The earliest snakes didn't have tunnel vision

Our understanding of what the most recent common ancestor of all modern snakes may have looked like has been brought into focus. 

Researchers from institutions across Europe, including the Museum, found that the ancestor of all living snakes was likely to have had much better vision than some of its descendants.

For many years, it had been argued that the earliest snakes would have been burrowers with poor vision. Recent work suggested ancestral snakes would have resembled the most dedicated burrowers known among living snakes, the scolecophidians.

But it has now been shown that living scolecophidians simplified their visual system after diverging from their ancestors, suggesting the earliest snakes were probably not as adapted to burrowing as some of their descendants.

Lead author Dr David Gower, head of vertebrates at the Museum, says, 'Our data and analytical results provide clear evidence for very substantial reduction of genetic components of the visual system of burrowing scolecophidian snakes.

'We already knew that snakes lost some vision genes and eye structures during their evolution from lizards, but most were nonetheless retained. It is highly unlikely that functional copies of a large number of vision genes were lost from the ancestral snake but subsequently re-evolved in most living snakes.  

'As a result, our study strongly suggests that the ancestor of all living snakes was unlikely to have been as extreme a burrower as living scolecophidian snakes.' 

The study was published in the journal Genome Biology and Evolution. 

A farewell to arms 

While there are almost 4000 species of snake around today, the origin of the group is somewhat contentious.

While snakes are known to have evolved from limbed reptiles, the rest of their evolution is a subject of intense debate. Some scientists have suggested that fossils found in rocks dating to 165 million years ago are the earliest snakes, but many others settle on remains found in 100-million-year-old sediments.

In any event, this group of reptiles gradually became more recognisably snake-like until 65 million years ago, when the meteor which wiped out the dinosaurs struck. As few as six living snake lineages survived the meteor strike, one or two of which were the burrowing snakes that might have survived because they were better suited to finding shelter underground. 

The scolecophidians, or blindsnakes, are thought to have evolved from the ancestor of the rest of all living snakes shortly before this mass extinction. Although they are probably not completely blind, their common name reflects their small, simplified eyes, which have become reduced over time because vision is less important underground. 

Because the rest of the snake family tree is more closely related to each other than to the scolecophidians, scientists believed the blindsnakes would have kept more of the primitive features that were likely to have been present in the earliest snakes. If proven, this would have meant the sharper vision of later snakes would have evolved later on.

In order to test if living scolecophidians are good representatives of the earliest snakes, the researchers decided to investigate blindsnake genetics to discover whether or not they inherited all of the vision genes likely to have been present in the ancestor of all living snakes. 

A goodbye to genes 

The scientists found that while the ancestor of modern snakes had lost some visual pigment genes, such as that for detecting green light, even more visual genes were lost during blindsnake evolution after this lineage had branched off from the rest of the modern snakes. 

Overall, seven of the 12 genes associated with bright-light and colour vision in most lizards and snakes were missing in scolecophidians. The remaining bright-light vision genes may have since been adapted for other purposes, such as resetting the reptiles' body clock. 

Due to the large number of gene losses, the researchers believe it is much more likely that these vision genes were lost in blindsnakes after they diverged from the modern snake ancestor. The alternative explanation would be that non-scolecophidian snakes lost all of these genes before then re-evolving them, which is considered extremely unlikely. 

Dr Bruno Simões, co-author on the paper, says that their findings help provide clues to what the most recent snake ancestor would have looked like. 

'Our research provides new information on the likely ecology and environment of the ancestral snake and suggests that the highly subterranean lives and reduced eyes of living scolecophidian snakes likely evolved as specialisms,' he says, 'rather than being the retention of primitive characteristics.

'This study also shows convergent evolution between subterranean snakes and burrowing mammals with the loss of similar genes, especially those associated with bright-light and colour vision.' 

The study adds to previous research which found that snake vision became more simple after evolving from lizards, and that the ancestor of modern snakes was perhaps semi-fossorial, spending part of its life above ground and the rest below it.