Some 30,000 years ago, kangaroos were too tall and heavy, without the proper bone structure, to hop around on their hind legs as the modern marsupials do today. They just walked around with a big, heavy gait, researchers from the United States and Spain say.

In a study, published this week in the journal PLOS ONE, scientists at Brown University and Spain's Universidad de Malaga were able to show that some of the modern kangaroo's larger ancestors had a skeleton built for a tall, upright posture for walking -- one foot in front of the other.

In detailing the differences in skeletal structure among 45 different species of kangaroos -- analysis which included a detailed study of the now extinct giant kangaroo, Sthenurus stirlingi -- researchers found these ancient species had much larger knee and hip bones, as well as big, bulky ankle joints. These structures were essential, not just in promoting walking over hopping, but in supporting a frame that stretched more than ten feet tall and body mass that often reached 500 pounds.

Sthenurus stirlingi and a few other past species also had a more rigid backbone and less flexible spinal muscles, the kind of anatomical makeup ill-suited for hunching over all day, propelling oneself forward two feet at a time.

Of course, scientists don't have any video footage of Sthenurus stirlingi moving about; they simply had to make their best guess for how the species made its way around the landscape, given the limits of its size and skeleton.

"I don't think they could have gotten that large unless they were walking," Christine Janis, an evolutionary biologist at Brown University, told the Telegraph. "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 four legged locomotion, then what do you have left?"

Perhaps the most important lesson from the new study is not that an animal forewent hopping in favor of an upright stride, but that sometimes modern animal behavior clouds a scientist's ability to interpret the past. All but one species of kangaroo today hops. But that doesn't mean they always did before.

"We need to consider that extinct animals may have been doing something different from any of the living forms," said Janis, "and the bony anatomy provides great clues."