Scientists have long been trying to figure out why early humans became bipedal, and one popular explanation has now been crossed off the list of possibilities. There was a popular notion that we started walking upright to free up our arms so we could carry objects, and babies—apes don't need to worry about this because their offspring can cling to them using their hands and feet. It's like a built-in Baby Bjorn.

But now University of Manchester researchers say the baby-carrying theory doesn't hold, mainly because supporting an infant on your hip or in your arms is too tiring to have been a legitimate factor in the transition to bipedalism.

It's striding toward us from the kitchen, smoothly and silently. As I set down my overnight bag and turn to question my friend Jack, it ambles gracefully into the foyer. I can sense Jack watching me out of the corner of his eye, looking for a reaction as his newest purchase stops and stands beside us on two thin mechanical legs and clasps two four-fingered hands behind its back. It's smaller than the average person, lithe, entirely unthreatening–I could take it in a fight. The face isn't human, but it's not the face of an appliance either.