Scientists investigate how snakes became an evolutionary success story

• Fieldwork, 60,000 museum specimens and cutting-edge technology combine to answer how snakes became one of evolution’s biggest success stories
• The study suggests that the answer to their success was speed, with snakes evolving up to three times faster than lizards
• The shift in snakes’ feeding ecology allowed them to become versatile hunters and able to specialize on prey that other groups, including lizards, cannot

An international team of scientists have generated the largest, most comprehensive evolutionary tree of snakes and lizards to answer how snakes went from small lizards to the highly diverse and sophisticated species we see today.     

More than 100 million years ago, the ancestors of the first snakes were small lizards that lived alongside other small, nondescript lizards in the shadow of the dinosaurs. In a sudden burst of innovation in form and function, they evolved legless bodies, highly sophisticated chemical detection systems to find and track prey, and flexible skulls that enabled them to swallow large animals.

Those changes set the stage for the spectacular diversification of snakes over the past 66 million years.

To discover what triggered the evolutionary explosion of snake diversity - a phenomenon known as an adaptive radiation - researchers from universities and museums around the world, including the Natural History Museum, London, completed a genetic and dietary study of snakes.

This was achieved through sequencing partial genomes for nearly 1,000 species and compiling a dataset on lizard and snake diets, from field observations and through examining records of stomach contents from over 60,000 museum specimens.

The study suggested that the answer to how snakes evolved so successfully was speed. Snakes evolved up to three times faster than lizards, with massive shifts in traits associated with feeding, locomotion and sensory processing.

‘Fundamentally, this study is about what makes an evolutionary winner. We found that snakes have been evolving faster than lizards in some important ways, and this speed of evolution has let them take advantage of new opportunities that other lizards could not,’ said University of Michigan evolutionary biologist and curator, Daniel Rabosky, senior author of the paper.

“Snakes evolved faster and—dare we say it—better than some other groups. They are versatile and flexible and able to specialize on prey that other groups cannot use,” said Rabosky.

The Natural History Museum's Curator of Fossil Reptiles, Dr Marc Jones’, expertise was called upon to create a geological timeline of the evolutionary events in the study, a step named time calibration.

Branching events were timed using 31 key fossils, including Dorsetisaurus purbeckensis, from the Early Cretaceous of Dorset, England, which sits within the collections at the Museum. The fossil represents the oldest known relative of the group that includes the Komodo dragon and slow worm.

Marc said of the findings, ‘This is the most comprehensive study of snakes and lizards, with respect to the number and representation of species, and their diets, that I’ve encountered in my career.

‘The study really highlights the differences between snakes and legless lizards.’

‘The analysis we’ve been able to put together from so many specimens and fossils from around the world is fascinating – especially as the fossil record for lizards and snakes is relatively patchy. We’ve been able to see how rapidly snakes responded after their divergence from lizards in the Cretaceous period, and the stark differences they exhibit.’

This dataset created in the study informed mathematical and statistical models which revealed that while other reptiles have evolved many snakelike traits—25 different groups of lizards also lost their limbs, for instance—only snakes experienced an ‘evolutionary explosion’.

This pointed the team to believe that there is something genetically unique about snakes that allowed them to be evolutionarily flexible while other groups of organisms are much more constrained.

The authors refer to this once-in-evolutionary-history event as a macroevolutionary singularity or a sudden shift into a higher evolutionary gear.  

In the case of snakes, the singularity started with the nearly simultaneous (from an evolutionary perspective) acquisition of elongated legless bodies, advanced chemical detection systems, and flexible skulls.

Those crucial changes allowed snakes to pursue a much broader array of prey types, while simultaneously enabling individual species to evolve extreme dietary specialization.

‘One of our key results is that snakes underwent a profound shift in feeding ecology that completely separates them from other reptiles,’ Rabosky said. ‘If there is an animal that can be eaten, it’s likely that some snake, somewhere, has evolved the ability to eat it.’

‘Museum specimens give us this incredible window into how organisms make a living in nature. For secretive animals like snakes, it’s almost impossible to get this kind of data any other way because it’s hard to observe a lot of their behavior directly,’ said study co-lead author Pascal Title of Stony Brook University.

The findings of the study “The macroevolutionary singularity of snakes” have been pubished in the Science journal and can be accessed here.

Notes to editors

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