titanoboa
Titanoboa scientists put forward new theory on snake evolution.Getty Images

The theory on snakes evolved into limbless creatures has been "turned on its head" by a genetic study that suggests they are not a simplified lizard.

Two of the scientists involved previously collaborated on a study charting the discovery of the biggest ever species of snake - Titanoboa. The creature, from 60 million years ago, would have weighed 2,500lbs and measured 43ft.

Previously, snakes were thought to have lost their limbs around 150 million years ago (when they began to evolve) as a result of a disrupted Hox gene which governs the boundaries of the neck, trunk, sacral, and tail regions of limbed animals.

However, the latest study published in the journal Nature, suggests that rather than snakes becoming simplified lizards, other animals gained more complex vertebral columns as they evolved.

Scientists from the University of Nebraska-Lincoln and Indiana University Bloomington found distinctions among snake vertebral bones that matched the backbones found in four-legged lizards.

It had been thought that when snakes lost their limbs, they lost regional distinctions that separated their backbones into neck, trunk and lumber – but this is not the case. Instead, scientists found snakes had the same regional differences as lizards.

snake evolution
Snake skeletons are just as regionalized as lizard skeletons, despite loss of limbs and an increase in the number of vertebrae.Craig Chandler, Angie Fox, Jason Head, University of Nebraska-Lincoln

Author Jason Head said: "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. 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."

When the researchers compared the regions in snakes with Hox gene expression, they also found the two matched, suggesting the Hox genes function in the evolution of 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", Head said.

Another study author, David Polly said: "Our findings turn the sequence of evolutionary events on its head. 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.