Talk about a southpaw—some kangaroos are almost exclusively lefties, a new study says.

That may not seem so revolutionary. But for decades, researchers had believed that handedness—the idea that most members of a species will use the same hand to do nearly everything—existed only in great apes, including humans. (Read " Unique Among Animals, Kangaroos Use Tail as Fifth Leg, Scientists Find .")

Not anymore.

By combining hundreds of observations of wild marsupials, scientists report that red kangaroos and eastern gray kangaroos —two iconic Australian species—almost always use their left paws.

This handedness, says study leader Yegor Malashichev , a zoologist at Saint Petersburg State University in Russia, likely emerged after these kangaroo species started walking upright, just as it did in humans.

"It seems we're no longer quite so unique," said Malashichev, whose study was published June 18 in Current Biology .

Balancing the Scales

Among quadruped animals, handedness has never been documented, presumably because they need all their limbs to move.

But in species such as humans and kangaroos, which have evolved the ability to walk on two legs, the hands are freed to perform other tasks. (Also watch video: "Right-Handed Chimps Provide Clues .")

Malashichev began his research after learning that a type of marsupial called the red-necked wallaby showed a hand preference when walking on two legs, but not when on all fours. Those findings were published in Animal Behaviour in 2012 .

For his study, whose fieldwork was funded by National Geographic's Committee for Research and Exploration , Malashichev and colleagues recorded limb preference in three wild marsupial species in Tasmania and continental Australia: the red-necked wallaby, the red kangaroo, and the eastern gray kangaroo.

The team also observed captive Goodfellow's tree kangaroos at zoos in Sydney and Europe. That species, which is native to Papua New Guinea's forests and generally needs all four limbs to climb, did not show any signs of handedness. (Watch video: " Elusive Tree Kangaroos Get Cameras .")

Red-necked wallabies, the team observed, used their left hands when standing on two limbs and feeding or grooming.

But red and gray kangaroos were the stand-outs: They preferred to groom themselves and eat using their left hands, regardless of whether they were standing on two legs or four.

Watch a video of kangaroos kickboxing—fights that can sometimes turn deadly.

"It's no doubt important work, and I was quite surprised at how many of these animals were left-handed, considering how shrunken and useless their forepaws are," said Paul MacNeilage , a biologist at the University of Texas at Austin who wasn't involved in the study.

"But I would have liked to see them analyze a broader range of animal species, and not just focus on marsupials."

Giving Evolution a Hand

Malashichev believes that handedness in humans and kangaroos evolved due to ancient differences in how the brain 's right and left sides function.

For instance, many animals, even those without handedness, generally prefer to use their right side for everyday tasks, which is controlled by the left hemisphere of the brain.

So-called emergency tasks, such as fleeing from predators, are delegated to the body's left side and the brain's right hemisphere.

These neural differences became exaggerated when the ancestors of humans and kangaroos began walking upright. ( Read "Secrets of the Brain" in National Geographic magazine .)

So why do these marsupials prefer their left hands when around 90 percent of humans are righties? MacNeilage has a hypothesis.

The tree-dwelling ancestors of red and gray kangaroos would have primarily relied on their right side for navigating the treetops, which meant only their left hands were free for grooming and feeding. (Also see " Watch: Kickboxing Kangaroos and 4 of Nature's Most Impressive Fighters .")

So if you ever find yourself around a kangaroo, keep an eye out for its mean left hook.