The mice hopped about like kangaroos, using both their forelimbs and hind limbs.

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Both had long tails to counterbalance large skulls, and, proportionally, they had the smallest forelimbs of any apex predator.

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"Comparative anatomy" - in which, for example, the forelimbs of a number of fossils are compared to establish which are most closely related - was the entire toolbox for the earliest palaeontologists.

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Studying these creatures, Dr Zoghbi saw that the rodents with a mutant form of the gene were affected similarly to people: they developed tremors and spasticity in their forelimbs, became susceptible to seizures and suffered breathing problems.

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Most had wings attached to the forelimbs.

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