The study found that long before vertebrates walked on land, some organisms moved with an asymmetrical gait.
When cheetahs are chasing prey, they move in an asymmetrical gait—more precisely, a gallop, just like horses—their fore and hind limbs move in pairs. Salamanders, on the other hand, run with a symmetrical gait – their left and right limbs move opposite each other.
Historically, scientists believed that the symmetrical gait was evolutionarily older – salamanders were a model for how the first land animals moved. Conversely, asymmetrical gaits were thought to have evolved independently across species over time.
But a new study offers a different version, according to which the asymmetrical gait existed in animals that lived more than 400 million years ago in ancient oceans, long before vertebrates came to land.
Asymmetrical gaits underlie the speeds achieved by cheetahs, greyhounds and kangaroos.
“That’s why so many people thought it was purely mammalian ‘innovation’,” explained Michael Granatsky, an evolutionary biologist at the New York Institute of Technology and one of the study’s authors.
However, evidence has been accumulating to suggest that asymmetrical gait may not have developed as recently as previously thought, and that it certainly was not exclusive to mammals. So, the researchers found that some species of crocodiles move at a gallop, there is also at least one species of sea turtles that “jumps” under water, and there are also fish that walk along the bottom of the ocean.
“African lungfish have little “noodles” instead of legs, but they are able to walk on the bottom. And out of her 10 steps, half will be symmetrical and half asymmetrical.
This prompted researchers to rethink their understanding of how asymmetric movements evolved. From a sample of 308 living species of jawed vertebrates, including mammals, reptiles and other animals, the team compiled a tree of evolutionary relationships between species. Each species that could not move asymmetrically was given a score of 0, and a score of 1 if it could. They then tested a range of potential models for the evolution of asymmetrical gait to see which one best fits the data.
The model that turned out to be the most likely did not impose any restrictions on how asymmetrical gaits could develop, and the “gains” and “losses” of asymmetrical gaits occurred freely over time.
According to Eric McElroy, a biologist at the College of Charleston and co-author of the study, the resulting model showed about a 75% probability that the ancestor of jawed vertebrates more than 400 million years ago had an asymmetrical gait, and that it could be both lost and acquired as evolution.
At the same time, the researchers admit that their theory is still difficult to confirm.
“When you try to estimate how something that was dead for 400 million years moved, you have to speculate a bit,” said McElroy.