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For years it's been thought that snakes lost regions in their bodies such as their neck, as they evolved from four-legged lizard ancestors into their sinuous shape.

But by looking at subtle differences in the shapes of snakes' vertebrae bones, palaeontologists believe they have shed new light on how the creatures evolved their legless bodies.

Researchers claim that snakes didn't lose distinct regions, but that instead mammals and birds independently gained them, turning snake evolution on its head.

Scientists have dispelled the theory that hox genes, which establish the boundaries of the neck and trunk, lin birds, lizards, crocodiles and mammals, were disrupted in snakes, resulting in a loss of regions in their seemingly simplified body form as they evolved from four-legged lizard ancestors (illustrated)

Hox genes, which establish the boundaries of the neck, trunk (body), lumbar and tail regions in birds, lizards, crocodiles and mammals, were previously thought to have been disrupted in snakes, resulting in a loss of defined regions, forming their elongated, legless shape.

To investigate whether the theory was right, P. David Polly of Indiana University, Bloomington and Jason Head of the University of Nebraska-Lincoln examined regional differences in the shapes of individual vertebral bones in snakes, lizards, alligators and mice.

Snakes are different from other amniote groups – those that descended from four-limbed animals with backbones that lay eggs on land – because they lack forelimbs, shoulder girdles and sternal skeletons.

Until now, it had been assumed that snake vertebrae became less regionalised, or defined, when the limbs were lost. 

By studying the vertebrae of snakes, (a skeleton is pictured,) researchers have concluded that the reptiles didn't lose distinct regions, but that instead mammals and birds independently gained them

'If the evolution of the snake body was driven by simplification or loss of Hox genes, we would expect to see fewer regional differences in the shapes of vertebrae,' Dr Head explained.

'Instead, what we found was the exact opposite. Snakes have the same number of regions and in the same places in the vertebral column as limbed lizards.'

The duo discovered that the snakes they studied were as varied as lizards.

When they compared regions in snakes with Hox gene expression - the process by which information from a gene is used to make a protein - they found that the two matched, according to the study in Nature.

'This suggests that Hox genes are functioning in the evolution and development of the vertebral column in snakes - but instead of patterning distinct, rib-less regions like the neck and lumbar spine of mice, they control more subtle, graded changes in shape,' the said.

When the findings were combined with information from fossils, the researchers concluded that the direction of snake evolution is the opposite of what was previously thought based on genetics alone.

'Our findings turn the sequence of evolutionary events on its head, Dr Polly said.

Dr Polly said that the study's findings 'turn the sequence of evolutionary events [in snakes] on its head'. She observed that snakes have lost the shoulder girdle but as just as regionalised as lizards

'It isn't that snakes have lost regions and Hox expression, it is that mammals and birds have independently gained distinct regions by augmenting the ordinary Hox expression shared by early amniotes.

'Snakes have a lot more vertebrae compared to lizards and they have lost the shoulder girdle, but they are just as regionalised,' Polly said.

To come to their conclusion, the researchers used a method called geometric morphometrics, which marks individual points on a plane using a pair of numerical coordinates to analyse the shape and size of vertebral structures.

They also used a statistical method called 'maximum likelihood estimation' to work out where one segment ends and the next begins.

Dr Head said: 'Analysis of gene functions are necessary, but not sufficient in studying evolutionary transitions.

'In order to fully understand the mechanisms by which new body forms evolve, it is crucial to study the anatomy of modern and fossil organisms.'