Scientists have hypothesized that some ancient kangaroos moved with a lumbering walk rather than bouncing due to their enormous size.

Detailing their findings in PLOS ONE, the researchers studied kangaroo fossils dated to around 100,000 years ago, discovering that the animals' hind limbs and spines were distinct from those of the modern kangaroo, the Telegraph reported.

The extinct Sthenurine kangaroo family was the subject of the study, which came from research that started in 2005 as part of a joint effort between Brown University and the University of Malaga in Spain, PBS Newshour reported.

The enormous ancient kangaroos weighed around 550 pounds, about three times the size of their modern equivalent. According to the new study, the large kangaroos were big-boned, something that would have made it difficult for them to jump; other physical differences from modern kangaroos also pointed to slow walking rather than bouncing.

"I don't think they could have gotten that large unless they were walking," said Christine Janis, professor of ecology and evolutionary biology at Brown University, as quoted by the Telegraph. "We need to consider that extinct animals may have been doing something different from any of the living forms, and the bony anatomy provides great clues."

Scientists say that modern humans came to Australia between 40,000 and 60,000 years ago. Their arrival may have spelled the end of the slow-moving, large kangaroos, which are believed to have been extinct for 30,000 years.  

Sthenurine kangaroos likely had sturdy backbones and moved one foot in front of the other to get around.

"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?" asked Janis, as quoted by PBS.

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