Baby steps. Human infants take a very long time learning how to walk, yet they fit a general rule.

All Mammals March to the Same Beat

Many animals test their legs and totter forth only hours after they are born, but humans need a year before they take their first, hesitant steps. Is something fundamentally different going on in human babies? Maybe not. A new study shows that the time it takes for humans and all other mammals to start walking fits closely with the size of their brains.

In past studies to develop a new animal model for the brain events that support motor development, neurophysiologist Martin Garwicz of Lund University in Sweden and his colleagues discovered that the schedules by which ferrets and rats acquire various motor skills, such as crawling and walking, are strikingly similar to each other; the progress simply happens faster for rats. That made them wonder how similar the timing of motor development might be among mammals in general.

They compared the time between conception and walking in 24 species and looked at how well this duration correlated with a range of variables, including gestation time, adult body mass, and adult brain mass. As they report in this week's issue of PNAS, brain mass accounts for the vast majority (94%) of the variance in walking time between species.

Species with larger brains, such as humans, tend to take longer to learn to walk. Strikingly, a model based on adult brain mass and walking time in the other 23 species almost perfectly predicts when humans begin to walk. "We've always considered humans the exception," Garwicz says, "But in fact, we start walking at exactly the time that would be expected from all other walking mammals."

Two other variables--gestation time and brain mass at birth--also correlate nicely with age of walking for most animals, but not for humans. That makes sense, the researchers say: Humans spend an unusually small portion of their development--and build an unusually small fraction of their brain mass--in the womb. The model is able to accommodate this quirk of human development because it uses the time it takes babies to learn to walk from conception, not birth. (At the other extreme, animals such as horses, who have a long gestation and then walk almost immediately after they are born, also fit the model.)

Barbara Finlay, a neuroscientist at Cornell University, says the findings support the existence of a kind of a development "clock" for mammals. In her own work, Finlay has found that various mammals have similar timetables for brain development before birth. But she had imagined that a postnatal milestone such as walking would be more idiosyncratic. "I was surprised," she says. "I thought the clock would start to fracture." It will be interesting, she says, to see if the clock will keep time for later milestones, such as events related to reproduction.