Snakes do it faster, better: How a group of scaly, legless lizards hit the evolutionary jackpot

What triggered the evolutionary explosion of snake diversity—a
phenomenon known as adaptive radiation—that led to nearly 4,000 living
species and made snakes one of evolution's biggest success stories?

A large new genetic and dietary study of snakes, published in Science, suggests that speed is the answer. Snakes evolved up to three times faster than lizards, with massive shifts in traits associated with feeding, locomotion and sensory processing. 

The study was conducted by an international team of top-tier experts in the field of snake evolution and ecology from the United States, the United Kingdom, Australia, Brazil, and Finland.

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," says University of Michigan evolutionary biologist Daniel Rabosky, senior author of the article.

“Various morphological features, including data on skull shape and complexity collected from our laboratory, indicate that snakes are highly distinctive and have evolved faster than lizards, aligning with ecological changes” explains Nicolas Di-Poi, study co-author and Research Director at the Institute of Biotechnology, University of Helsinki.

Evolutionary tree of snakes and lizards

For the study, researchers generated the largest, most comprehensive evolutionary tree of snakes and lizards by sequencing partial genomes for nearly 1,000 species. In addition, they compiled a huge dataset on lizard and snake diets, examining records of stomach contents from tens of thousands of preserved museum specimens.

They fed this mountain of data into sophisticated mathematical and statistical models, backed by massive amounts of computer power, to analyze the history of snake and lizard evolution through geological time and to study how various traits, such as limblessness and skull complexity, evolved.

This multipronged approach revealed that while other reptiles have evolved many snakelike traits—25 different groups of lizards also lost their limbs, for instance—only snakes experienced this level of explosive diversification.

So, it seems there is something special about snakes that enabled them to hit the evolutionary jackpot. Maybe something in their genes that allowed them to be evolutionarily flexible while other groups of organisms are much more constrained.

Macroevolutionary singularity 

The ultimate causes, or triggers, of adaptive radiations is one of the big mysteries in biology. In the case of snakes, it's likely there were multiple contributing factors, and it may never be possible to tease them apart.

The authors of the study refer to this once-in-evolutionary-history event as a macroevolutionary singularity with "unknown and perhaps unknowable" causes.

A macroevolutionary singularity can be viewed as a sudden shift into a higher evolutionary gear, and biologists suspect these outbursts have happened repeatedly throughout the history of life on Earth. 

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, as a group, to pursue a much broader array of prey types, while simultaneously enabling individual species to evolve extreme dietary specialization.

Today, there are cobras that strike with lethal venom, giant pythons that constrict their prey, shovel-snouted burrowers that hunt desert scorpions, slender tree snakes called "goo-eaters" that prey on snails and frog eggs high above the ground, paddle-tailed sea snakes that probe reef crevicesfor fish eggs and eels, and many more.

Original article 

Pascal O. Title et al., The macroevolutionary singularity of snakes.Science 383, 918-923(2024). DOI:10.1126/science.adh2449

Diversification of snakes

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.

Then, in a burst of innovation in form and function, the ancestors of snakes evolved legless bodies that could slither across the ground, 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, allowing them to quickly exploit new opportunities that emerged after an asteroid impact wiped out roughly three-quarters of the planet's plant and animal species.