More than 30,000 years ago, giant kangaroos walked the Earth. Three times the size of today’s largest species, these colossal beasts were over 10 feet tall and weighed 240 kg — or 529 lbs. Because of their size, it was hard to tell if they would have moved around like kangaroos do today: by hopping at high speeds and walking on five limbs (arms and tail included) when they slowed down. Now, a study published in PLOS One suggests that these giants may have developed an upright posture — a posture that would have allowed them to put one powerful, gigantic foot in front of the other. In short, they probably didn’t hop at all.