A new study by scientists in Sweden has shed light on the mystery, finding that the time it takes for all mammals to start walking closely correlates with the size of their adult brains. The researchers from Lund University found that motor development milestones in rats and ferrets, such as crawling and walking, followed the same timetable but at different rates, being faster for rats. The team then wondered if similar results would be found for other mammals. Then they used a multiple-regression model to study the time between conception and walking for 24 mammal species, including sheep, chimpanzees, guinea pigs, camels, and aardvarks.
They found that mammals with larger brains, such as humans, take longer to master walking than species with smaller brains and those that walk on the heels (like humans) take the longest time to learn to walk, which the scientists thought might also be related to the brain since this kind of walking is more complex and probably takes more brain power.
One remarkable result of the research is that the model of the walking time’s relationship to adult brain mass for the other 23 species leads to an almost perfect prediction of when humans will begin to walk. The effect of gestation and the birth brain mass were also analyzed and found to correlate with walking time for most of the animals studied, but not for humans, and the scientists believe this is because humans spend a much smaller percentage of development time in the uterus, and more of the brain mass is developed after birth than it is in many of the other species studied.
For more detail about this study you can read and download this pdf type article.
Source : http://www.pnas.org