In 2005, Christine Janis, professor of ecology and evolutionary biology at Brown University, was looking at the skeleton of an ancient kangaroo relative in a museum. Janis noticed that the sthenurine skeleton didn’t look like it belonged to an animal that hopped.

Over the last decade Janis and her colleagues have examined the skeletons of more than 140 individual kangaroos of various species past and present. They took over 100 measurements of various parts of the animals skeletons and determined that, at the very least, the shenurine does not appear to have the skeleton of an animal that hops or walks on all fours.

The sthenurine kangaroos, which became extinct about 30,000 years ago, were larger than their modern descendants. The animals were about 6 feet tall and weighed more than 500 pounds. They had a round face, more rabbit-like than modern kangaroos.

According to the researchers, whose work is published in the journal PLOS ONE, the animals ankles had flange that wraps over the lower end of the tibia. Modern kangaroos do not have this but modern horses and dogs, animals to whom running is important, do.

The Sthenurines also had larger hip and knee joints and a broad and flared pelvis that would have allowed for larger gluteal muscles, important for balance in animals that walk. The animal’s “hands” were not designed to support the animals massive weight and its inflexible spine would have made walking on all fours difficult.

“If it is not possible in terms of biomechanics to hop at very slow speeds, particularly if you are a big animal, and you cannot easily do pentapedal locomotion, then what do you have left? You’ve got to move somehow,” said Janis in a statement.

If the sthenurines walked on two legs it may, ironically, have been that which doomed them. While considered a considerable advantage in humans, animals that walk on two legs can’t typically move as far or as fast as their cousins who walk on four. Their limited range may have made food difficult to find given their limited range. It also would have made them easy prey.

Although they were large the sthenurines were not made for hunting or fighting. That means that when their “fight or flight” instinct kicked in they would have had limited ability to do either. Humans, facing food shortages of their own, may have hunted the animals to extinction.