after birth and it takes us such a long time?"

His previous work on rats and ferrets had hinted at the relationship between brain development and walking onset. But he wondered if this link was an exception to the rule.

To find out, Garwicz and his colleagues looked at the relationship between various factors, such as brain size and limb biomechanics, and the onset of walking for 24 mammal species, including aardvarks, chimpanzees, guinea pigs, sheep, hippos and camels. Together, such animals belonged to 11 of the 14 orders of terrestrial mammals that walk.

And rather than the conventional way in which people talk about the onset of walking, the researchers started the clock at conception. For humans, that would add about nine months to this walking clock.

Sure enough, they saw a pattern that could mostly be explained by differences in brain mass. The fact that the pattern only showed up when looking at the time from conception suggests brain development occurs along this continuum that extends from conception through early development out of the womb, Garwicz said.

They also found limb biomechanics was involved in the timing of walking onset, though not as important of a factor as brain mass. Specifically, animals that stand on the full length of their hind feet (like us) take longer to reach those first steps.

The researchers suspect this link is also related to the brain, because the hind limbs of this so-called plantigrade stance are more complex biomechanically than those of horses, say, that don't place their heels on the ground. That biomechanical complexity likely requires more brain power to operate, and thus more time to get moving in early development.

Human brains not so special

The finding could help to explain why human babies are helpless for such a long time following their birth. Until now, one idea has been that our brains are so large and complex and we learn so many other things while also developing our motor skills that it takes longer for us to gain our footing.

"With respect to walking onset, those assumptions are wrong," Garwicz said. "It is possible using our model and data from other mammals you can predict when a human baby will start walking despite the fact that we walk on two legs, despite the fact that we have a large brain, and despite the fact that we learn many other things."

The finding also suggests the human noggin is not just the result of an advancement of the brain in non-human primates. Rather, our brains may be very similar to various other animals with the only exception really being time — how long our brains are allotted for development.

"By increasing the time of development we grow a brain that is so much larger and so much more complex, and at first glance would seem so different from other species," Garwicz said. "But maybe the underlying principles and building blocks of development are similar in different species."

Garwicz's colleagues included Maria Christensson of Lund University and Elia Psouni of Lund University and Kristianstad University in Sweden.