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Kangaroos are descended from giant ten-foot (three metre tall) creatures that were too heavy to hop, new research suggests.

Their extinct ancestors strode around the Australian outback 100,000 years ago, walking on two legs.

It was only when kangaroos shrunk from 40 stone to a weight closer to their current seven-stone frame that they learned to bounce.

Hop-less! Kangaroos are descended from giant ten-foot (three metres) creatures that were too heavy to hop, new research suggests. An illustration is pictured. Their extinct ancestors strode around the Australian outback 100,000 years ago, walking on two legs

Professor Christine Janis, from Brown University in Providence, US, said: 'I don't think they could have gotten that large unless they were walking.'

The team, whose research was published last night in the journal PLOS One, analysed bones from more than 140 past and present kangaroo and wallaby skeletons, making nearly 100 measurements of each animal.

Unlike modern red and grey kangaroos, the giant animals called sthenurine kangaroos that preceded them lacked specialised features geared for rapid hopping.

Their bone structure indicated they had an upright posture and were able to support their weight on one foot at a time. 

It was only when kangaroos shrunk from 40 stone to a weight closer to their current seven-stone frame that they learned to bounce. Modern kangaroos are thought to have developed their distinctive hopping motion as an efficient way to travel the large distances needed to find food on the arid Australian outback (pictured)

The creatures had large hips and knees and stable ankle joints that would have helped them walk on two legs.

Their hands were poorly suited for moving on all fours, but adapted for foraging.

Modern kangaroos are thought to have developed their distinctive hopping motion as an efficient way to travel the large distances needed to find food on the arid Australian outback.

Using elasticity in the tendons of their large legs to bounce is an effective, low-energy way to move at speed for long periods. 

But it only remains efficient if body weight remains low. The giant ancient kangaroos would simply have been too heavy, the scientists suggest.

Professor Janis said: 'People often interpret the behaviour of extinct animals as resembling that of the ones known today, but how would we interpret a giraffe or an elephant known only from the fossil record?

'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.'

Whether or not reliance on walking rather than faster and more efficient hopping led to the ancient kangaoroos becoming extinct is unknown.

Professor Janis said they may have struggled to elude human hunters, or been unable to migrate far enough to find food as the climate became more arid.

So how DOES a kangaroo use it's tail to help it get around?

...KANGAROOS USE THEIR TAILS AS AN EXTRA LIMB TO HELP THEM MOVE

In July, scientists from Colorado, Sydney and Burnaby in Canada discovered that a kangaroo's tail provide as much driving force as their front and hind legs combined.

'We found that when a kangaroo is walking, it uses its tail just like a leg,' said Associate Professor Maxwell Donelan of Simon Fraser University in in Burnaby, Canada.

'They use it to support, propel and power their motion. In fact, they perform as much mechanical work with their tails as we do with one of our legs.'

'What we didn't expect to find was how much power the tails of the kangaroos were producing. It was pretty darn surprising,' said Associate Professor Rodger Kram at the University of Colorado Boulder.

This summer, scientists discovered that a red kangaroo's tail provides as much driving force as their front and hind legs combined. The tail skeleton of a kangaoo is pictured right

Red kangaroos are the largest of the kangaroo species in Australia.

When grazing on grass, they move both hind feet forward 'paired limb' style while using their tails and front limbs together to support their bodies.

'They appear to be awkward and ungainly walkers when one watches them moseying around in their mobs looking for something to eat,' said Professor Kram.

'But it turns out it is not really that awkward, just weird.'

In human movement, the back foot acts as the gas pedal and the front foot acts as a brake, which is not especially efficient, said Professor Kram.

Professor Donelan said no animal other than the kangaroo uses its tail like a leg. 'Their tails have more than 20 vertebrae, taking on the role of our foot, calf, and thigh bones.'

The kangaroo tail also boosts balance when male kangaroos grab each other by the chests or shoulders, then rear back and kick each other in the stomach in an attempt to assert dominance for the purpose of reproduction.