Speed and Agility in Mammals Restricts Evolution of the Vertebral Column

Scientists have unraveled one of the riddles of mammal evolution. They've discovered why mammals have a strong conservation in the number of their trunk vertebrae; it all has to do with the essential role of speed and agility.

The mammal vertebral column differs widely between species, reflecting adaptations to a wide range of lifestyles. Yet as a rule of thumb, the number of trunk vertebrae varies very little between most mammal species. The vertebral column in general is considered to be centrally important to the evolution of vertebrates, which is why this consistency in trunk vertebrae is puzzling. That's why researchers decided to investigate a bit further.

"The stiffness of the back of a mammal is key to whether evolutionary change is possible or not," said Frietson Galis, one of the researchers, in a news release. "A combination of developmental, biochemical and evolutionary insights and a large dataset were necessary to solve this puzzle of mammal evolution."

The researchers analyzed 774 skeletons of 90 different species of mammals. These skeletons belonged to collections of 9 European natural history museums.

In the end, they found that evolutionary change is virtually impossible in fast running and agile mammals, but only marginally affects slow and sturdy mammals. This is because several mutations are necessary to change the number of trunk vertebrae, with single mutations leading to irregularly shaped transitional lumbosacral vertebrae that are asymmetrically and incompletely fused to the sacrum. This reduces flexibility and can hamper running and jumping, which means that the initial changes are evolutionarily undesirable.

"The locomotion of slow mammals with a stiff back is only marginally affected by irregular lumbosacral joints, but for fast running mammals such joints are fatal," said Clara ten Broek, one of the authors of the new study.

The findings reveal how mammal vertebrae evolved and why changes in the trunk vertebrae weren't forthcoming.

The new study is published in the journal Proceedings of the National Academy of Sciences.