A Russian biologist reports that marsupials lean left-handed rather than right, potentially shedding light on how such traits were developed. “It seems like bipedalism is a triggering factor that pushes forward the evolution of handedness,” said the scientist, Yegor Malashichev. “Standing on your hind legs frees up your forelimbs, and you can do with them what you like.” Kangaroos and wallabies, like humans, are bipeds. Primates in captivity have been shown to favor the right hand, though some suggest they picked up the behavior from humans. Scientists suggested other animal species would see a 50-50 split.